Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue12.Dec2000:
Organic Geochemistry 31 (2000) 1267±1284
www.elsevier.nl/locate/orggeochem
The La Luna formation: chemostratigraphy and organic
facies in the Middle Magdalena Basin
A. Rangel a,*, P. Parra b, C. NinÄo b
b
a
ECOPETROL-ICP, PBX 4185, Bucaramanga, Colombia
Universidad Industrial de Santander, Colciencias, GEMS Ltda, PBX 4185, Bucaramanga, Colombia
Abstract
A detailed geochemical study and a sequence stratigraphic interpretation have been conducted on a sedimentary
sequence of the Upper Cretaceous La Luna Formation, in a section outcropping in the eastern ¯ank of the Middle
Magdalena Basin (MMB), Colombia. The goals were to evaluate geochemical variability related to lithofacies and
organic facies changes, characterize depositional environment and investigate the possible relationship between geochemical data and sequence stratigraphic cycles. The La Luna Formation is composed of organic-rich sediments of
monotonous appearance, with good to excellent potential for oil generation. Most of the bulk, petrographic and biomarker parameters display a relatively narrow range of variation. However, the geochemical variations are sucient to
dierentiate organic facies types B, BC and C in the Salada Member, B and D in the Pujamana Member and B in the
Galembo Member. Certain biomarker ratios are consistent within the La Luna Formation and are characteristic of its
depositional environment, for example, average ratios of diasterane/sterane are lower than 1, Ts/Tm averages are less
than 0.33, the C35/C34 hopane ratio is more than 0.92, and oleanane/C30 hopane ratios range from 0.02 to 0.19.
Regarding depositional condition indicators, the C35/C34 hopane ratio shows a good positive correlation with HI. This
suggests that in carbonate environment changes in this parameter are more strongly related to redox condition than to
changes in carbonate content. Regarding the possible relationship between organic matter characteristics and sea level
changes, in regressive carbonate shelves during shallow stages, HI tends to increase and TOC tends to decrease, while
in regressive siliciclastic shelves, both TOC and HI decrease continuously. Some biomarker ratios (oleanane/C30
hopane, C20/C23 tricyclic, Ts/Tm) increase during base level falls. Regarding d 13C/12C isotope composition, the aromatic fraction and whole bitumen display an isotopic shift associated to the main deepening event in the section.
# 2000 Elsevier Science Ltd. All rights reserved.
Keywords: La Luna Formation; Sequence stratigraphy; Molecular geochemistry; Depositional environments; Carbonates; Source rock
1. Introduction
The La Luna Formation has been considered to be
the main hydrocarbon source rock in the Middle Magdalena Basin (MMB) by Zumberge (1984), Rangel et al.
(1996), as well as in other important basins such as
Maracaibo Basin (Talukdar et al., 1986). Nevertheless,
the existing knowledge about depositional processes
controlling the Upper Cretaceous-La Luna Formation
* Corresponding author.
E-mail address: arangel@ecopetrol.com.co (A. Rangel).
deposit in the MMB is minimal and there are few integrated stratigraphic and geochemical studies on this
formation.
Zumberge (1984) addressed the hydrocarbon potential of the La Luna Formation in the La Sorda Creek.
Rangel et al. (1996) identi®ed four oil families, and
based on oil characteristics he suggested that two of
them are possibly derived from the La Luna Formation.
Ramon and Dzou (1999) discussed some geochemical
processes in the MMB, based on oil-derived parameters.
A clear understanding of geochemical characteristics of
the La Luna Formation and associated organic facies
would increase con®dence in the oil±La Luna source
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00127-3
1268
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
rock correlation, as well as better delineate the contribution of each part of the sequence to oil generation
in this basin. The relationship of geochemical parameters to local depositional processes and stratigraphic
cycles would help identify the best source rock areas and
allow us to better understand the spatial relationship
between source rock and migration conduits for oils
related to the La Luna Formation in the MMB.
Bulk, petrographic, carbon-isotopic and molecular
geochemical data were used in this study, along with a
stratigraphic interpretation. The objectives were to evaluate geochemical changes related to existing lithofacies and
organic facies, to characterize depositional environment
and to investigate possible relationships between geochemical parameters and sequence stratigraphy cycles.
2. Methodology and sampling
Outcrop samples (160) were systematically collected
from an estimated 240 m vertical interval of the La
Luna Formation in La Sorda Creek, on the western
¯ank of the Nuevo Mundo Syncline.
The sequence stratigraphic interpretation utilized in
this study follows the methods developed by the Genetic
Stratigraphy Research Group (GSRG) of Colorado
School of Mines (Cross, 1988; Cross et al. 1993; Cross
and Lessenger, 1995). This methodology identi®es unidirectional trends of increasing and decreasing ratio of
accommodation space to sediment ¯ux (A/S ratio).
Stratigraphic cycles register the time in both rise and fall
of A/S.
Regarding the geochemical study, all samples were
submitted for bulk geochemical analysis. Organic carbon
analysis (in a Leco Carbon Analyzer) and Rock Eval
pyrolysis analysis (Espitalie et al., 1977) were performed
on all samples along with determination of carbonate
content by acid treatment.
The screening results were followed by analyses of 40
samples by gas chromatography and gas chromatography±mass spectrometry of rock extracts. Kerogen
was isolated by consecutive HCl and HF treatment, and
¯oated in ZnBr. Powdered samples were Soxhlet
extracted with chloroform to remove extractable
organic matter. The hexane soluble material was then
separated by liquid chromatography into saturate, aromatic and NSO fractions on an alumina and silica column.
The whole bitumen, saturate and aromatic fractions were
prepared for carbon isotope composition by a modi®ed
method of Sofer (1984) and measured on a Finnigan
MAT Delta S instrument The saturate fractions were
subjected to GC and GC±MS analyses. Total alkane
fractions or branched/cyclic sub-fractions were analyzed
in the selected ion recording mode on an HP 5890 GC±
MS system. The GC column was a 30 m HP-5 temperature programmed from 60 to 320 C at 4 C/min and
helium carrier gas at 1.5 ml/min. Selected ion recording
was performed on the 5890 MSD monitoring ions 177,
191, 217, 218, 259 for saturate fractions.
3. Geological framework
The La Sorda Creek section is located approximately
20 km West of Bucaramanga (Fig. 1). The sedimentary
column of the MMB, consists of Jurassic sandstones of
¯uvial origin, Cretaceous limestones and shales of shallow
marine to paludal origin, and Tertiary sedimentary
rocks of predominantly ¯uvial origin (Fig. 2).
Morales (1958) subdivided the La Luna Formation
into three members, which from base to top are, Salada
(Turonian), Pujamana (upper Turonian-lower Coniacian)
and Galembo (uppermost Turonian-Coniacian and
possibly Santonian).
The tectonic evolution of the eastern edge of the MMB
is closely related to the tectonic evolution of the Eastern
Cordillera, as widely discussed by several authors
(Campbell and BuÈrgl, 1965; Macellari, 1988; Colleta et
al., 1990; Dengo and Covey, 1993; Cooper et al., 1995).
The Eastern Cordillera consists of predominantly
clastic material and carbonates overlaid on a Precambrian and Paleozoic basement. During Triassic±
Jurassic time, rifting and magmatic events produced by
Paci®c plate subduction, was responsible for the uplift
of the Central Cordillera and the deposition of continental and volcanic rocks in the backarc setting. During the Early Cretaceous, a marine transgression led to
the backarc basin to be ®lled with a prograding
sequence. Maximum transgression during the Turonian±Santonian period, led to the deposition of the La
Luna Formation and its equivalent rocks, namely the
Villeta (in the Upper Magdalena Valley) and Chipaque
or Gacheta Formations (in the Llanos basin and the
Eastern Cordillera), both also with excellent source
rocks. During the latest Cretaceous (Maastrichtian), the
beginning of marine regression allowed deposition of a
transitional sequence (the Umir Formation); by accretion
in the Western Cordillera. Finally, during Tertiary, the
rising of the Eastern Cordillera (Andean Orogeny) was
responsible for the development of a whole continental
sequence. This event reached its maximum during the
Miocene-Pliocene period and is still continuing at present.
4. Results and discussion
4.1. Lithofacies and organic facies in the La Luna
Formation (La Sorda Creek section)
The La Luna Formation is a calcareous petroleum
source rock with good to excellent potential for oil.
Most of the sedimentologic and geochemical parameters
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
1269
Fig. 1. Location of the La Sorda creek section in the Middle Magdalena Basin, Colombia.
of the La Luna Formation in La Sorda Creek display a
relatively narrow range (Table 1). However, the sedimentologic and geochemical variations are sucient to
dierentiate several lithofacies and organic facies (Table
2 and Fig. 3).
4.1.1. Lithofacies and organic facies in the Salada
Member
This member consists of 102.2 m of foraminiferal
wackestones interbedded with occasionally-cherty calcareous shales. The lithologies observed in this member
are grouped in four sedimentary lithofacies.
. Poorly laminated wackestones (plW): This lithofacies consists mainly of planktonic foraminiferal
wackestones with some vertebrae and bones of
®sh, as well as pyrite traces.
. Muddy laminated wackestones and calcareous
shales (mlW): This lithofacies consists of dark
gray, thin bedded foraminiferal wackestones and
calcareous shales, both with planoparallel laminations and small nodules.
. Phosphatic calcareous shales and laminated mudstones (pcSM): This lithofacies consists of calcareous, slightly phosphatic and ®nely laminated
shales and claystones, with abundant foraminifera
(Fig. 4)
. Crystalline limestone (cL): This lithofacies consists
of two layers of 40 cm of greenish-gray crystalline
limestone with laminae of organic matter.
Fig. 2. Cretaceous stratigraphic units in the Middle Magdalena
Basin.
Organic facies type B, BC, and C, sensu Jones (1984),
was identi®ed in the Salada Member (Table 2). Organic
facies B is related to mlW and pcSM lithofacies. This
facies is composed of organic matter with average values
of HI around 428 (mg HC/g TOC), TOC around 4.3
(wt.%) and S2 between 15.6 and 22.2 mg HC/g sample.
The saturated hydrocarbons of this organic facies is
1270
Table 1
Average values (meanstandard deviation) of bulk geochemical parameters for each lithofacies of the La Luna Formation
%
TOC
Tmax
( C)
S1
(mg HC/g
rock)
S2
(mg HC/g
rock)
S3
(mg CO2/g
rock)
Hl
(mg HC/g
TOC)
1.4
3.9
3.2
8.5
15
15
14
44
2.61.0
2.30.8
2.41.0
2.40.9
4344
4364
4383
4364
4.612.25
3.721.64
3.791.88
4.041.94
13.465.23
11.334.52
11.365.22
12.074.98
0.590.17
0.470.15
0.400.08
0.490.16
pcSM2b
B
mlW2
pcSM2,
B, mlW2
49.3
0.5
1.4
51.1
63
3
3
69
3.21.1
0.30.1
3.70.9
3.11.2
4353
437
4383
4363
4.432.52
0.04
4.410.66
4.242.58
14.415.69
0.040.02
15.373.65
13.826.23
plW
mlW3
pcSM3b
cL
plW, mlW3,
pcSM3, cL
18.4
10.9
10.5
0.5
40.4
18
11
8
7
44
1.81.3
4.21.3
4.61.2
0.90.9
2.81.9
4353
4383
4344
4329
4355
1.741.33
2.950.98
5.863.47
0.810.95
2.602.41
7.065.85
15.553.18
22.509.66
3.474.33
11.288.85
Organic
facies
Lithofacies
Galembo
B
B
B
Organic
facies B
pPh
phPW
pcSM1b
pPH, phPW,
pcSM1
B
D
B
Organic facies
B and D
BC
B
B
C
Organic facies
B, BC and C
Average
Pujamana
Average
Salada
Average
a
b
c
%
thickness
n samples analyzed.
Total lithofacies pcSM=pcSM1+pcSM2+pcSM3=62.96%.
n.d., no data
OI
(mg CO2/g
rock)
%
Carbonate
%
Bitumen
52363
49162
46772
49568
259
229
2010
229
31.99.7
52.010.7
27.414.5
37.315.8
2.340.32
2.350.98
1.080.77
1.780.90
0.470.11
0.160.11
0.690.08
0.460.14
43488
1412
42251
415121
1817
5230
205
2018
22.313.1
0.30.5
49.83.7
22.614.6
1.551.15
n.d.c
1.680.76
1.571.10
0.470.12
0.560.11
0.520.05
0.310.22
0.480.15
36077
39584
474132
234190
366132
3827
56.910.2
136
5332
3026
75.112.3
0.89
28.111.3
66.211.0
59.920.7
0.640.41
3.692.83
n.d.
1.601.98
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
na
Member
1271
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Table 2
Characteristics of the organic facies in the La Luna Formation (La Sorda Creek section)
Member
Galembo
Pujamana
Salada
Organic facies
B
B
D
B
BC
C
% in the section
TOC (wt.%)
HI (mg HC/g TOC)
OI (mg CO2/g TOC)
Tmax ( C)
Carbonate (%)
8.48
2.4 (0.9)
495 (68)
22 (9)
436 (4)
37.3 (15.8)
50.62
3.2 (1.1)
434 (87)
18 (16)
436 (3)
23.6 (14.1)
0.51
0.3 (0.1)
14 (12)
52 (30)
437
0.3 (0.5)
21.42
4.3 (1.3)
428 (111)
15 (9)
436 (4)
44.8 (17.9)
18.44
1.8 (1.3)
360 (77)
38 (27)
435 (3)
75.1 (12.3)
0.53
0.9 (0.9)
234 (190)
53 (32)
432 (9)
66.2 (10.7)
Fig. 3. Stratigraphic pro®le, cycles interpretation and lithofacies and organic facies substitution diagrams of the La Sorda Creek
section. The percentages of each lithofacies are shown in the diagram.
characterized by the high ratios of C30/C29 sterane and
relatively low ratios of C29/C30 hopane and C23 tricyclic/
C24 tetracyclic (Fig. 5; Tables 2 and 3).
The organic facies BC is related to the plW lithofacies. This facies is characterized by average values of
HI around 360, TOC around 1.8%, and an average of
S2 of 7 mg HC/g of rock. This facies has the highest
values of oleanane/C30 hopane and tricyclic C20/C23
ratios, and relatively low values of diasterane/sterane
(0.46). The other geochemical parameters are within
the range of the La Luna Formation (Tables 2 and 3;
Fig. 5).
1272
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Fig. 4. Photomicograph of a thin section from mlW (a) and pcSM2 (b) lithofacies. Sample CD-152 and sample CD-90. Lamination
corresponds to alternation of foraminifer tests, organic matter and clay minerals. Magni®cation 2.5. Plane light.
An organic Facies C associated with the cL lithofacies, displays values of HI around 234 and TOC
around 0.9%. This organic facies has the lowest average
ratios of oleanane/C30 hopane (0.02), C30/C29 sterane
(0.17), gammacerane/C30 sterane (0.09), diasterane/sterane
(0.18) and C35/C34 hopane (0.92) (Tables 2 and 3; Fig. 5).
4.1.2. Lithofacies and organic facies in the Pujamana
Member
This member consists mainly of calcareous phosphatic shales with abundant foraminifera, phosphatic calcareous mudstones, cherts and bentonites. Calcareous
nodules as large as 1 m in diameter are observed. The
abundance of pyrite is greater than in Salada Member.
Three lithofacies were identi®ed in this member (Fig. 3).
. Phosphatic calcareous shales and laminated mudstones (pcSM): This lithofacies consists of shales
and calcareous mudstones that are ®nely laminated and slightly phosphatic, with abundant foraminifera and bones of ®sh.
. Bentonites (B): This lithofacies is composed of
yellowish-gray, greenish-gray and grayish-orange
clays. (smectite±illite). The lithofacies generally
appears in tabular layers thinner than 35 cm.
. Muddy laminated wackestones and calcareous
shales (mlW): This lithofacies consists of dark
gray, thin-bedded foraminiferal wackestones and
calcareous shales.
Organic facies type B and D were observed in this
member. The organic facies B, the most abundant, is
associated with the pcSM and mlW lithofacies. Averages
values of HI and TOC are around 434 and 3.2, respectively, and S2 varies between 14,41 and 15,37. This
organic facies shows very little variation in the average
values of Ts/Tm (0.16±0.19), C35/C34 hopane (1.10±1.32),
C24 tetracyclic/C26 tricyclic (1.08±1.09) (Tables 1, 2 and
3; Fig. 5).
Organic facies C, associated with the bentonite lithofacies, has HI values of about 14, and TOC of about
0.3%. This is a minor organic facies (0.5% of the geological column by volume) (Tables 1 and 2).
4.1.3. Lithofacies and organic facies in the Galembo
Member
The lower part of this member (19 m) was studied.
The section consists of a series of packstone phosphorites (sensu Greensmith, 1989), wackestones and phosphatic packstones, chert and calcareous and phosphatic
shale. Layers are tabular, with a thickness range
between 5 and 30 cm and concretions up to 2 m in diameter are present toward the base of this member. Three
lithofacies were identi®ed in this member (Fig. 3).
. Phosphatic calcareous shales and laminated mudstones (pcSM): This lithofacies consists of calcareous, slightly phosphatic and ®nely laminated
shales and claystones, with abundant foraminifera.
. Packstone phosphorites (pPh): This facies consists
of packstone phosphorites with abundant foraminifera, pellets, ®sh bone fragments, and
oolites. Wavy and lenticular lamination is common and in some samples ¯aser lamination can be
observed.
. Phosphatic packstones and wackestones (phPW):
This lithofacies was observed towards the top of
the Galembo member. It consists mainly of
slightly phosphatic foraminiferal packstones, with
lenticular and wavy lamination. This lithofacies
also contain ®sh fragments, pellets, oolites and
Planktonic fossils.
The organic matter of Galembo Member exhibits the
greatest HI around 495, OI average of 22, TOC of about
2.4% and S2 of 12. These geochemical parameters are
typical of organic facies B. This organic facies displays
Table 3
Average values (meanstandard deviation) of biomarker parameters for each lithofacies and organic facies in La Luna Formation
Organic Lithofacies na Ts/Tm
facies
Diasterane/ C35/C34
regular
extended
steraneb
hopanes
Tricyclic
terpanes/
hopanesc
%C27
sterane
%C29
sterane
C30/C29
sterane
C24 tetracyclic/ C23 tricyclic/ C29 norhopane/ Oleanane/ Gammacerane/ Steranes/
C26 tricyclic
C24 tetracyclic C30 hopane
C30 hopane C30 hopane
hopanesd
Galembo
B
B
B
Organic
facies B
phPW
pPh
pcSM1
pPH,
phPW,
pcSM1
0.71
0.350.07
0.590.49
0.510.36
3.132.06
2.020.08
2.850.53
2.630.93
28.678.40
38.529.59
38.976.40
36.538.19
24.427.98
29.107.28
26.881.98
27.075.10
0.26
0.230.03
0.270.02
0.260.03
0.980.40
0.590.07
1.120.57
0.910.45
8.225.59
14.551.17
10.175.92
11.204.93
2.130.58
1.400.66
0.950.33
1.360.65
0.05
0.040.03
0.130.11
0.080.09
n.d.e
0.100.04
0.330.37
0.250.31
0.150.07
0.340.08
0.290.09
0.280.10
Pujamana B
B
Average Organic
facies B
pcSM2
mlW2
pcSM2,
mlW2
23 0.160.05 0.450.44
8 0.190.02 1.200.60
31 0.170.05 0.520.49
1.100.18 2.331.06 44.576.12 23.465.71 0.290.08 1.090.56
1.320.34 2.601.23 39.122.67 26.204.50 0.350.11 1.080.50
1.130.22 2.361.05 43.946.06 23.775.58 0.300.08 1.090.54
11.865.01
11.005.47
11.764.96
1.270.49
0.940.65
1.230.51
0.080.11
0.030.02
0.080.11
0.320.28
0.300.28
0.310.27
0.200.09 0.280.16 0.85
0.100.04 0.220.02 n.d.
0.190.09 0.280.16 0.85
Salada
plW
mlW3
pcSM3
mlW3,
pcSM3
cL
6
8
6
14
0.460.28
0.310.19
0.640.49
0.460.38
1.030.09
1.100.09
1.240.38
1.170.28
3.010.78
2.730.93
1.780.72
2.320.95
38.994.16
41.745.29
38.914.98
40.535.17
27.364.89
26.122.88
24.235.81
25.314.29
0.310.04
0.210.05
0.760.66
0.460.51
0.980.36
0.930.24
2.972.31
1.801.78
9.547.68
12.325.90
5.706.60
9.496.86
1.380.25
1.430.32
0.780.43
1.150.49
0.190.15
0.050.06
0.030.01
0.040.05
0.260.15
0.230.23
0.150.06
0.200.19
0.240.05
0.280.11
0.210.12
0.260.11
0.18
0.92
1.90
41.22
26.63
0.17
0.65
17.00
1.31
0.02
0.09
0.270.00 0.21
Average
Average
a
b
c
d
e
BC
B
B
Organic
facies B
C
2
3
4
7
0.28
0.25
0.260.01
0.260.02
0.310.17
0.190.04
0.330.22
0.250.16
1 0.16
n, Samples analyzed.
C27 ba diasterane (20S)/C27 aaa sterane (20R).
Sum tri/sum hopanes.
Sum sterane/sum hopanes.
n.d., no data.
n.d.
1.130.16
1.350.32
1.280.28
C20/C23
tricyclic
Pr/Ph
0.610.45
0.200.02
0.520.51
0.420.37
1.01
n.d.
0.60
0.81
0.790.71
0.380.23
0.250.05
0.340.21
0.52
0.98
0.49
0.73
n.d.
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Member
1273
1274
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Fig. 5. Histograms showing the average values and standard deviation of some geochemical bulk parameters and representative biomarker ratios for each lithofacies in the La Luna Formation.
1275
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Fig. 6. Schematic evolution of the carbonate ramp.
Table 4
Carbon isotopes ratios in whole bitumen, saturates and aromatic fractions for dierent samples of the La Luna formation
d13C saturates
13
ÿ27.40
ÿ27.43
ÿ27.24
ÿ27.33
ÿ28.45
ÿ27.42
ÿ27.35
ÿ27.32
ÿ27.38
ÿ27.37
ÿ27.21
ÿ27.36
25.9
34.0
34.3
58.2
74.4
102.5
112.0
130.0
ÿ27.76
ÿ27.27
ÿ27.33
ÿ27.90
ÿ26.91
ÿ26.08
ÿ26.61
ÿ26.47
ÿ27.91
ÿ27.57
ÿ27.27
n.d.
ÿ27.52
ÿ28.18
ÿ27.78
ÿ27.41
ÿ27.40
ÿ27.28
ÿ27.31
ÿ27.94
ÿ26.99
ÿ26.78
ÿ26.34
ÿ26.49
142.0
179.5
192.5
204.0
221.0
228.0
233.0
235.0
ÿ26.13
ÿ27.51
ÿ27.62
ÿ27.61
ÿ27.46
ÿ27.41
ÿ27.39
ÿ27.96
ÿ27.98
ÿ27.99
ÿ28.43
ÿ27.55
ÿ28.31
ÿ28.31
ÿ27.56
ÿ28.33
ÿ26.56
ÿ27.47
ÿ27.84
ÿ27.67
ÿ27.60
ÿ27.43
ÿ27.79
ÿ27.86
ÿ27.24
ÿ27.82
ÿ27.30
Member
Organic facies
Lithofacies
Sample ID
Galembo
B
B
B
B
pcSM1
phPW
pPh
pcSM1
CD07
CD15
CD22
CD41
3.0
9.0
14.0
18.2
Pujamana
B
B
B
B
B
B
B
B
pcSM2
pcSM2
pcSM2
pcSM2
pcSM2
pcSM2
mlW2
pcSM2
CD50
CD59
CD60
CD82
CD98
CD104
CD110
CD114
Salada
B
BC
BC
B
BC
B
BC
B
pcSM3
plW
plW
mlW3
plW
pcSM3
plW
pcSM3
CD119
CD129
CD135
CD143
CD154
CD157
CD158
CD160
Average
Cummulative thickness
13
C Bitumen
C Aromatics
1276
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
little variation of geochemical parameters such as Ts/Tm
(0.25±0.28), C35/C34 hopane (1.13±1.35) and C30/C29
sterane (0.23±0.27). The oleanane/C30 hopane ratio is
high, varying between 0.04±0.13 (Tables 2 and 3; Fig. 5).
The phosphatic packstone lithofacies of the Galembo
member shows the highest values of HI, the highest
ratios of sterane/hopane (0.34) and C23 tricyclic/C24
tetracyclic (14.55) and the lowest ratios of C20/C23 tricyclic (0.20) and C24 tetracyclic/C26 tricyclic (0.59) of the
La Luna Formation. This parameter could correlate
with increased algal material within this lithofacies.
These characteristics are consistent with the observed
abundance of phosphates indicating an upwelling event.
The HI versus OI plot, the modi®ed Van Krevelen
diagram (Fig. 7), shows that organic matter is predominantly Type II, amorphous (algal and bacterial),
and Type I organic matter. A very small proportion of
Type II/III and III/IV kerogen is also observed. Talukdar
et al. (1986) also noted, based on microscopic analyses
and molecular geochemistry, that the bulk of the
organic matter is algal and bacterial in origin.
Summarizing, Organic Facies B is the most representative organic facies of the La Sorda Creek section
with a total percentage of 80.5% in volume. This facies
is related to phPW, pPh, pcSM, and mlW lithofacies.
The organic facies BC represent 18.4% of the section by
volume. Organic facies C and D represent 0.5 and 0.5%
of the stratigraphic column, respectively.
4.2. Sedimentary cycles
Based on lithologic and sedimentological characteristics, a facies substitution diagrams (Fig. 3) for the La
Luna formation in La Sorda Creek was interpreted.
This diagram provides information about the natural
succession and substitution of lithofacies under increasing
or decreasing accommodation conditions. The area of
each lithofacies is proportional to its abundance within
the section. This diagram helps to identify stratigraphic
cycles of high, intermediate, and low frequency, for
sequence stratigraphic analysis.
The intermediate and low frequency cycles for the La
Luna Formation in La Sorda Creek section de®ned in
this work, using lithologic and stratigraphic attributes,
correlate with cycles described for this formation by
Reyes et al. (1998), using well logs and cores.
From base to top, the transition of the argillaceous
shales of the SimitõÂ Formation to calcareous shales, and
the appearance of thin layers of foraminiferal wackestones (Salada Member), indicates a generalized low
frequency hemicycle of base level fall. This hemicycle
reaches a maximum fall (minimum in A/S) where the
limestone layers have the greatest thickness, up to 170 m
of measured thickness (Fig. 6).
From this point of maximum progradation, a deepening
event began, evidenced by the decrease in thickness of the
wackestones layers, and the increase in thickness of
shales and calcareous mudstones (pcSM lithofacies).
This increase in A/S ratio ends in a surface of maximum
¯ooding up to 75 m thick, with high contents of organic
matter and low percentages of carbonates.
After the maximum deepening within the Pujamana
Member, the amount of phosphates progressively
increases. Packstone phosphorites (lithofacies pPh) and
phosphatic packstones (lithofacies phPW) appear. This
demonstrates a base level fall in the basin (Fig. 5). The
shallowing is not completely recorded in the column of
the La Sorda Creek, because of a fault at the top of the
La Sorda Creek section. Stratigraphic events in the
basin are shown in detail by the intermediate frequency
cycles (third order cycles, sensu Vail et al., 1977) (Fig. 3).
4.3. Paleoceanographic events and environmental
considerations
Fig. 7. Modi®ed van Krevelen diagram showing lithofacies and
organic facies.
The ®ne lamination and the calcareous character of
Salada Member and the presence of very ®ne planktonic
foraminifera arranged in laminae, demonstrate a low
energy marine environment of deposition.
The shale sequence and the minor calcareous character, which characterize the Pujamana Member, evidence deeper conditions in the carbonate platform than
those during the deposition of the Salada Member (Fig.
6). The increase of phosphates towards the top is consistent with the occurrence of upwelling currents. The
high contents of organic matter, as well as the presence
of pyrite are evidence of high productivity and suboxic
to anoxic condition, which favored accumulation and
preservation of organic matter. The bentonites represent
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
sedimentary input from volcanic activity during this
period.
The packstone phosphorites and phosphatic packstones that characterize the Galembo Member indicate a
¯ow regime greater than the one during the sedimentation of the Salada Member and the Pujamana Member.
The presence of wavy and lenticular lamination, ripples,
¯aser laminations, reworking of ®sh fragments and presence of quartz demonstrates a shallow water environment within wave base. These lithofacies correspond to
the shallowest level within the carbonate platform in
which the La Luna Formation was deposited (Fig. 6).
The increase of phosphates indicates upwelling and high
primary productivity in a suboxic environment.
The accumulation of organic rich sediments in this
area, during Late Cretaceous, was favored by interaction
of important regional and global paleogeographic events,
such as Ekman water transport (closely related to
upwelling regimens), and oceanic anoxic events described
by Macellari (1998), Martinez and HernaÂndez (1992) and
Villamil (1998) for the north corner of South America.
Taking into account the data of this study, and the
regional paleogeographic framework, the La Luna Formation was deposited on a large carbonate platform. On
this platform, sedimentation took place in a through,
limited to the West by the submerged Central Cordillera, which restricted water circulation and contributed
to an anoxic environment of deposition. During some
less restricted periods, upwelling currents favored high
primary productivity, and organic matter accumulation
and preservation that characterize the sediments of the
La Luna Formation. According to Martinez and HernaÂndez (1992), the deepest parts of the ramp were located near the present area of Maracaibo, where a
maximum depth of approximately 600 m was attained
during the Campanian.
4.4. Maturity level
Because of the predominance of amorphous organic
matter, the samples contain little vitrinite, and therefore
the vitrinite re¯ectance measures are unreliable. The
maturation level in this study was determined using RockEval pyrolysis. Tmax values (average 436 C) listed in Table
1. This suggests the section is early mature to mature.
Average Tmax and S1 values suggest that the section is
not greatly aected by oil migration, and that geochemical variations can largely be considered as indicative of depositional conditions and associated variations
in the type of organic matter.
4.5. Relationship between geochemical parameters and
sequence stratigraphy
Several authors have described the relationship
between the characteristics of organic matter and sea
1277
level changes (e.g. Middleburg et al., 1991; Pasley et al.,
1993). The relationship between some geochemical
parameters commonly used to characterize petroleum
source rocks and interpreted sequence stratigraphy
cycles in the La Sorda Creek section are graphically
shown in Fig. 8. The trends in variations of geochemical
parameters in this ®gure are generalized. In the regressive carbonate shelf (Salada Member and Galembo
members) an increase in the relative values of HI and a
decrease in TOC contents is observed, while in the
regressive siliciclastic shelf (Pujamana-Salada Member),
both TOC and HI values decrease continuously during
the shallowing stage. Some biomarker parameters such
as oleanane/C30hopane and C20/C23 tricyclic ratios
usually used to re¯ect relative contribution of continental organic matter, increase during regression
cycles. This occurs on both carbonate and siliciclastic
shelves. Other parameters suggested by some authors
(e.g. Waples and Machihara, 1990) as sensitive to
lithology, such as Ts/Tm ratios, increase in carbonate
levels. In the carbonate shelves, HI values increase
despite the higher oleanane/C30 hopane and C20/C23
tricyclic ratios, probably suggesting that algal productivity and preservation are predominant processes in
this depositional environment. Based on these results,
organic geochemistry could be considered as an important
tool to support the sequence stratigraphy architecture of
a sedimentary sequence. Conversely, sequence stratigraphy is a useful tool to follow oil prone strata.
4.6. Sedimentological controls on geochemical composition
Biomarker fragmentograms appear similar for the
dierent organic facies upon initial inspection (Fig. 9).
The average of certain biomarker ratios (Table 4 and
Fig. 5), show a narrow range. Therefore, these values
can be considered typical for the La Luna Formation
and its depositional conditions. This is useful for oilsource rock correlation in the MMB.
The pristane/phytane (Pr/Ph) ratio is lower than 1.01
con®rming an anoxic/reducing depositional environment for the La Luna Formation. As was noted by
Zumberge (1984), the most abundant cyclic compounds
throughout the La Luna formation are the tricyclic terpanes and hopanes while steranes abundance are relatively
low. The sterane distributions display a predominance
of C27 steranes. Low concentration of rearranged C27
steranes relative to regular steranes are also characteristic (Fig. 9).
Most extracts display diasterane/sterane ratios less
than 1, except those from the mlW lithofacies (Table 3).
According to Rubinstein et al. (1975), Mello et al.
(1988), and Moldowan et al. (1986), the relatively low
abundance of diasteranes over regular steranes should
be related to a carbonate/anoxic environment, typical of
the La Luna Formation.
1278
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Fig. 8. Variation of geochemical bulk parameters and some typical biomarkers correlated with stratigraphic sequence in the La Luna Formation, La Sorda creek.
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
The extended hopanes have relative abundance of C35
hopanes (C35/C34 hopane ratios higher than 0.92). High
C35 hopane are commonly associated with marine carbonate environment (Mello et al., 1988; Clark and
Philp, 1989). Additionally, Peters and Moldowan (1993)
interpret this phenomenon as a general indicator of a
highly reducing marine condition during deposition.
1279
The high relative abundance of C35 over C34 hopane in
the La Luna Formation con®rms its association to carbonate environment.
The C35/C34 hopane ratios show a correlation with HI
(Fig. 10a) indicating that in a carbonate environment,
changes in these parameters correlate with a redox conditions rather than with changes in the carbonate content.
Fig. 9. Hopanes and tricyclic terpanes (m/z 191) and steranes (m/z 217) in typical samples from organic facies B and C.
1280
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
The lower abundance of the C35 hopanes in the bitumen
of the cL lithofacies might indicate more oxic conditions
during deposition.
The abundance of C24 tetracyclic terpane/C26 tricyclic
ranges from an average of 0.59 in the pPh lithofacies to
2.97 in the pcSM lithofacies. The C24 tetracyclic/C26
tryciclic ratio is in general less than 1, except in the more
siliciclastic lithofacies (pcSM). Ekweozor et al. (1981),
reported abundance of C24 tetracyclic terpanes in oils of
deltaic origin. According to Mello et al. (1988) and
Fig. 10. Crossplots and some typical geochemical parameters. The symbols correspond to average values for each lithofacies.
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Fig. 11. Isotope correlations: (a) relative sea level changes according to stratigraphic and sedimentologic analyses; (b) pro®le of carbon isotopic composition of whole bitumen and
aromatic hydrocarbon fraction; (c) d 13C saturates versus d 13C aromatic hydrocarbons. Sofer (1984) cross-plot.
1281
1282
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Philp and Gilbert (1986), the abundance of the C24 tetracyclic may indicate higher plant marker. In extracts of
the La Luna Formation (calcareous platform), variations in the abundance of this compound could not be
clearly related to higher plant input. Instead, it seems to
increase in more siliciclastic facies.
The tricyclic terpanes (C19±C30) display a high relative
abundance over hopanes (Table 3). Mello et al. (1988)
indicated that samples of lacustrine saline environments
and marine carbonate related environments are characterized by high relative abundance of tricyclic terpanes. Fig. 10c shows a relatively good correlation
between carbonate content and tricyclic terpanes/
hopanes ratio.
The Ts/Tm ratios are less than 0.33 (Table 3), and the
relative abundance of Ts increases systematically during
the regressive cycles (Fig. 8). According to Peters and
Moldowan (1993), many authors have associated
anomalously low (Ts/Ts+Tm) ratios to carbonate
source rock, which is also observed in this study.
Oleanane (triterpane of higher plant origin; Ekweozor
et al., 1979) is present in low abundance. The oleanane/
C30 hopane ratio range from 0.02 to 0.19 (Table 3).
Most of the extracts derived from sediments deposited
in deeper environments such as the pcSM PujamanaSalada lithofacies, have lower oleanane/hopane ratios
than those derived from carbonate shallow facies. Based
on the sequence stratigraphy cycles and geochemical
logs (Fig. 8), oleanane/C30 hopane can be proposed as a
good indicator of rise and fall of sea level in third order
sequence stratigraphy cycles.
The cross plots of oleanane/C30 hopane,%C29 steranes,
and C20/C23 tricyclic versus TOC show inverse correlation (Figs. 10d±f). This inverse correlation is consistent
with the opposite trends displayed for this parameter in
Fig. 8. Higher oleanane/C30 hopane,%C29 sterane and
C20/C23 tricyclic are considered to re¯ect relatively
greater contribution of higher plant material, in this
case associated with a shallow platform environment.
These plots help to delineate the three organic facies
grouped according to Jones' (1987) criteria (Fig. 10).
The organic facies C, related to cL lithofacies, always
presents an anomalous trend.
4.7. Isotope composition
The average values of the saturate fractions is
ÿ27.82% PDB and the range of variation is 1.18% in
the saturate fractions (Table 4). In the aromatic fraction
and whole bitumen, the range of variation is 1.6 and
1.88%, respectively, greater than observed in saturate
fractions. The isotope log of the aromatic fraction and
whole bitumen displays an isotopic shift associated with
the main sea level fall in the section (Fig. 11a).
Perez-Infante et al. (1996) also observed a marked d
13
C Corg isotopic excursion in the middle part of a
Maraca Creek section of the La Luna Formation, which
was interpreted as a global depletion in 12C around the
Cenomanian /Turonian and the Coniacian/Santonian
boundary.
Using the plot of Sofer (1984) (Fig. 11c), isotopically
heavier samples correspond to extracts from pcSM
lithofacies (more siliciclastic) and isotopically lighter
extracts are related to more calcareous lithofacies. All
extracts plot in the marine area of Sofer (1984).
5. Conclusions
The La Luna Formation is a petroleum source rock
with good to excellent potential for oil. About 63% of
the volume of this formation is composed of phosphatic
calcareous shales and laminated mudstones with abundant foraminifers (pcSM lithofacies).
Three low frequency hemicycles were identi®ed in the
La Luna Formation: A generalized base level fall, during
the deposition of the Salada Member; a base level rise or
a deepening of the basin during the sedimentation of the
Pujamana Member, and a second base level fall that
permitted the deposition of the calcareous and phosphatic lithologies of the Galembo member. Regarding
the relationship between organic matter characteristics and
sea level changes, during shallowing stages in carbonate
shelves (Salada Member and Galembo Member), HI
tends to increase and TOC to decrease. In the siliciclastic
shelf, during shallowing stages, (Pujamana-Salada
Member), both TOC and HI decrease continuously.
Certain biomarker ratios such as oleanane/C30 hopane,
C20/C23 tricyclic, Ts/Tm show an increasing trend during
base level falls and could be proposed as a good indicator of rise and fall of sea level in third order sequence
stratigraphy cycles.
Sedimentation of the La Luna Formation occurred
on a large carbonate ramp, with restricted water circulation and anoxity. During certain periods, upwelling
favored high primary productivity and accumulation
and preservation of organic matter. Some biomarker
ratios can be considered typical values for the depositional environment of the La Luna Formation (e.g.
diasterane/sterane ratios < 1, Ts/Tm average < 0.33,
C35/C34 hopane >0.92, and oleanane/C30 hopane ranging
from 0.02 to 0.19). The C35/C34 hopane ratio correlates
with HI; suggesting that in carbonate environments,
changes in this parameter are more strongly related to
redox condition rather than to changes in carbonate
content.
It is possible to dierentiate organic facies type B, BC
and C in the Salada Member, organic facies type B and
D in the Pujamana Member and organic facies type B in
the Galembo Member. The d 13C isotope composition of
aromatic fractions and whole bitumens display an isotopic
shift associated with the main deepening event in the
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
section. Based on these results, organic geochemistry
could be considered as an important tool to support the
sequence stratigraphy architecture of a sedimentary
succession.
Acknowledgements
We are grateful to Drs. R. Sassen and S. Talukdar for
helpful suggestions and constructive reviews and Drs.
M.N. YalcËyÂn and S. YÂnan for major linguistic and editorial corrections.
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www.elsevier.nl/locate/orggeochem
The La Luna formation: chemostratigraphy and organic
facies in the Middle Magdalena Basin
A. Rangel a,*, P. Parra b, C. NinÄo b
b
a
ECOPETROL-ICP, PBX 4185, Bucaramanga, Colombia
Universidad Industrial de Santander, Colciencias, GEMS Ltda, PBX 4185, Bucaramanga, Colombia
Abstract
A detailed geochemical study and a sequence stratigraphic interpretation have been conducted on a sedimentary
sequence of the Upper Cretaceous La Luna Formation, in a section outcropping in the eastern ¯ank of the Middle
Magdalena Basin (MMB), Colombia. The goals were to evaluate geochemical variability related to lithofacies and
organic facies changes, characterize depositional environment and investigate the possible relationship between geochemical data and sequence stratigraphic cycles. The La Luna Formation is composed of organic-rich sediments of
monotonous appearance, with good to excellent potential for oil generation. Most of the bulk, petrographic and biomarker parameters display a relatively narrow range of variation. However, the geochemical variations are sucient to
dierentiate organic facies types B, BC and C in the Salada Member, B and D in the Pujamana Member and B in the
Galembo Member. Certain biomarker ratios are consistent within the La Luna Formation and are characteristic of its
depositional environment, for example, average ratios of diasterane/sterane are lower than 1, Ts/Tm averages are less
than 0.33, the C35/C34 hopane ratio is more than 0.92, and oleanane/C30 hopane ratios range from 0.02 to 0.19.
Regarding depositional condition indicators, the C35/C34 hopane ratio shows a good positive correlation with HI. This
suggests that in carbonate environment changes in this parameter are more strongly related to redox condition than to
changes in carbonate content. Regarding the possible relationship between organic matter characteristics and sea level
changes, in regressive carbonate shelves during shallow stages, HI tends to increase and TOC tends to decrease, while
in regressive siliciclastic shelves, both TOC and HI decrease continuously. Some biomarker ratios (oleanane/C30
hopane, C20/C23 tricyclic, Ts/Tm) increase during base level falls. Regarding d 13C/12C isotope composition, the aromatic fraction and whole bitumen display an isotopic shift associated to the main deepening event in the section.
# 2000 Elsevier Science Ltd. All rights reserved.
Keywords: La Luna Formation; Sequence stratigraphy; Molecular geochemistry; Depositional environments; Carbonates; Source rock
1. Introduction
The La Luna Formation has been considered to be
the main hydrocarbon source rock in the Middle Magdalena Basin (MMB) by Zumberge (1984), Rangel et al.
(1996), as well as in other important basins such as
Maracaibo Basin (Talukdar et al., 1986). Nevertheless,
the existing knowledge about depositional processes
controlling the Upper Cretaceous-La Luna Formation
* Corresponding author.
E-mail address: arangel@ecopetrol.com.co (A. Rangel).
deposit in the MMB is minimal and there are few integrated stratigraphic and geochemical studies on this
formation.
Zumberge (1984) addressed the hydrocarbon potential of the La Luna Formation in the La Sorda Creek.
Rangel et al. (1996) identi®ed four oil families, and
based on oil characteristics he suggested that two of
them are possibly derived from the La Luna Formation.
Ramon and Dzou (1999) discussed some geochemical
processes in the MMB, based on oil-derived parameters.
A clear understanding of geochemical characteristics of
the La Luna Formation and associated organic facies
would increase con®dence in the oil±La Luna source
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00127-3
1268
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
rock correlation, as well as better delineate the contribution of each part of the sequence to oil generation
in this basin. The relationship of geochemical parameters to local depositional processes and stratigraphic
cycles would help identify the best source rock areas and
allow us to better understand the spatial relationship
between source rock and migration conduits for oils
related to the La Luna Formation in the MMB.
Bulk, petrographic, carbon-isotopic and molecular
geochemical data were used in this study, along with a
stratigraphic interpretation. The objectives were to evaluate geochemical changes related to existing lithofacies and
organic facies, to characterize depositional environment
and to investigate possible relationships between geochemical parameters and sequence stratigraphy cycles.
2. Methodology and sampling
Outcrop samples (160) were systematically collected
from an estimated 240 m vertical interval of the La
Luna Formation in La Sorda Creek, on the western
¯ank of the Nuevo Mundo Syncline.
The sequence stratigraphic interpretation utilized in
this study follows the methods developed by the Genetic
Stratigraphy Research Group (GSRG) of Colorado
School of Mines (Cross, 1988; Cross et al. 1993; Cross
and Lessenger, 1995). This methodology identi®es unidirectional trends of increasing and decreasing ratio of
accommodation space to sediment ¯ux (A/S ratio).
Stratigraphic cycles register the time in both rise and fall
of A/S.
Regarding the geochemical study, all samples were
submitted for bulk geochemical analysis. Organic carbon
analysis (in a Leco Carbon Analyzer) and Rock Eval
pyrolysis analysis (Espitalie et al., 1977) were performed
on all samples along with determination of carbonate
content by acid treatment.
The screening results were followed by analyses of 40
samples by gas chromatography and gas chromatography±mass spectrometry of rock extracts. Kerogen
was isolated by consecutive HCl and HF treatment, and
¯oated in ZnBr. Powdered samples were Soxhlet
extracted with chloroform to remove extractable
organic matter. The hexane soluble material was then
separated by liquid chromatography into saturate, aromatic and NSO fractions on an alumina and silica column.
The whole bitumen, saturate and aromatic fractions were
prepared for carbon isotope composition by a modi®ed
method of Sofer (1984) and measured on a Finnigan
MAT Delta S instrument The saturate fractions were
subjected to GC and GC±MS analyses. Total alkane
fractions or branched/cyclic sub-fractions were analyzed
in the selected ion recording mode on an HP 5890 GC±
MS system. The GC column was a 30 m HP-5 temperature programmed from 60 to 320 C at 4 C/min and
helium carrier gas at 1.5 ml/min. Selected ion recording
was performed on the 5890 MSD monitoring ions 177,
191, 217, 218, 259 for saturate fractions.
3. Geological framework
The La Sorda Creek section is located approximately
20 km West of Bucaramanga (Fig. 1). The sedimentary
column of the MMB, consists of Jurassic sandstones of
¯uvial origin, Cretaceous limestones and shales of shallow
marine to paludal origin, and Tertiary sedimentary
rocks of predominantly ¯uvial origin (Fig. 2).
Morales (1958) subdivided the La Luna Formation
into three members, which from base to top are, Salada
(Turonian), Pujamana (upper Turonian-lower Coniacian)
and Galembo (uppermost Turonian-Coniacian and
possibly Santonian).
The tectonic evolution of the eastern edge of the MMB
is closely related to the tectonic evolution of the Eastern
Cordillera, as widely discussed by several authors
(Campbell and BuÈrgl, 1965; Macellari, 1988; Colleta et
al., 1990; Dengo and Covey, 1993; Cooper et al., 1995).
The Eastern Cordillera consists of predominantly
clastic material and carbonates overlaid on a Precambrian and Paleozoic basement. During Triassic±
Jurassic time, rifting and magmatic events produced by
Paci®c plate subduction, was responsible for the uplift
of the Central Cordillera and the deposition of continental and volcanic rocks in the backarc setting. During the Early Cretaceous, a marine transgression led to
the backarc basin to be ®lled with a prograding
sequence. Maximum transgression during the Turonian±Santonian period, led to the deposition of the La
Luna Formation and its equivalent rocks, namely the
Villeta (in the Upper Magdalena Valley) and Chipaque
or Gacheta Formations (in the Llanos basin and the
Eastern Cordillera), both also with excellent source
rocks. During the latest Cretaceous (Maastrichtian), the
beginning of marine regression allowed deposition of a
transitional sequence (the Umir Formation); by accretion
in the Western Cordillera. Finally, during Tertiary, the
rising of the Eastern Cordillera (Andean Orogeny) was
responsible for the development of a whole continental
sequence. This event reached its maximum during the
Miocene-Pliocene period and is still continuing at present.
4. Results and discussion
4.1. Lithofacies and organic facies in the La Luna
Formation (La Sorda Creek section)
The La Luna Formation is a calcareous petroleum
source rock with good to excellent potential for oil.
Most of the sedimentologic and geochemical parameters
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
1269
Fig. 1. Location of the La Sorda creek section in the Middle Magdalena Basin, Colombia.
of the La Luna Formation in La Sorda Creek display a
relatively narrow range (Table 1). However, the sedimentologic and geochemical variations are sucient to
dierentiate several lithofacies and organic facies (Table
2 and Fig. 3).
4.1.1. Lithofacies and organic facies in the Salada
Member
This member consists of 102.2 m of foraminiferal
wackestones interbedded with occasionally-cherty calcareous shales. The lithologies observed in this member
are grouped in four sedimentary lithofacies.
. Poorly laminated wackestones (plW): This lithofacies consists mainly of planktonic foraminiferal
wackestones with some vertebrae and bones of
®sh, as well as pyrite traces.
. Muddy laminated wackestones and calcareous
shales (mlW): This lithofacies consists of dark
gray, thin bedded foraminiferal wackestones and
calcareous shales, both with planoparallel laminations and small nodules.
. Phosphatic calcareous shales and laminated mudstones (pcSM): This lithofacies consists of calcareous, slightly phosphatic and ®nely laminated
shales and claystones, with abundant foraminifera
(Fig. 4)
. Crystalline limestone (cL): This lithofacies consists
of two layers of 40 cm of greenish-gray crystalline
limestone with laminae of organic matter.
Fig. 2. Cretaceous stratigraphic units in the Middle Magdalena
Basin.
Organic facies type B, BC, and C, sensu Jones (1984),
was identi®ed in the Salada Member (Table 2). Organic
facies B is related to mlW and pcSM lithofacies. This
facies is composed of organic matter with average values
of HI around 428 (mg HC/g TOC), TOC around 4.3
(wt.%) and S2 between 15.6 and 22.2 mg HC/g sample.
The saturated hydrocarbons of this organic facies is
1270
Table 1
Average values (meanstandard deviation) of bulk geochemical parameters for each lithofacies of the La Luna Formation
%
TOC
Tmax
( C)
S1
(mg HC/g
rock)
S2
(mg HC/g
rock)
S3
(mg CO2/g
rock)
Hl
(mg HC/g
TOC)
1.4
3.9
3.2
8.5
15
15
14
44
2.61.0
2.30.8
2.41.0
2.40.9
4344
4364
4383
4364
4.612.25
3.721.64
3.791.88
4.041.94
13.465.23
11.334.52
11.365.22
12.074.98
0.590.17
0.470.15
0.400.08
0.490.16
pcSM2b
B
mlW2
pcSM2,
B, mlW2
49.3
0.5
1.4
51.1
63
3
3
69
3.21.1
0.30.1
3.70.9
3.11.2
4353
437
4383
4363
4.432.52
0.04
4.410.66
4.242.58
14.415.69
0.040.02
15.373.65
13.826.23
plW
mlW3
pcSM3b
cL
plW, mlW3,
pcSM3, cL
18.4
10.9
10.5
0.5
40.4
18
11
8
7
44
1.81.3
4.21.3
4.61.2
0.90.9
2.81.9
4353
4383
4344
4329
4355
1.741.33
2.950.98
5.863.47
0.810.95
2.602.41
7.065.85
15.553.18
22.509.66
3.474.33
11.288.85
Organic
facies
Lithofacies
Galembo
B
B
B
Organic
facies B
pPh
phPW
pcSM1b
pPH, phPW,
pcSM1
B
D
B
Organic facies
B and D
BC
B
B
C
Organic facies
B, BC and C
Average
Pujamana
Average
Salada
Average
a
b
c
%
thickness
n samples analyzed.
Total lithofacies pcSM=pcSM1+pcSM2+pcSM3=62.96%.
n.d., no data
OI
(mg CO2/g
rock)
%
Carbonate
%
Bitumen
52363
49162
46772
49568
259
229
2010
229
31.99.7
52.010.7
27.414.5
37.315.8
2.340.32
2.350.98
1.080.77
1.780.90
0.470.11
0.160.11
0.690.08
0.460.14
43488
1412
42251
415121
1817
5230
205
2018
22.313.1
0.30.5
49.83.7
22.614.6
1.551.15
n.d.c
1.680.76
1.571.10
0.470.12
0.560.11
0.520.05
0.310.22
0.480.15
36077
39584
474132
234190
366132
3827
56.910.2
136
5332
3026
75.112.3
0.89
28.111.3
66.211.0
59.920.7
0.640.41
3.692.83
n.d.
1.601.98
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
na
Member
1271
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Table 2
Characteristics of the organic facies in the La Luna Formation (La Sorda Creek section)
Member
Galembo
Pujamana
Salada
Organic facies
B
B
D
B
BC
C
% in the section
TOC (wt.%)
HI (mg HC/g TOC)
OI (mg CO2/g TOC)
Tmax ( C)
Carbonate (%)
8.48
2.4 (0.9)
495 (68)
22 (9)
436 (4)
37.3 (15.8)
50.62
3.2 (1.1)
434 (87)
18 (16)
436 (3)
23.6 (14.1)
0.51
0.3 (0.1)
14 (12)
52 (30)
437
0.3 (0.5)
21.42
4.3 (1.3)
428 (111)
15 (9)
436 (4)
44.8 (17.9)
18.44
1.8 (1.3)
360 (77)
38 (27)
435 (3)
75.1 (12.3)
0.53
0.9 (0.9)
234 (190)
53 (32)
432 (9)
66.2 (10.7)
Fig. 3. Stratigraphic pro®le, cycles interpretation and lithofacies and organic facies substitution diagrams of the La Sorda Creek
section. The percentages of each lithofacies are shown in the diagram.
characterized by the high ratios of C30/C29 sterane and
relatively low ratios of C29/C30 hopane and C23 tricyclic/
C24 tetracyclic (Fig. 5; Tables 2 and 3).
The organic facies BC is related to the plW lithofacies. This facies is characterized by average values of
HI around 360, TOC around 1.8%, and an average of
S2 of 7 mg HC/g of rock. This facies has the highest
values of oleanane/C30 hopane and tricyclic C20/C23
ratios, and relatively low values of diasterane/sterane
(0.46). The other geochemical parameters are within
the range of the La Luna Formation (Tables 2 and 3;
Fig. 5).
1272
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Fig. 4. Photomicograph of a thin section from mlW (a) and pcSM2 (b) lithofacies. Sample CD-152 and sample CD-90. Lamination
corresponds to alternation of foraminifer tests, organic matter and clay minerals. Magni®cation 2.5. Plane light.
An organic Facies C associated with the cL lithofacies, displays values of HI around 234 and TOC
around 0.9%. This organic facies has the lowest average
ratios of oleanane/C30 hopane (0.02), C30/C29 sterane
(0.17), gammacerane/C30 sterane (0.09), diasterane/sterane
(0.18) and C35/C34 hopane (0.92) (Tables 2 and 3; Fig. 5).
4.1.2. Lithofacies and organic facies in the Pujamana
Member
This member consists mainly of calcareous phosphatic shales with abundant foraminifera, phosphatic calcareous mudstones, cherts and bentonites. Calcareous
nodules as large as 1 m in diameter are observed. The
abundance of pyrite is greater than in Salada Member.
Three lithofacies were identi®ed in this member (Fig. 3).
. Phosphatic calcareous shales and laminated mudstones (pcSM): This lithofacies consists of shales
and calcareous mudstones that are ®nely laminated and slightly phosphatic, with abundant foraminifera and bones of ®sh.
. Bentonites (B): This lithofacies is composed of
yellowish-gray, greenish-gray and grayish-orange
clays. (smectite±illite). The lithofacies generally
appears in tabular layers thinner than 35 cm.
. Muddy laminated wackestones and calcareous
shales (mlW): This lithofacies consists of dark
gray, thin-bedded foraminiferal wackestones and
calcareous shales.
Organic facies type B and D were observed in this
member. The organic facies B, the most abundant, is
associated with the pcSM and mlW lithofacies. Averages
values of HI and TOC are around 434 and 3.2, respectively, and S2 varies between 14,41 and 15,37. This
organic facies shows very little variation in the average
values of Ts/Tm (0.16±0.19), C35/C34 hopane (1.10±1.32),
C24 tetracyclic/C26 tricyclic (1.08±1.09) (Tables 1, 2 and
3; Fig. 5).
Organic facies C, associated with the bentonite lithofacies, has HI values of about 14, and TOC of about
0.3%. This is a minor organic facies (0.5% of the geological column by volume) (Tables 1 and 2).
4.1.3. Lithofacies and organic facies in the Galembo
Member
The lower part of this member (19 m) was studied.
The section consists of a series of packstone phosphorites (sensu Greensmith, 1989), wackestones and phosphatic packstones, chert and calcareous and phosphatic
shale. Layers are tabular, with a thickness range
between 5 and 30 cm and concretions up to 2 m in diameter are present toward the base of this member. Three
lithofacies were identi®ed in this member (Fig. 3).
. Phosphatic calcareous shales and laminated mudstones (pcSM): This lithofacies consists of calcareous, slightly phosphatic and ®nely laminated
shales and claystones, with abundant foraminifera.
. Packstone phosphorites (pPh): This facies consists
of packstone phosphorites with abundant foraminifera, pellets, ®sh bone fragments, and
oolites. Wavy and lenticular lamination is common and in some samples ¯aser lamination can be
observed.
. Phosphatic packstones and wackestones (phPW):
This lithofacies was observed towards the top of
the Galembo member. It consists mainly of
slightly phosphatic foraminiferal packstones, with
lenticular and wavy lamination. This lithofacies
also contain ®sh fragments, pellets, oolites and
Planktonic fossils.
The organic matter of Galembo Member exhibits the
greatest HI around 495, OI average of 22, TOC of about
2.4% and S2 of 12. These geochemical parameters are
typical of organic facies B. This organic facies displays
Table 3
Average values (meanstandard deviation) of biomarker parameters for each lithofacies and organic facies in La Luna Formation
Organic Lithofacies na Ts/Tm
facies
Diasterane/ C35/C34
regular
extended
steraneb
hopanes
Tricyclic
terpanes/
hopanesc
%C27
sterane
%C29
sterane
C30/C29
sterane
C24 tetracyclic/ C23 tricyclic/ C29 norhopane/ Oleanane/ Gammacerane/ Steranes/
C26 tricyclic
C24 tetracyclic C30 hopane
C30 hopane C30 hopane
hopanesd
Galembo
B
B
B
Organic
facies B
phPW
pPh
pcSM1
pPH,
phPW,
pcSM1
0.71
0.350.07
0.590.49
0.510.36
3.132.06
2.020.08
2.850.53
2.630.93
28.678.40
38.529.59
38.976.40
36.538.19
24.427.98
29.107.28
26.881.98
27.075.10
0.26
0.230.03
0.270.02
0.260.03
0.980.40
0.590.07
1.120.57
0.910.45
8.225.59
14.551.17
10.175.92
11.204.93
2.130.58
1.400.66
0.950.33
1.360.65
0.05
0.040.03
0.130.11
0.080.09
n.d.e
0.100.04
0.330.37
0.250.31
0.150.07
0.340.08
0.290.09
0.280.10
Pujamana B
B
Average Organic
facies B
pcSM2
mlW2
pcSM2,
mlW2
23 0.160.05 0.450.44
8 0.190.02 1.200.60
31 0.170.05 0.520.49
1.100.18 2.331.06 44.576.12 23.465.71 0.290.08 1.090.56
1.320.34 2.601.23 39.122.67 26.204.50 0.350.11 1.080.50
1.130.22 2.361.05 43.946.06 23.775.58 0.300.08 1.090.54
11.865.01
11.005.47
11.764.96
1.270.49
0.940.65
1.230.51
0.080.11
0.030.02
0.080.11
0.320.28
0.300.28
0.310.27
0.200.09 0.280.16 0.85
0.100.04 0.220.02 n.d.
0.190.09 0.280.16 0.85
Salada
plW
mlW3
pcSM3
mlW3,
pcSM3
cL
6
8
6
14
0.460.28
0.310.19
0.640.49
0.460.38
1.030.09
1.100.09
1.240.38
1.170.28
3.010.78
2.730.93
1.780.72
2.320.95
38.994.16
41.745.29
38.914.98
40.535.17
27.364.89
26.122.88
24.235.81
25.314.29
0.310.04
0.210.05
0.760.66
0.460.51
0.980.36
0.930.24
2.972.31
1.801.78
9.547.68
12.325.90
5.706.60
9.496.86
1.380.25
1.430.32
0.780.43
1.150.49
0.190.15
0.050.06
0.030.01
0.040.05
0.260.15
0.230.23
0.150.06
0.200.19
0.240.05
0.280.11
0.210.12
0.260.11
0.18
0.92
1.90
41.22
26.63
0.17
0.65
17.00
1.31
0.02
0.09
0.270.00 0.21
Average
Average
a
b
c
d
e
BC
B
B
Organic
facies B
C
2
3
4
7
0.28
0.25
0.260.01
0.260.02
0.310.17
0.190.04
0.330.22
0.250.16
1 0.16
n, Samples analyzed.
C27 ba diasterane (20S)/C27 aaa sterane (20R).
Sum tri/sum hopanes.
Sum sterane/sum hopanes.
n.d., no data.
n.d.
1.130.16
1.350.32
1.280.28
C20/C23
tricyclic
Pr/Ph
0.610.45
0.200.02
0.520.51
0.420.37
1.01
n.d.
0.60
0.81
0.790.71
0.380.23
0.250.05
0.340.21
0.52
0.98
0.49
0.73
n.d.
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Member
1273
1274
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Fig. 5. Histograms showing the average values and standard deviation of some geochemical bulk parameters and representative biomarker ratios for each lithofacies in the La Luna Formation.
1275
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Fig. 6. Schematic evolution of the carbonate ramp.
Table 4
Carbon isotopes ratios in whole bitumen, saturates and aromatic fractions for dierent samples of the La Luna formation
d13C saturates
13
ÿ27.40
ÿ27.43
ÿ27.24
ÿ27.33
ÿ28.45
ÿ27.42
ÿ27.35
ÿ27.32
ÿ27.38
ÿ27.37
ÿ27.21
ÿ27.36
25.9
34.0
34.3
58.2
74.4
102.5
112.0
130.0
ÿ27.76
ÿ27.27
ÿ27.33
ÿ27.90
ÿ26.91
ÿ26.08
ÿ26.61
ÿ26.47
ÿ27.91
ÿ27.57
ÿ27.27
n.d.
ÿ27.52
ÿ28.18
ÿ27.78
ÿ27.41
ÿ27.40
ÿ27.28
ÿ27.31
ÿ27.94
ÿ26.99
ÿ26.78
ÿ26.34
ÿ26.49
142.0
179.5
192.5
204.0
221.0
228.0
233.0
235.0
ÿ26.13
ÿ27.51
ÿ27.62
ÿ27.61
ÿ27.46
ÿ27.41
ÿ27.39
ÿ27.96
ÿ27.98
ÿ27.99
ÿ28.43
ÿ27.55
ÿ28.31
ÿ28.31
ÿ27.56
ÿ28.33
ÿ26.56
ÿ27.47
ÿ27.84
ÿ27.67
ÿ27.60
ÿ27.43
ÿ27.79
ÿ27.86
ÿ27.24
ÿ27.82
ÿ27.30
Member
Organic facies
Lithofacies
Sample ID
Galembo
B
B
B
B
pcSM1
phPW
pPh
pcSM1
CD07
CD15
CD22
CD41
3.0
9.0
14.0
18.2
Pujamana
B
B
B
B
B
B
B
B
pcSM2
pcSM2
pcSM2
pcSM2
pcSM2
pcSM2
mlW2
pcSM2
CD50
CD59
CD60
CD82
CD98
CD104
CD110
CD114
Salada
B
BC
BC
B
BC
B
BC
B
pcSM3
plW
plW
mlW3
plW
pcSM3
plW
pcSM3
CD119
CD129
CD135
CD143
CD154
CD157
CD158
CD160
Average
Cummulative thickness
13
C Bitumen
C Aromatics
1276
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
little variation of geochemical parameters such as Ts/Tm
(0.25±0.28), C35/C34 hopane (1.13±1.35) and C30/C29
sterane (0.23±0.27). The oleanane/C30 hopane ratio is
high, varying between 0.04±0.13 (Tables 2 and 3; Fig. 5).
The phosphatic packstone lithofacies of the Galembo
member shows the highest values of HI, the highest
ratios of sterane/hopane (0.34) and C23 tricyclic/C24
tetracyclic (14.55) and the lowest ratios of C20/C23 tricyclic (0.20) and C24 tetracyclic/C26 tricyclic (0.59) of the
La Luna Formation. This parameter could correlate
with increased algal material within this lithofacies.
These characteristics are consistent with the observed
abundance of phosphates indicating an upwelling event.
The HI versus OI plot, the modi®ed Van Krevelen
diagram (Fig. 7), shows that organic matter is predominantly Type II, amorphous (algal and bacterial),
and Type I organic matter. A very small proportion of
Type II/III and III/IV kerogen is also observed. Talukdar
et al. (1986) also noted, based on microscopic analyses
and molecular geochemistry, that the bulk of the
organic matter is algal and bacterial in origin.
Summarizing, Organic Facies B is the most representative organic facies of the La Sorda Creek section
with a total percentage of 80.5% in volume. This facies
is related to phPW, pPh, pcSM, and mlW lithofacies.
The organic facies BC represent 18.4% of the section by
volume. Organic facies C and D represent 0.5 and 0.5%
of the stratigraphic column, respectively.
4.2. Sedimentary cycles
Based on lithologic and sedimentological characteristics, a facies substitution diagrams (Fig. 3) for the La
Luna formation in La Sorda Creek was interpreted.
This diagram provides information about the natural
succession and substitution of lithofacies under increasing
or decreasing accommodation conditions. The area of
each lithofacies is proportional to its abundance within
the section. This diagram helps to identify stratigraphic
cycles of high, intermediate, and low frequency, for
sequence stratigraphic analysis.
The intermediate and low frequency cycles for the La
Luna Formation in La Sorda Creek section de®ned in
this work, using lithologic and stratigraphic attributes,
correlate with cycles described for this formation by
Reyes et al. (1998), using well logs and cores.
From base to top, the transition of the argillaceous
shales of the SimitõÂ Formation to calcareous shales, and
the appearance of thin layers of foraminiferal wackestones (Salada Member), indicates a generalized low
frequency hemicycle of base level fall. This hemicycle
reaches a maximum fall (minimum in A/S) where the
limestone layers have the greatest thickness, up to 170 m
of measured thickness (Fig. 6).
From this point of maximum progradation, a deepening
event began, evidenced by the decrease in thickness of the
wackestones layers, and the increase in thickness of
shales and calcareous mudstones (pcSM lithofacies).
This increase in A/S ratio ends in a surface of maximum
¯ooding up to 75 m thick, with high contents of organic
matter and low percentages of carbonates.
After the maximum deepening within the Pujamana
Member, the amount of phosphates progressively
increases. Packstone phosphorites (lithofacies pPh) and
phosphatic packstones (lithofacies phPW) appear. This
demonstrates a base level fall in the basin (Fig. 5). The
shallowing is not completely recorded in the column of
the La Sorda Creek, because of a fault at the top of the
La Sorda Creek section. Stratigraphic events in the
basin are shown in detail by the intermediate frequency
cycles (third order cycles, sensu Vail et al., 1977) (Fig. 3).
4.3. Paleoceanographic events and environmental
considerations
Fig. 7. Modi®ed van Krevelen diagram showing lithofacies and
organic facies.
The ®ne lamination and the calcareous character of
Salada Member and the presence of very ®ne planktonic
foraminifera arranged in laminae, demonstrate a low
energy marine environment of deposition.
The shale sequence and the minor calcareous character, which characterize the Pujamana Member, evidence deeper conditions in the carbonate platform than
those during the deposition of the Salada Member (Fig.
6). The increase of phosphates towards the top is consistent with the occurrence of upwelling currents. The
high contents of organic matter, as well as the presence
of pyrite are evidence of high productivity and suboxic
to anoxic condition, which favored accumulation and
preservation of organic matter. The bentonites represent
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
sedimentary input from volcanic activity during this
period.
The packstone phosphorites and phosphatic packstones that characterize the Galembo Member indicate a
¯ow regime greater than the one during the sedimentation of the Salada Member and the Pujamana Member.
The presence of wavy and lenticular lamination, ripples,
¯aser laminations, reworking of ®sh fragments and presence of quartz demonstrates a shallow water environment within wave base. These lithofacies correspond to
the shallowest level within the carbonate platform in
which the La Luna Formation was deposited (Fig. 6).
The increase of phosphates indicates upwelling and high
primary productivity in a suboxic environment.
The accumulation of organic rich sediments in this
area, during Late Cretaceous, was favored by interaction
of important regional and global paleogeographic events,
such as Ekman water transport (closely related to
upwelling regimens), and oceanic anoxic events described
by Macellari (1998), Martinez and HernaÂndez (1992) and
Villamil (1998) for the north corner of South America.
Taking into account the data of this study, and the
regional paleogeographic framework, the La Luna Formation was deposited on a large carbonate platform. On
this platform, sedimentation took place in a through,
limited to the West by the submerged Central Cordillera, which restricted water circulation and contributed
to an anoxic environment of deposition. During some
less restricted periods, upwelling currents favored high
primary productivity, and organic matter accumulation
and preservation that characterize the sediments of the
La Luna Formation. According to Martinez and HernaÂndez (1992), the deepest parts of the ramp were located near the present area of Maracaibo, where a
maximum depth of approximately 600 m was attained
during the Campanian.
4.4. Maturity level
Because of the predominance of amorphous organic
matter, the samples contain little vitrinite, and therefore
the vitrinite re¯ectance measures are unreliable. The
maturation level in this study was determined using RockEval pyrolysis. Tmax values (average 436 C) listed in Table
1. This suggests the section is early mature to mature.
Average Tmax and S1 values suggest that the section is
not greatly aected by oil migration, and that geochemical variations can largely be considered as indicative of depositional conditions and associated variations
in the type of organic matter.
4.5. Relationship between geochemical parameters and
sequence stratigraphy
Several authors have described the relationship
between the characteristics of organic matter and sea
1277
level changes (e.g. Middleburg et al., 1991; Pasley et al.,
1993). The relationship between some geochemical
parameters commonly used to characterize petroleum
source rocks and interpreted sequence stratigraphy
cycles in the La Sorda Creek section are graphically
shown in Fig. 8. The trends in variations of geochemical
parameters in this ®gure are generalized. In the regressive carbonate shelf (Salada Member and Galembo
members) an increase in the relative values of HI and a
decrease in TOC contents is observed, while in the
regressive siliciclastic shelf (Pujamana-Salada Member),
both TOC and HI values decrease continuously during
the shallowing stage. Some biomarker parameters such
as oleanane/C30hopane and C20/C23 tricyclic ratios
usually used to re¯ect relative contribution of continental organic matter, increase during regression
cycles. This occurs on both carbonate and siliciclastic
shelves. Other parameters suggested by some authors
(e.g. Waples and Machihara, 1990) as sensitive to
lithology, such as Ts/Tm ratios, increase in carbonate
levels. In the carbonate shelves, HI values increase
despite the higher oleanane/C30 hopane and C20/C23
tricyclic ratios, probably suggesting that algal productivity and preservation are predominant processes in
this depositional environment. Based on these results,
organic geochemistry could be considered as an important
tool to support the sequence stratigraphy architecture of
a sedimentary sequence. Conversely, sequence stratigraphy is a useful tool to follow oil prone strata.
4.6. Sedimentological controls on geochemical composition
Biomarker fragmentograms appear similar for the
dierent organic facies upon initial inspection (Fig. 9).
The average of certain biomarker ratios (Table 4 and
Fig. 5), show a narrow range. Therefore, these values
can be considered typical for the La Luna Formation
and its depositional conditions. This is useful for oilsource rock correlation in the MMB.
The pristane/phytane (Pr/Ph) ratio is lower than 1.01
con®rming an anoxic/reducing depositional environment for the La Luna Formation. As was noted by
Zumberge (1984), the most abundant cyclic compounds
throughout the La Luna formation are the tricyclic terpanes and hopanes while steranes abundance are relatively
low. The sterane distributions display a predominance
of C27 steranes. Low concentration of rearranged C27
steranes relative to regular steranes are also characteristic (Fig. 9).
Most extracts display diasterane/sterane ratios less
than 1, except those from the mlW lithofacies (Table 3).
According to Rubinstein et al. (1975), Mello et al.
(1988), and Moldowan et al. (1986), the relatively low
abundance of diasteranes over regular steranes should
be related to a carbonate/anoxic environment, typical of
the La Luna Formation.
1278
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Fig. 8. Variation of geochemical bulk parameters and some typical biomarkers correlated with stratigraphic sequence in the La Luna Formation, La Sorda creek.
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
The extended hopanes have relative abundance of C35
hopanes (C35/C34 hopane ratios higher than 0.92). High
C35 hopane are commonly associated with marine carbonate environment (Mello et al., 1988; Clark and
Philp, 1989). Additionally, Peters and Moldowan (1993)
interpret this phenomenon as a general indicator of a
highly reducing marine condition during deposition.
1279
The high relative abundance of C35 over C34 hopane in
the La Luna Formation con®rms its association to carbonate environment.
The C35/C34 hopane ratios show a correlation with HI
(Fig. 10a) indicating that in a carbonate environment,
changes in these parameters correlate with a redox conditions rather than with changes in the carbonate content.
Fig. 9. Hopanes and tricyclic terpanes (m/z 191) and steranes (m/z 217) in typical samples from organic facies B and C.
1280
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
The lower abundance of the C35 hopanes in the bitumen
of the cL lithofacies might indicate more oxic conditions
during deposition.
The abundance of C24 tetracyclic terpane/C26 tricyclic
ranges from an average of 0.59 in the pPh lithofacies to
2.97 in the pcSM lithofacies. The C24 tetracyclic/C26
tryciclic ratio is in general less than 1, except in the more
siliciclastic lithofacies (pcSM). Ekweozor et al. (1981),
reported abundance of C24 tetracyclic terpanes in oils of
deltaic origin. According to Mello et al. (1988) and
Fig. 10. Crossplots and some typical geochemical parameters. The symbols correspond to average values for each lithofacies.
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Fig. 11. Isotope correlations: (a) relative sea level changes according to stratigraphic and sedimentologic analyses; (b) pro®le of carbon isotopic composition of whole bitumen and
aromatic hydrocarbon fraction; (c) d 13C saturates versus d 13C aromatic hydrocarbons. Sofer (1984) cross-plot.
1281
1282
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
Philp and Gilbert (1986), the abundance of the C24 tetracyclic may indicate higher plant marker. In extracts of
the La Luna Formation (calcareous platform), variations in the abundance of this compound could not be
clearly related to higher plant input. Instead, it seems to
increase in more siliciclastic facies.
The tricyclic terpanes (C19±C30) display a high relative
abundance over hopanes (Table 3). Mello et al. (1988)
indicated that samples of lacustrine saline environments
and marine carbonate related environments are characterized by high relative abundance of tricyclic terpanes. Fig. 10c shows a relatively good correlation
between carbonate content and tricyclic terpanes/
hopanes ratio.
The Ts/Tm ratios are less than 0.33 (Table 3), and the
relative abundance of Ts increases systematically during
the regressive cycles (Fig. 8). According to Peters and
Moldowan (1993), many authors have associated
anomalously low (Ts/Ts+Tm) ratios to carbonate
source rock, which is also observed in this study.
Oleanane (triterpane of higher plant origin; Ekweozor
et al., 1979) is present in low abundance. The oleanane/
C30 hopane ratio range from 0.02 to 0.19 (Table 3).
Most of the extracts derived from sediments deposited
in deeper environments such as the pcSM PujamanaSalada lithofacies, have lower oleanane/hopane ratios
than those derived from carbonate shallow facies. Based
on the sequence stratigraphy cycles and geochemical
logs (Fig. 8), oleanane/C30 hopane can be proposed as a
good indicator of rise and fall of sea level in third order
sequence stratigraphy cycles.
The cross plots of oleanane/C30 hopane,%C29 steranes,
and C20/C23 tricyclic versus TOC show inverse correlation (Figs. 10d±f). This inverse correlation is consistent
with the opposite trends displayed for this parameter in
Fig. 8. Higher oleanane/C30 hopane,%C29 sterane and
C20/C23 tricyclic are considered to re¯ect relatively
greater contribution of higher plant material, in this
case associated with a shallow platform environment.
These plots help to delineate the three organic facies
grouped according to Jones' (1987) criteria (Fig. 10).
The organic facies C, related to cL lithofacies, always
presents an anomalous trend.
4.7. Isotope composition
The average values of the saturate fractions is
ÿ27.82% PDB and the range of variation is 1.18% in
the saturate fractions (Table 4). In the aromatic fraction
and whole bitumen, the range of variation is 1.6 and
1.88%, respectively, greater than observed in saturate
fractions. The isotope log of the aromatic fraction and
whole bitumen displays an isotopic shift associated with
the main sea level fall in the section (Fig. 11a).
Perez-Infante et al. (1996) also observed a marked d
13
C Corg isotopic excursion in the middle part of a
Maraca Creek section of the La Luna Formation, which
was interpreted as a global depletion in 12C around the
Cenomanian /Turonian and the Coniacian/Santonian
boundary.
Using the plot of Sofer (1984) (Fig. 11c), isotopically
heavier samples correspond to extracts from pcSM
lithofacies (more siliciclastic) and isotopically lighter
extracts are related to more calcareous lithofacies. All
extracts plot in the marine area of Sofer (1984).
5. Conclusions
The La Luna Formation is a petroleum source rock
with good to excellent potential for oil. About 63% of
the volume of this formation is composed of phosphatic
calcareous shales and laminated mudstones with abundant foraminifers (pcSM lithofacies).
Three low frequency hemicycles were identi®ed in the
La Luna Formation: A generalized base level fall, during
the deposition of the Salada Member; a base level rise or
a deepening of the basin during the sedimentation of the
Pujamana Member, and a second base level fall that
permitted the deposition of the calcareous and phosphatic lithologies of the Galembo member. Regarding
the relationship between organic matter characteristics and
sea level changes, during shallowing stages in carbonate
shelves (Salada Member and Galembo Member), HI
tends to increase and TOC to decrease. In the siliciclastic
shelf, during shallowing stages, (Pujamana-Salada
Member), both TOC and HI decrease continuously.
Certain biomarker ratios such as oleanane/C30 hopane,
C20/C23 tricyclic, Ts/Tm show an increasing trend during
base level falls and could be proposed as a good indicator of rise and fall of sea level in third order sequence
stratigraphy cycles.
Sedimentation of the La Luna Formation occurred
on a large carbonate ramp, with restricted water circulation and anoxity. During certain periods, upwelling
favored high primary productivity and accumulation
and preservation of organic matter. Some biomarker
ratios can be considered typical values for the depositional environment of the La Luna Formation (e.g.
diasterane/sterane ratios < 1, Ts/Tm average < 0.33,
C35/C34 hopane >0.92, and oleanane/C30 hopane ranging
from 0.02 to 0.19). The C35/C34 hopane ratio correlates
with HI; suggesting that in carbonate environments,
changes in this parameter are more strongly related to
redox condition rather than to changes in carbonate
content.
It is possible to dierentiate organic facies type B, BC
and C in the Salada Member, organic facies type B and
D in the Pujamana Member and organic facies type B in
the Galembo Member. The d 13C isotope composition of
aromatic fractions and whole bitumens display an isotopic
shift associated with the main deepening event in the
A. Rangel et al. / Organic Geochemistry 31 (2000) 1267±1284
section. Based on these results, organic geochemistry
could be considered as an important tool to support the
sequence stratigraphy architecture of a sedimentary
succession.
Acknowledgements
We are grateful to Drs. R. Sassen and S. Talukdar for
helpful suggestions and constructive reviews and Drs.
M.N. YalcËyÂn and S. YÂnan for major linguistic and editorial corrections.
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