Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue12.Dec2000:
Organic Geochemistry 31 (2000) 1495±1507
www.elsevier.nl/locate/orggeochem
Generation and hydrocarbon entrapment within
Gondwanan sediments of the Mandapeta area,
Krishna-Godavari Basin, India
M.S. Raza Khan *, A.K. Sharma, S.K. Sahota, M. Mathur
RCL, ERBC, ONGC, Sibsagar, Assam 785640, India
Abstract
The discovery of hydrocarbons (mainly gas) in commercial quantities from Gondwanan sediments in the Mandapeta
®eld of Krishna-Godavari Basin, India, provided impetus for intensi®ed exploration in Mandapeta and the adjoining
Kommugudem, Draksharama and Endamuru ®elds. Both oil and gas have been found in the reservoirs of Mandapeta
(Triassic) and Golapalli (Early Cretaceous) formations. Mature, localised, basal shales (1.0±1.1% Ro) in the Mandapeta formation have sourced the oils from the Mandapeta Sandstone reservoir (Triassic). The oils being produced from
Golapalli Sandstone reservoir (Early Cretaceous) are relatively less mature and have been sourced by the underlying
shales in the Mandapeta Formation at a maturity level of 0.80±0.85% Ro. The source and maturity data preclude
liquid hydrocarbon sourcing from the Kommugudem (Permian) sequence. Permian coals and shales of the Kommugudem Formation are the major source rocks for gaseous hydrocarbons in this area. The hydrocarbon generation
started in Early Cretaceous in the Kommugudem Formation, but the intermittent tectonic activity (with associated
structural developments) has resulted in reorientation and redistribution of the then existing trap con®gurations. The
present day maturity level of the Permian sediments in the Mandapeta ®eld is 1.2% Ro or greater, capable of generating gas dominantly. The Raghavapuram shale in the Mandapeta area is adequately mature and has good hydrocarbon potential for oil generation. The probability of ®nding hydrocarbon reserves in the sands of Raghavapuram
shales and other suitable traps is high. Modern seismic information together with geologic models can give new
exploration leads. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Gondwanan sediments; Mandapeta ®eld; Petroleum generation; Maturation and isomerisation
1. Introduction
The discovery of gaseous hydrocarbons in commercial
quantities from Gondwanan sediments (Golapalli Formation of Early Cretaceous and Mandapeta Formation
of Triassic age) in the Mandapeta area of Krishna-Godavari onland basin provided impetus for intensi®ed
exploration in Mandapeta (MDP) and adjoining areas.
The discovery well Mandapeta-1 (MDP-A, Fig. 1) was
drilled to a depth of 4302 m and encountered the top of
gneissic basement at a depth of 4263 m. It penetrated
Permian to Recent sediments and produced gas from
Gondwanan sediments in the interval 2804±2740 m
(Mandapeta Sandstone).
* Corresponding author. Tel.: +91-3772-23560.
To date, more than 16 wells have been drilled on the
Mandapeta structure (Fig. 1). Out of these about seven
are hydrocarbon bearing in Mandapeta Sandstone
reservoir and three are hydrocarbon bearing in Golapalli
Sandstone reservoir (Fig. 2). As Kommugudem (KMG),
Draksharama (DRK) and Endamuru (END) structures
like Mandapeta (MDP) structure have also been identi®ed to possess Gondwanan sediments, they are considered
together for this study.
Seven wells KMG-A, MDP-A, -D, -L, -O, DRK-A
and END-A have been studied along with oil samples
from MDP-A, -C, -I, -L and-O wells for molecular level
characterisation (see in Fig. 1 for well locations).
The aim of the present study was to identify source
units and areas of generation and to investigate the subsequent migration and accumulation of hydrocarbons in
relation to the geological framework of the area.
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00132-7
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M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
Fig. 1. Location map of wells considered in present study.
2. Tectonics and stratigraphy
Mandapeta graben, lying in the East Godavari subbasin, was formed during Early Palaeozoic rift phase.
This extensive track of Lower Gondwanan deposition is
collinear with Pranhita Godavari Graben, which hosts
the complete sequence of Gondwanan Super Group
ranging in age from Permian to Lower Cretaceous. The
generalised stratigraphic succession of the Mandapeta
area based on interpreted lithology and electrolog correlation is given in Fig. 2.
In the Mandapeta area Kommugudem formations of
Lower Gondwanan comprising coal/shale/sandstone were
deposited in the lower deltaic to lacustrine environment
over an Archean basement. A major erosional unconformity has been found at the top of the Kommugudem
Formation over which Mandapeta Fluvial Sandstone of
Triassic age were deposited. The top of this sandstone
represents a hiatus overlain by Red Bed interval of
Triassic age comprising reddish to dark claystone.
The Mandapeta Sandstone together with the Red
Beds belongs to the Lower Gondwanan. At this juncture, the NW dip of the basin was reversed to SE
because of thermal doming associated with rifting. This
led to the erosion of elevated areas and deposition of
eroded material mainly as alluvial fans in adjacent lows.
Subsequent cooling and simultaneous crustal subsidence
part heralded the ®rst transgression where paralic to
marine sediments of Cretaceous age (Golapalli Sandstone, Raghavapuram Shale and Tirupati Sandstone
formations) were deposited. The end of Cretaceous
sedimentation is marked by marine regression. This was
followed by widespread volcanic activity where basaltic
¯ows with interbedded sediments like limestone, sandstone and claystone were poured. The Paleocene basalts
(Rajahamundry Traps) are unconformably overlain by
sandstone, claystone and basal limestone of Eocene age.
The sediments are then succeeded by Rajahmundry
Sandstone of Mio-Pliocene age followed by Pliestocene
to Recent sediments.
3. Experimental
Rock samples were crushed in a shatterbox at room
temperature. The crushed samples were soxhlet extracted
for 48 h with chloroform. The solvent was removed in a
rotary evaporator and the extract separated by column
chromatography using alumina and silica gel into three
fractions: alkane hydrocarbons, aromatic hydrocarbons
and polar compounds. The oils were separated into their
respective fractions (alkanes, aromatics and polar compounds) by column chromatography following the same
procedure. Fractionation of aromatic hydrocarbons into
mono-, di- and triaromatic was carried out on Waters
840 HPLC system in a normal phase isocratic mode
M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
1497
Fig. 2. The generalised lithostratigraphy. The gas/liquid hydrocarbon occurrences have been highlighted by asterisks.
using Energy Analysis NH2 columns with n-hexane as
the mobile phase.
The saturated hydrocarbon fractions were analysed
on a Shimadzu GC 9A system using a 30 m long, 0.25
mm i.d. OV-101, fused silica column with helium as the
carrier gas. GC conditions were: 100 C, heating rate 4 C/
min, ®nal temperature 280 C and injector temperature
280 C.
The triaromatic hydrocarbon fractions were analysed
on a Shimadzu GC 9A system using a 60 m long, 0.25
mm i.d. SE-54, fused silica column with helium as the
carrier gas. GC conditions were : 60 C, heating rate 3 C/
min, ®nal temperature 260 C and injector temperature
280 C.
The Rc was calculated as per the scheme of Radke
and Welte (1983).
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M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
4. Results and discussion
4.1. Occurrence of hydrocarbons
Hydrocarbon accumulations have been found in
Mandapeta Formation (Triassic) and Golapalli Formation (Early Cretaceous) in Mandapeta area. Regional
cap rock for Golapalli Sandstone reservoir is provided
by Raghavapuram shale and the regional cap rock for
Mandapeta Sandstone reservoir is provided by the Red
Bed.
Hydrocarbons have been found in the Mandapeta
Sandstone in seven wells (MDP-A, -C, -E, -F, -H and -K)
and in the Golapalli Sandstone in three wells (MDP-K,
-L and -O). This is dominantly a gas-prone area. The
presence of oil has been indicated in MDP-A, -C (Mandapeta Sandstone) and MDP-I, -L and -O (Golapalli
Sandstone). There is no prominent hydrocarbon ®nd in
Draksharama or Kommugudem area.
these oils from terrestrial organic matter deposited in
oxic environment.
Two aromatic biomarker ratios (the ratio of the concentration of 1-methyl phenanthrene to that of 9-methyl
phenanthrene and the ratio of the concentration of 1,7dimethylphenanthrene to that of a peak labelled x, which
is due to an unresolved mixture of 1,3-dimethylphenanthrene, 3,9-DMP, 2,10-DMP and 3,10-DMP have been
calculated which provide information about the biological
origins (Alexander et al., 1992). Alexander et al. (1992),
have termed them as age speci®c biomarkers and have
reported that these ratios are helpful for correlation studies at moderate maturities (the signi®cance of the values of
these ratios has been discussed in the following text under
sub-title `source rocks'). The values of these ratios for oil
samples are shown in Fig. 3. Both the ratios are very much
similar for oils from the Mandapeta reservoir as well as
the Golapalli reservoir, indicating that the source sequences which have given rise to these ¯uids received the same/
similar type of organic matter (Fig. 3).
4.2. Hydrocarbon type and characteristics
4.3. Maturity
The oils from MDP-A and MDP-C (produced from
Mandapeta Sandstone) are light coloured with medium
API gravities (Table 1). The oils from MDP-I, -L and
-O, being produced from Golapalli Sandstone, are also
of medium API gravities (Table 1). Oils from MDP-I, -L
are dark coloured while the oil from MDP-O is light
coloured. All liquid hydrocarbons are paranic in nature. The oils found in MDP-A, -C and -O, though light
in colour, have high yields of 300 C+ residue (around
40%) and substantial amounts of wax (more than 8%).
These types of oils may result from deasphalting.
The oils from Mandapeta Sandstone have values of
pristane/phytane (Pr/Ph) and pristane/n-heptadecane
(Pr/nC17) ratios 2.4:2.6 and 0.5:0.6, respectively. The
oils from Golapalli Sandstone have Pr/Ph and Pr/nC17
ratios ranging from 3.1 to 3.4 and 0.6 to 0.7 (Fig. 3).
The high values of Pr/Ph ratio indicate the generation of
The maturity of the oils has been assessed using aromatic compounds. The aromatic based maturity parameters are considered quite reliable as these compounds
are present in substantial concentration. The assessment
is based on the fact that during geological times the
thermodynamic less stable methyl phenanthrene isomers
are converted into more stable isomers. The Rc was
calibrated with Ro by plotting the MPI of rock extracts
against the observed Ro. It was found that the equation
of Radke and Welte (1983) is valid in this area. The
maturity derived from calibrated Rc also matches well
with other maturity parameters.
The oils found in the Mandapeta reservoir are more
mature than the oils found in the Golapalli Reservoir. The
maturity of Mandapeta and Golapalli reservoired oils is
0.95±1.0% Rc and 0.80±0.85% Rc, respectively (Fig. 3).
Table 1
Characteristics of oils of the Mandapeta area
Well no.
Depth (m)
Formation
Reservoir age
Density (15 C)
API gravity
Gross comp.
Saturates%
Aromatics%
NSO%
Sat/arom
MDP-A
MDP-C
MDP-I
MDP-L
MDP-O
2804±2795
Mandapeta
Triassic
0.7814
49.5
2835±2831
Mandapeta
Triassic
0.8068
43.8
2271±2275
Golapalli
Cretaceous
0.8019
44.9
2249±2246
Golapalli
Cretaceous
0.8204
41.0
2373±2359
Golapalli
Cretaceous
0.7742
51.2
83.54
12.61
3.85
6.62
81.24
13.61
5.15
5.97
75.25
15.92
8.83
4.73
74.34
18.41
7.25
4.04
76.62
16.91
6.47
4.53
M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
1499
4.5. Permian source sequence (Kommugudem
Formation)
4.4. Source rocks
Fair to good, locally rich source rocks occur
throughout the Permian, Triassic and Cretaceous units.
Source rock pyrolysis logs, maturation, extract data and
molecular level parameters have been compiled from
many wells throughout the sub-basin. Fig. 4 shows the
correlation of dierent stratigraphic units of the KMGMDP-DRK-END belt.
The Permian Gondwanan coal/coaly shale sediments
were deposited in ¯uvial-lower deltaic- lacustrine environment. The total organic carbon of these sediments is
high but S2 (mg HC/g rock) and HI (mg HC/g TOC)
values indicate hydrogen de®cient organic matter (Fig.
4). The pattern and values of Pr/Ph and Pr/nC17 ratios
Fig. 3. Hydrocarbon occurrences and their characteristics.
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Fig. 4. Source rock potential of the Kommugudem, Mandapeta, Draksharama and Endamuru belt (given values are ranges over
depths).
of Permian section (Kommugudem Formation) are
shown in Figs. 5 and 6, respectively.
In the Permian Gondwanan sediments of this basin, the
values of the ratios of 1 MP/9 MP and 1,7 DMP/x are less
than 0.65 and 0.35 respectively, which show the major
contribution of Glossopteris Pteridosperms. This is also
supported by palynological information (Prasad et al.,
1995).
Alexander et al. (1992) have shown that in Cooper/
Eromanga Basin, low values of 1 MP/9 MP and 1,7 DMP/
x ratios (less than 0.65 and 0.35, respectively) in oils and
rock extracts are indicator of Permian ¯ora (mainly Pteridosperms). The values of these ratios increase remarkably
as the ¯ora changes.
The Permian Gondwanan coals/coaly shales formed
from relatively simple Glossopteris pteridosperms ¯ora
in a cold to cool temperate climate. The Glossopteris are
pteridosperms gymnosperms (Gould and Shibaoka,
1980; Prasad et al., 1995), the preserved components of
which are rich in cellulose and lignin, which form the
hydrogen-poor, woody parts of trees (Cooper and
Murchison, 1969). Permian ¯ora, itself poor in exinite,
was also highly susceptible to dessication and oxidation
during the peat forming process. Hence the organic
matter, which was mainly hydrogen de®cient initially,
was subjected to further degradation in unfavourable
sites of deposition (Thomas, 1982). Since the Glassopteris ¯ora does not seem to have been rich in cuticle or
resin, major oil accumulations of land plant origin are not
considered likely. The organic character of the Permian is,
therefore, mainly gas prone, possibly with low yields in
areas where the primary inertinite content is high.
4.6. Triassic sediments (Mandapeta Formation)
This sequence is sand dominated with little signi®cance as a source rock. These sediments however,
contain many thin, non-coal, locally rich source rock
intervals. This sequence was deposited in a ¯uvial environment. The intraformational shales in the Kommugudem area are thick and of good quality as compared to the
Mandapeta area, there being a great variation in source
rock thickness, quality and potential. Inertinite is the
main maceral with a subordinate amount of vitrinite.
The sequence is absent in Draksharama and Endamuru
(Fig. 4). The maturity is around 0.8±1.1% Ro. The
values of Pr/Ph and Pr/nC17 ratios of this sequence
(Mandapeta Formation) are shown in Fig. 6. The
source parameters (1 MP/9 MP and I,7-DMP/x) based
on aromatic biomarkers are dierent from those of the
Permian. The explanation is that the ¯ora had changed
(Figs. 6 and 7). The palynological data indicates that the
Dicroidium Pteridosperms (Pteridospermous gymnosperms) had become dominant (along with some early
conifers) over Glossopteris ¯ora (Prasad et al., 1995).
The Triassic sequence contains some exinite fraction
M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
1501
Fig. 5. Saturated hydrocarbon capillary gas chromatograms of selected Cretaceous, Triassic, and Permian source rocks. (B) and (C)
show the lateral variation of values of Pr/Ph ratio from 2.4 to 3.4 Mandapeta Formation (localised source rocks).
which might have been derived from thick cutinite of
Dicroidium (Cook and Taylor, 1963; Cook, 1975) and
some early conifers (Prasad et al., 1995).
4.7. Cretaceous sequence
The organic richness in terms of TOC increases from
Kommugudem to Mandapeta to Draksharama to Endamuru (Fig. 4). The quality of organic matter deteriorates
from Kommugudem to Mandapeta area, while the
organic matter in Mandapeta and Draksharama areas is
almost similar in quality. Angiosperms appeared in the
Cretaceous. The abundance of resins, cuticle and spores,
together with the generally resin rich character of the
woody parts has led to high survival rate of organic
matter which has a high potential for liquid yield. This
sequence was deposited in a shallow marine environ-
ment. The aromatic biomarkers are dierent (1MP/
9MP>1.2 and 1,7-DMP/x>0.65) from the Triassic and
Permian sequences. (Figs. 6 and 7). During this period
the ¯ora had the dominance of conifers (Prasad et al.,
1995).
4.8. Organic maturity
The degree of maturity of source rocks has been
determined from Rc, spore discoloration, Rock Eval
Tmax. Solvent extracts of source rocks provide additional
data on chemical maturity as well as providing a means of
oil-source correlation. All these methods have practical
de®ciencies and integrated approach to maturity estimation has been adopted (Fig. 8). As discussed above MPI
of the rock extracts has been calibrated with observed
Ro to check validity of these parameters in this area.
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According to Tissot and Welte (1984), the onset of oil
generation occurs between 0.5 and 0.7% Ro depending
on the organic type. The transformation ratio derived
from hydrous pyrolysis of immature rock sample of this
area indicates that the onset of oil generation has
occurred at around 0.65% Ro.
The Permian sequence is post oil mature in the
Kommugudem area. The maturity increases towards
Fig. 6. Variation of source speci®c parameters in the study area.
Fig. 7. GC traces of aromatic fractions of typical oil and typical Cretaceous, Triassic, and Permian rock samples.
M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
1503
Fig. 8. Maturity levels at dierent stratigraphic levels in the KGM-MDP-DRK-END belt.
Fig. 9. Probable source rocks of the oils found in well MDP-K and MDP-O. Source rocks in the Mandapeta Formation have sourced
these oils, as evident from source and maturity parameters shown in the ®gure.
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Fig. 10. Probable source rocks of the oils found in well MDP-C. Source and maturity data, as shown in the ®gure, indicate that source
rocks in the Mandapeta Formation have sourced MDP-C oil.
Mandapeta. The maturity decreases towards Draksharama and Endamuru. In the Triassic sequence, maturity
increases from Kommugudem to Mandapeta. Around
Kommugudem, the sequence is moderately mature to
mature and around Mandapeta the sequence is mature.
Signi®cant maturation commenced around 2000 m in
Cretaceous sediments in Kommugudem and Mandapeta. The Cretaceous sections are marginal to moderately
mature in the studied area. The maturity increases from
Kommugudem to Mandapeta and decreases towards
Draksharama and Endamuru.
4.9. Oil±source rock correlation
Fig. 3 shows values of the aromatic hydrocarbon
based maturity indicators on oils recovered from the
Mandapeta ®eld. The striking feature of this data is the
progressive increase in maturity of oils with increasing
age (depth) of the reservoir formation based on Rc from
methyl phenanthrenes. The maturity indicators show
that the oils in the reservoir have similar maturities to
those of indigenous hydrocarbons contained in shales in
similar stratigraphic locations (Figs. 3 and 8).
The oils found in the Mandapeta and Golapalli
reservoirs have been derived from a similar type of
source organics based on aromatic biomarker ratios (1
MP/9 MP and 1,7-DMP/x). The oils from the Golapalli
reservoir are slightly less mature than the oils from the
Mandapeta reservoir.
A detailed study on oil to source correlation suggests
that the underlying shales at the depth interval of 2700±
2800 m in the Mandapeta Formation, of moderate
maturity, have sourced the oils reservoired in the Golapalli sandstone based on source and maturity parameters
shown in Fig. 9.
Mandapeta sandstone oils (from wells A and C) have
been generated by underlying and comparatively more
mature source rocks at the depth interval 2850±3150 m in
the Mandapeta Formation as evident from the close
values of pristane/phytane ratio and aromatic parameters
(1 MP/9 MP and 1,7-DMP/x) of Mandapeta reservoir oils
and probable rock extracts shown in Figs. 10 and 11. The
maturity of ¯uids is around 0.95±1.0% Rc which matches
well with the maturity of basal shales in Mandapeta Formation (Figs. 10 and 11). The origin of these oils from
underlying Permian sequence is ruled out on the basis of
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M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
Fig. 11. Probable source rocks of the oils found in well MDP-A. Source and maturity data, as shown in the ®gure, indicate that source
rocks in Mandapeta Formation have sourced MDP-A and-C oils.
maturity as well as aromatic biomarker dierences (Figs. 6
and 7). Moreover, Permian sequence is post oil mature
and hydrogen-de®cient which can generate only gaseous
hydrocarbons. The ¯ora available during Permian time
was of poor quality (as discussed earlier) and must have
generated only gaseous hydrocarbons during peak oil
generation as evident from a number of gas occurrences
in the sands of Kommugudem (Permian) Formation.
4.10. Gas composition and source
The composition of gases recovered from the Golapalli and Mandapeta sandstone reservoirs is given in
Table 2 and Fig. 3. The simple composition of gases and
factors such as multiple source, maturity and fractionation during migration make it dicult to correlate them
with their source rocks. The gas generating potential of
Table 2
Characteristics of gases of the Mandapeta area
Well no.
Depth (m)
Formation
Age
d 13C1
d 13C2
C1 (vol. %)
C2+ (vol. %)
C1/C2+C3
C2/C3+
iC4/nC4
C1/ Cn
MDP-A
MDP-C
MDP-G
MDP-L
2804±2795
Mandapeta
Triassic
ÿ32.6
ÿ25.6
89.13
10.3
9.66
2.42
0.80
0.90
2835±2777
Mandapeta
Triassic
ÿ32.6
ÿ24.1
85.18
9.32
9.68
3.63
0.81
0.90
2925±2895
Mandapeta
Triassic
ÿ32.5
ÿ25.0
85.2
11.1
8.67
2.17
1.12
0.88
2249±2246
Golapalli
Cretaceous
ÿ38.9
ÿ29.0
53.13
41.15
1.77
0.39
0.49
0.56
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M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
Permian sediments in the Mandapeta sub-basin has long
been recognized (unpublished work).
Gas reservoired in Mandapeta sandstone is moderately
wet with low C4+ hydrocarbon content (Fig. 3). The gas
composition and stable carbon isotopic studies point to a
mixed maturity source (catagenetic and metagenetic) with
a methane stable carbon isotope ratio of ÿ32.5 to
ÿ32.6%. The bulk of the gas is from the metagenetic
stage and appears to be sourced by Kommugudem
Formation. The maturity of these gases is equivalent to
1.2% Ro.
Gas reservoired in the overlying Golapalli Formation
is dierent from the Mandapeta Sandstone gas, in that
it is wetter and isotopically lighter, the gas composition
suggests the gas to be a typical oil-associated gas.
4.11. Origin, migration and entrapment of hydrocarbons
The Triassic reservoir (Mandapeta sandstone) is overlying the Permian coal-shale sequence (Kommugudem
Formation), resulting in extremely favourable entrapment conditions with the Red Bed being the regional
cap rock. Vertical and lateral migration is facilitated by
communication of the predominantly channel point bar
sandstone and local syndepositional faulting of the
Kommugudem Formation.
The Lower Cretaceous (Golapalli) reservoir is overlying
the Red Bed. The observed migration of hydrocarbons
from underlying source rocks will have occurred due to
erosion or non-deposition of the regional seal over these
trends, together with faulting.
The basin has experienced (Fig. 2) continuous subsidence interrupted by episodes of non-deposition
(example of heating with a constant rise in temperature
punctuated by isothermal heating). The two commonly
used methods of maturity modeling, namely TTI
method and Easy Ro method, have been applied in this
basin. It has been found that Waples TTI method
(Waples, 1980) overestimates maturation while the Ro
values calculated from Easy Ro of Sweeney and Burnham (1990) are in close agreement with the observed
ones in marginal to high maturity range (0.5±1.35%
Ro). Fig. 12 depicts the source rock potential and
maturity level of dierent layers in the KMG-MDPDRK-END belt. It has been argued that the Permian
coals and shales of the Kommugudem Formation are
the major source rocks for gas in this area. The hydrocarbon generation started in Early Cretaceous in the
Kommugudem Formation as estimated by Easy Ro of
Sweeney and Burnham (1990). The traps were available
during this time, but the intermittent tectonic activity
has resulted into reorientation and redistribution of the
original trap geometries. The present day maturity level
of the Permian in the Mandapeta area is Ro 1.2% or
greater, which is consistent with the maturity of gases
encountered in this area. They are thermogenic (mixed
i.e. derived at late catagenetic as well as early metagenetic stages) in origin.
Localised shales in the Mandapeta Formation have
sourced the Mandapeta and the Golapalli oils. The oils
being produced from Golapalli Formation have been
generated at lower maturity levels.
Fig. 12. Source rock potential (along A±A0 ) based on organofacies and its maturity level.
M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
4.12. Hydrocarbon occurrence and fault blocks
The fault blocks in the Mandapeta area are the result
of tectonic activity, which was probably initiated prior
to the Permian and has continued intermittently. This
has in¯uenced the generation and entrapment of hydrocarbons by:
1. Providing conditions conducive to the deposition
of potential Permian, Triassic and Cretaceous
source rocks, sometimes in considerable thickness,
in close proximity to potential reservoir/seal pairs.
2. The formation of a signi®cant number of proven
structural traps and probably stratigraphic traps.
3. A favourable timing of trap development relative
to hydrocarbon expulsion.
4. Possibly allowing hydrocarbons to migrate up
faults to reservoirs.
1507
lish this work. Profound thanks are due to Shri Kuldeep
Chandra, Executive Director and Head, KDMIPE, for
his valuable guidance and suggestions during the preparation of this manuscript. Thanks are due to Shri
K.N. Misra, G.M. (GRG), KDMIPE, Dehradun, for
his valuable guidance and encouragement. The authors
are also grateful to Shri Lehamber Singh, G.M. (Exp),
ERBC, and Dr. B.K. Sharma, G.M. (Chem), ERBC, for
providing valuable guidance and a wonderful environment to complete this work. The authors are also grateful
to Dr. Rajiv Sharma, Sr. Chemist, for his contribution
in the preparation of this report. We thank the reviewers, Drs. R.G. Schaefer and C. Clayton, for their valuable criticism and suggestions.
References
5. Conclusions
Land plant rich source rocks are widely distributed
throughout the Permian and Triassic sections of the basin.
The Cretaceous sequence deposited in shallow marine
environment has substantial contribution of marine
organic matter. The Permian section is post mature for oil
generation in the Kommugudem and Mandapeta area
of the belt. The Triassic section is oil mature and the
Cretaceous section is early to moderately mature.
Hydrocarbon accumulations are mainly gas and gas/oil.
Gas in the Golapalli and Mandapeta reservoirs has been
generated from both the Triassic shales (Mandapeta Formation) and Permian coals/coaly shales (Kommugudem
Formation), although there is evidence that the Permian
is the principal source.
The associated small accumulations of oils encountered
in parts of this belt are attributed to the oil-prone shales in
the Mandapeta Formation. Most of the oils discovered
are paranic in nature and have mature character. The oil
to source correlation and the basin con®guration suggest
vertical and short distance migration.
The study indicates that Raghavapuram shale in the
Mandapeta area has adequate maturity and hydrocarbon potential for oil generation. Oil has been discovered in the interbedded sands of one well recently.
The probability of ®nding hydrocarbon reserves in
sands of Raghavapuram shale and other suitable traps
is high. Modern seismic information together with geologic models can give new exploration leads.
Acknowledgements
The authors are grateful to Director (Exploration) Shri
T.K.N. Gopalaswami, for according permission to pub-
Alexander, R., Larcher, A.V., Kagi, R.I., Price, P.L., 1992. An
oil correlation study using age-speci®c plant-derived aromatic biomarkers. In: Moldowan, J.M., Albrecht, P., Philp,
R.P. (Eds.), Biological Markers in Sediments and Petroleums. Prentice Hall, NJ, pp. 201±221.
Cook, A.C., 1975. The spatial and temporal variation of the
type and rank of Australian coals. In: Cook, A.C. (Ed.),
Australian Black Coal. Australasian Institute of Mining and
Metallurgy, Illawarra Branch, Wollongong, pp. 63±84.
Cook, A.C., Taylor, G.H., 1963. The petrography of some
Triassic Ipswich coals. Proceedings of the Australasian
Institute of Mining and Metallurgy 205, 35±55.
Cooper, B.S., Murchison, D.G., 1969. Organic geochemistry of
coal. In: Eglinton G., Murphy M.T.J. (Eds.), Organic Geochemistry, Methods and Results. Springer Verlag, Berlin.
Gould, R., Shibaoka, M., 1980. Some aspects of the formation
and petrofabric features of coal members in Australia, with
special reference to the Tasman orogenic Zone. Coal Geology 2, 1±29.
Prasad, B., Jain, A.K., Mathur, Y.K., 1995. A standard palynozonation scheme for Cretaceous and Pre-Cretaceous subsurface sediments of K.G. Basin, India. Geoscience Journal
16, 155±233.
Radke, M., Welte, D.H., 1983. The methyl phenanthrene index
(MPI): a maturity parameter based on aromatic hydrocarbons. In: Bjoroy, M. et al. (Eds.), Advances in Organic
Geochemistry 1981. John Wiley and Sons, New York, pp.
504±512.
Sweeney, J.J., Burnham, A.K., 1990. Evaluation of a simple
model of vitrinite re¯ectance based on chemical kinetics.
American Association of Petroleum Geologists Bulletin 74,
1559±1570.
Thomas, B.M., 1982. Land plant source rocks for oils and their
signi®cance in Australian basins. APEA Journal 22, 164±178.
Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and
Occurrence, 2nd Edition. Springer Verlag, Berlin.
Waples, D.W., 1980. Time and temperature in petroleum formation, application of Lopatin's method to petroleum
exploration. American Association of Petroleum Geologists
Bulletin 64, 916±926.
www.elsevier.nl/locate/orggeochem
Generation and hydrocarbon entrapment within
Gondwanan sediments of the Mandapeta area,
Krishna-Godavari Basin, India
M.S. Raza Khan *, A.K. Sharma, S.K. Sahota, M. Mathur
RCL, ERBC, ONGC, Sibsagar, Assam 785640, India
Abstract
The discovery of hydrocarbons (mainly gas) in commercial quantities from Gondwanan sediments in the Mandapeta
®eld of Krishna-Godavari Basin, India, provided impetus for intensi®ed exploration in Mandapeta and the adjoining
Kommugudem, Draksharama and Endamuru ®elds. Both oil and gas have been found in the reservoirs of Mandapeta
(Triassic) and Golapalli (Early Cretaceous) formations. Mature, localised, basal shales (1.0±1.1% Ro) in the Mandapeta formation have sourced the oils from the Mandapeta Sandstone reservoir (Triassic). The oils being produced from
Golapalli Sandstone reservoir (Early Cretaceous) are relatively less mature and have been sourced by the underlying
shales in the Mandapeta Formation at a maturity level of 0.80±0.85% Ro. The source and maturity data preclude
liquid hydrocarbon sourcing from the Kommugudem (Permian) sequence. Permian coals and shales of the Kommugudem Formation are the major source rocks for gaseous hydrocarbons in this area. The hydrocarbon generation
started in Early Cretaceous in the Kommugudem Formation, but the intermittent tectonic activity (with associated
structural developments) has resulted in reorientation and redistribution of the then existing trap con®gurations. The
present day maturity level of the Permian sediments in the Mandapeta ®eld is 1.2% Ro or greater, capable of generating gas dominantly. The Raghavapuram shale in the Mandapeta area is adequately mature and has good hydrocarbon potential for oil generation. The probability of ®nding hydrocarbon reserves in the sands of Raghavapuram
shales and other suitable traps is high. Modern seismic information together with geologic models can give new
exploration leads. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Gondwanan sediments; Mandapeta ®eld; Petroleum generation; Maturation and isomerisation
1. Introduction
The discovery of gaseous hydrocarbons in commercial
quantities from Gondwanan sediments (Golapalli Formation of Early Cretaceous and Mandapeta Formation
of Triassic age) in the Mandapeta area of Krishna-Godavari onland basin provided impetus for intensi®ed
exploration in Mandapeta (MDP) and adjoining areas.
The discovery well Mandapeta-1 (MDP-A, Fig. 1) was
drilled to a depth of 4302 m and encountered the top of
gneissic basement at a depth of 4263 m. It penetrated
Permian to Recent sediments and produced gas from
Gondwanan sediments in the interval 2804±2740 m
(Mandapeta Sandstone).
* Corresponding author. Tel.: +91-3772-23560.
To date, more than 16 wells have been drilled on the
Mandapeta structure (Fig. 1). Out of these about seven
are hydrocarbon bearing in Mandapeta Sandstone
reservoir and three are hydrocarbon bearing in Golapalli
Sandstone reservoir (Fig. 2). As Kommugudem (KMG),
Draksharama (DRK) and Endamuru (END) structures
like Mandapeta (MDP) structure have also been identi®ed to possess Gondwanan sediments, they are considered
together for this study.
Seven wells KMG-A, MDP-A, -D, -L, -O, DRK-A
and END-A have been studied along with oil samples
from MDP-A, -C, -I, -L and-O wells for molecular level
characterisation (see in Fig. 1 for well locations).
The aim of the present study was to identify source
units and areas of generation and to investigate the subsequent migration and accumulation of hydrocarbons in
relation to the geological framework of the area.
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00132-7
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M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
Fig. 1. Location map of wells considered in present study.
2. Tectonics and stratigraphy
Mandapeta graben, lying in the East Godavari subbasin, was formed during Early Palaeozoic rift phase.
This extensive track of Lower Gondwanan deposition is
collinear with Pranhita Godavari Graben, which hosts
the complete sequence of Gondwanan Super Group
ranging in age from Permian to Lower Cretaceous. The
generalised stratigraphic succession of the Mandapeta
area based on interpreted lithology and electrolog correlation is given in Fig. 2.
In the Mandapeta area Kommugudem formations of
Lower Gondwanan comprising coal/shale/sandstone were
deposited in the lower deltaic to lacustrine environment
over an Archean basement. A major erosional unconformity has been found at the top of the Kommugudem
Formation over which Mandapeta Fluvial Sandstone of
Triassic age were deposited. The top of this sandstone
represents a hiatus overlain by Red Bed interval of
Triassic age comprising reddish to dark claystone.
The Mandapeta Sandstone together with the Red
Beds belongs to the Lower Gondwanan. At this juncture, the NW dip of the basin was reversed to SE
because of thermal doming associated with rifting. This
led to the erosion of elevated areas and deposition of
eroded material mainly as alluvial fans in adjacent lows.
Subsequent cooling and simultaneous crustal subsidence
part heralded the ®rst transgression where paralic to
marine sediments of Cretaceous age (Golapalli Sandstone, Raghavapuram Shale and Tirupati Sandstone
formations) were deposited. The end of Cretaceous
sedimentation is marked by marine regression. This was
followed by widespread volcanic activity where basaltic
¯ows with interbedded sediments like limestone, sandstone and claystone were poured. The Paleocene basalts
(Rajahamundry Traps) are unconformably overlain by
sandstone, claystone and basal limestone of Eocene age.
The sediments are then succeeded by Rajahmundry
Sandstone of Mio-Pliocene age followed by Pliestocene
to Recent sediments.
3. Experimental
Rock samples were crushed in a shatterbox at room
temperature. The crushed samples were soxhlet extracted
for 48 h with chloroform. The solvent was removed in a
rotary evaporator and the extract separated by column
chromatography using alumina and silica gel into three
fractions: alkane hydrocarbons, aromatic hydrocarbons
and polar compounds. The oils were separated into their
respective fractions (alkanes, aromatics and polar compounds) by column chromatography following the same
procedure. Fractionation of aromatic hydrocarbons into
mono-, di- and triaromatic was carried out on Waters
840 HPLC system in a normal phase isocratic mode
M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
1497
Fig. 2. The generalised lithostratigraphy. The gas/liquid hydrocarbon occurrences have been highlighted by asterisks.
using Energy Analysis NH2 columns with n-hexane as
the mobile phase.
The saturated hydrocarbon fractions were analysed
on a Shimadzu GC 9A system using a 30 m long, 0.25
mm i.d. OV-101, fused silica column with helium as the
carrier gas. GC conditions were: 100 C, heating rate 4 C/
min, ®nal temperature 280 C and injector temperature
280 C.
The triaromatic hydrocarbon fractions were analysed
on a Shimadzu GC 9A system using a 60 m long, 0.25
mm i.d. SE-54, fused silica column with helium as the
carrier gas. GC conditions were : 60 C, heating rate 3 C/
min, ®nal temperature 260 C and injector temperature
280 C.
The Rc was calculated as per the scheme of Radke
and Welte (1983).
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M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
4. Results and discussion
4.1. Occurrence of hydrocarbons
Hydrocarbon accumulations have been found in
Mandapeta Formation (Triassic) and Golapalli Formation (Early Cretaceous) in Mandapeta area. Regional
cap rock for Golapalli Sandstone reservoir is provided
by Raghavapuram shale and the regional cap rock for
Mandapeta Sandstone reservoir is provided by the Red
Bed.
Hydrocarbons have been found in the Mandapeta
Sandstone in seven wells (MDP-A, -C, -E, -F, -H and -K)
and in the Golapalli Sandstone in three wells (MDP-K,
-L and -O). This is dominantly a gas-prone area. The
presence of oil has been indicated in MDP-A, -C (Mandapeta Sandstone) and MDP-I, -L and -O (Golapalli
Sandstone). There is no prominent hydrocarbon ®nd in
Draksharama or Kommugudem area.
these oils from terrestrial organic matter deposited in
oxic environment.
Two aromatic biomarker ratios (the ratio of the concentration of 1-methyl phenanthrene to that of 9-methyl
phenanthrene and the ratio of the concentration of 1,7dimethylphenanthrene to that of a peak labelled x, which
is due to an unresolved mixture of 1,3-dimethylphenanthrene, 3,9-DMP, 2,10-DMP and 3,10-DMP have been
calculated which provide information about the biological
origins (Alexander et al., 1992). Alexander et al. (1992),
have termed them as age speci®c biomarkers and have
reported that these ratios are helpful for correlation studies at moderate maturities (the signi®cance of the values of
these ratios has been discussed in the following text under
sub-title `source rocks'). The values of these ratios for oil
samples are shown in Fig. 3. Both the ratios are very much
similar for oils from the Mandapeta reservoir as well as
the Golapalli reservoir, indicating that the source sequences which have given rise to these ¯uids received the same/
similar type of organic matter (Fig. 3).
4.2. Hydrocarbon type and characteristics
4.3. Maturity
The oils from MDP-A and MDP-C (produced from
Mandapeta Sandstone) are light coloured with medium
API gravities (Table 1). The oils from MDP-I, -L and
-O, being produced from Golapalli Sandstone, are also
of medium API gravities (Table 1). Oils from MDP-I, -L
are dark coloured while the oil from MDP-O is light
coloured. All liquid hydrocarbons are paranic in nature. The oils found in MDP-A, -C and -O, though light
in colour, have high yields of 300 C+ residue (around
40%) and substantial amounts of wax (more than 8%).
These types of oils may result from deasphalting.
The oils from Mandapeta Sandstone have values of
pristane/phytane (Pr/Ph) and pristane/n-heptadecane
(Pr/nC17) ratios 2.4:2.6 and 0.5:0.6, respectively. The
oils from Golapalli Sandstone have Pr/Ph and Pr/nC17
ratios ranging from 3.1 to 3.4 and 0.6 to 0.7 (Fig. 3).
The high values of Pr/Ph ratio indicate the generation of
The maturity of the oils has been assessed using aromatic compounds. The aromatic based maturity parameters are considered quite reliable as these compounds
are present in substantial concentration. The assessment
is based on the fact that during geological times the
thermodynamic less stable methyl phenanthrene isomers
are converted into more stable isomers. The Rc was
calibrated with Ro by plotting the MPI of rock extracts
against the observed Ro. It was found that the equation
of Radke and Welte (1983) is valid in this area. The
maturity derived from calibrated Rc also matches well
with other maturity parameters.
The oils found in the Mandapeta reservoir are more
mature than the oils found in the Golapalli Reservoir. The
maturity of Mandapeta and Golapalli reservoired oils is
0.95±1.0% Rc and 0.80±0.85% Rc, respectively (Fig. 3).
Table 1
Characteristics of oils of the Mandapeta area
Well no.
Depth (m)
Formation
Reservoir age
Density (15 C)
API gravity
Gross comp.
Saturates%
Aromatics%
NSO%
Sat/arom
MDP-A
MDP-C
MDP-I
MDP-L
MDP-O
2804±2795
Mandapeta
Triassic
0.7814
49.5
2835±2831
Mandapeta
Triassic
0.8068
43.8
2271±2275
Golapalli
Cretaceous
0.8019
44.9
2249±2246
Golapalli
Cretaceous
0.8204
41.0
2373±2359
Golapalli
Cretaceous
0.7742
51.2
83.54
12.61
3.85
6.62
81.24
13.61
5.15
5.97
75.25
15.92
8.83
4.73
74.34
18.41
7.25
4.04
76.62
16.91
6.47
4.53
M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
1499
4.5. Permian source sequence (Kommugudem
Formation)
4.4. Source rocks
Fair to good, locally rich source rocks occur
throughout the Permian, Triassic and Cretaceous units.
Source rock pyrolysis logs, maturation, extract data and
molecular level parameters have been compiled from
many wells throughout the sub-basin. Fig. 4 shows the
correlation of dierent stratigraphic units of the KMGMDP-DRK-END belt.
The Permian Gondwanan coal/coaly shale sediments
were deposited in ¯uvial-lower deltaic- lacustrine environment. The total organic carbon of these sediments is
high but S2 (mg HC/g rock) and HI (mg HC/g TOC)
values indicate hydrogen de®cient organic matter (Fig.
4). The pattern and values of Pr/Ph and Pr/nC17 ratios
Fig. 3. Hydrocarbon occurrences and their characteristics.
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M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
Fig. 4. Source rock potential of the Kommugudem, Mandapeta, Draksharama and Endamuru belt (given values are ranges over
depths).
of Permian section (Kommugudem Formation) are
shown in Figs. 5 and 6, respectively.
In the Permian Gondwanan sediments of this basin, the
values of the ratios of 1 MP/9 MP and 1,7 DMP/x are less
than 0.65 and 0.35 respectively, which show the major
contribution of Glossopteris Pteridosperms. This is also
supported by palynological information (Prasad et al.,
1995).
Alexander et al. (1992) have shown that in Cooper/
Eromanga Basin, low values of 1 MP/9 MP and 1,7 DMP/
x ratios (less than 0.65 and 0.35, respectively) in oils and
rock extracts are indicator of Permian ¯ora (mainly Pteridosperms). The values of these ratios increase remarkably
as the ¯ora changes.
The Permian Gondwanan coals/coaly shales formed
from relatively simple Glossopteris pteridosperms ¯ora
in a cold to cool temperate climate. The Glossopteris are
pteridosperms gymnosperms (Gould and Shibaoka,
1980; Prasad et al., 1995), the preserved components of
which are rich in cellulose and lignin, which form the
hydrogen-poor, woody parts of trees (Cooper and
Murchison, 1969). Permian ¯ora, itself poor in exinite,
was also highly susceptible to dessication and oxidation
during the peat forming process. Hence the organic
matter, which was mainly hydrogen de®cient initially,
was subjected to further degradation in unfavourable
sites of deposition (Thomas, 1982). Since the Glassopteris ¯ora does not seem to have been rich in cuticle or
resin, major oil accumulations of land plant origin are not
considered likely. The organic character of the Permian is,
therefore, mainly gas prone, possibly with low yields in
areas where the primary inertinite content is high.
4.6. Triassic sediments (Mandapeta Formation)
This sequence is sand dominated with little signi®cance as a source rock. These sediments however,
contain many thin, non-coal, locally rich source rock
intervals. This sequence was deposited in a ¯uvial environment. The intraformational shales in the Kommugudem area are thick and of good quality as compared to the
Mandapeta area, there being a great variation in source
rock thickness, quality and potential. Inertinite is the
main maceral with a subordinate amount of vitrinite.
The sequence is absent in Draksharama and Endamuru
(Fig. 4). The maturity is around 0.8±1.1% Ro. The
values of Pr/Ph and Pr/nC17 ratios of this sequence
(Mandapeta Formation) are shown in Fig. 6. The
source parameters (1 MP/9 MP and I,7-DMP/x) based
on aromatic biomarkers are dierent from those of the
Permian. The explanation is that the ¯ora had changed
(Figs. 6 and 7). The palynological data indicates that the
Dicroidium Pteridosperms (Pteridospermous gymnosperms) had become dominant (along with some early
conifers) over Glossopteris ¯ora (Prasad et al., 1995).
The Triassic sequence contains some exinite fraction
M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
1501
Fig. 5. Saturated hydrocarbon capillary gas chromatograms of selected Cretaceous, Triassic, and Permian source rocks. (B) and (C)
show the lateral variation of values of Pr/Ph ratio from 2.4 to 3.4 Mandapeta Formation (localised source rocks).
which might have been derived from thick cutinite of
Dicroidium (Cook and Taylor, 1963; Cook, 1975) and
some early conifers (Prasad et al., 1995).
4.7. Cretaceous sequence
The organic richness in terms of TOC increases from
Kommugudem to Mandapeta to Draksharama to Endamuru (Fig. 4). The quality of organic matter deteriorates
from Kommugudem to Mandapeta area, while the
organic matter in Mandapeta and Draksharama areas is
almost similar in quality. Angiosperms appeared in the
Cretaceous. The abundance of resins, cuticle and spores,
together with the generally resin rich character of the
woody parts has led to high survival rate of organic
matter which has a high potential for liquid yield. This
sequence was deposited in a shallow marine environ-
ment. The aromatic biomarkers are dierent (1MP/
9MP>1.2 and 1,7-DMP/x>0.65) from the Triassic and
Permian sequences. (Figs. 6 and 7). During this period
the ¯ora had the dominance of conifers (Prasad et al.,
1995).
4.8. Organic maturity
The degree of maturity of source rocks has been
determined from Rc, spore discoloration, Rock Eval
Tmax. Solvent extracts of source rocks provide additional
data on chemical maturity as well as providing a means of
oil-source correlation. All these methods have practical
de®ciencies and integrated approach to maturity estimation has been adopted (Fig. 8). As discussed above MPI
of the rock extracts has been calibrated with observed
Ro to check validity of these parameters in this area.
1502
M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
According to Tissot and Welte (1984), the onset of oil
generation occurs between 0.5 and 0.7% Ro depending
on the organic type. The transformation ratio derived
from hydrous pyrolysis of immature rock sample of this
area indicates that the onset of oil generation has
occurred at around 0.65% Ro.
The Permian sequence is post oil mature in the
Kommugudem area. The maturity increases towards
Fig. 6. Variation of source speci®c parameters in the study area.
Fig. 7. GC traces of aromatic fractions of typical oil and typical Cretaceous, Triassic, and Permian rock samples.
M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
1503
Fig. 8. Maturity levels at dierent stratigraphic levels in the KGM-MDP-DRK-END belt.
Fig. 9. Probable source rocks of the oils found in well MDP-K and MDP-O. Source rocks in the Mandapeta Formation have sourced
these oils, as evident from source and maturity parameters shown in the ®gure.
1504
M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
Fig. 10. Probable source rocks of the oils found in well MDP-C. Source and maturity data, as shown in the ®gure, indicate that source
rocks in the Mandapeta Formation have sourced MDP-C oil.
Mandapeta. The maturity decreases towards Draksharama and Endamuru. In the Triassic sequence, maturity
increases from Kommugudem to Mandapeta. Around
Kommugudem, the sequence is moderately mature to
mature and around Mandapeta the sequence is mature.
Signi®cant maturation commenced around 2000 m in
Cretaceous sediments in Kommugudem and Mandapeta. The Cretaceous sections are marginal to moderately
mature in the studied area. The maturity increases from
Kommugudem to Mandapeta and decreases towards
Draksharama and Endamuru.
4.9. Oil±source rock correlation
Fig. 3 shows values of the aromatic hydrocarbon
based maturity indicators on oils recovered from the
Mandapeta ®eld. The striking feature of this data is the
progressive increase in maturity of oils with increasing
age (depth) of the reservoir formation based on Rc from
methyl phenanthrenes. The maturity indicators show
that the oils in the reservoir have similar maturities to
those of indigenous hydrocarbons contained in shales in
similar stratigraphic locations (Figs. 3 and 8).
The oils found in the Mandapeta and Golapalli
reservoirs have been derived from a similar type of
source organics based on aromatic biomarker ratios (1
MP/9 MP and 1,7-DMP/x). The oils from the Golapalli
reservoir are slightly less mature than the oils from the
Mandapeta reservoir.
A detailed study on oil to source correlation suggests
that the underlying shales at the depth interval of 2700±
2800 m in the Mandapeta Formation, of moderate
maturity, have sourced the oils reservoired in the Golapalli sandstone based on source and maturity parameters
shown in Fig. 9.
Mandapeta sandstone oils (from wells A and C) have
been generated by underlying and comparatively more
mature source rocks at the depth interval 2850±3150 m in
the Mandapeta Formation as evident from the close
values of pristane/phytane ratio and aromatic parameters
(1 MP/9 MP and 1,7-DMP/x) of Mandapeta reservoir oils
and probable rock extracts shown in Figs. 10 and 11. The
maturity of ¯uids is around 0.95±1.0% Rc which matches
well with the maturity of basal shales in Mandapeta Formation (Figs. 10 and 11). The origin of these oils from
underlying Permian sequence is ruled out on the basis of
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M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
Fig. 11. Probable source rocks of the oils found in well MDP-A. Source and maturity data, as shown in the ®gure, indicate that source
rocks in Mandapeta Formation have sourced MDP-A and-C oils.
maturity as well as aromatic biomarker dierences (Figs. 6
and 7). Moreover, Permian sequence is post oil mature
and hydrogen-de®cient which can generate only gaseous
hydrocarbons. The ¯ora available during Permian time
was of poor quality (as discussed earlier) and must have
generated only gaseous hydrocarbons during peak oil
generation as evident from a number of gas occurrences
in the sands of Kommugudem (Permian) Formation.
4.10. Gas composition and source
The composition of gases recovered from the Golapalli and Mandapeta sandstone reservoirs is given in
Table 2 and Fig. 3. The simple composition of gases and
factors such as multiple source, maturity and fractionation during migration make it dicult to correlate them
with their source rocks. The gas generating potential of
Table 2
Characteristics of gases of the Mandapeta area
Well no.
Depth (m)
Formation
Age
d 13C1
d 13C2
C1 (vol. %)
C2+ (vol. %)
C1/C2+C3
C2/C3+
iC4/nC4
C1/ Cn
MDP-A
MDP-C
MDP-G
MDP-L
2804±2795
Mandapeta
Triassic
ÿ32.6
ÿ25.6
89.13
10.3
9.66
2.42
0.80
0.90
2835±2777
Mandapeta
Triassic
ÿ32.6
ÿ24.1
85.18
9.32
9.68
3.63
0.81
0.90
2925±2895
Mandapeta
Triassic
ÿ32.5
ÿ25.0
85.2
11.1
8.67
2.17
1.12
0.88
2249±2246
Golapalli
Cretaceous
ÿ38.9
ÿ29.0
53.13
41.15
1.77
0.39
0.49
0.56
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M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
Permian sediments in the Mandapeta sub-basin has long
been recognized (unpublished work).
Gas reservoired in Mandapeta sandstone is moderately
wet with low C4+ hydrocarbon content (Fig. 3). The gas
composition and stable carbon isotopic studies point to a
mixed maturity source (catagenetic and metagenetic) with
a methane stable carbon isotope ratio of ÿ32.5 to
ÿ32.6%. The bulk of the gas is from the metagenetic
stage and appears to be sourced by Kommugudem
Formation. The maturity of these gases is equivalent to
1.2% Ro.
Gas reservoired in the overlying Golapalli Formation
is dierent from the Mandapeta Sandstone gas, in that
it is wetter and isotopically lighter, the gas composition
suggests the gas to be a typical oil-associated gas.
4.11. Origin, migration and entrapment of hydrocarbons
The Triassic reservoir (Mandapeta sandstone) is overlying the Permian coal-shale sequence (Kommugudem
Formation), resulting in extremely favourable entrapment conditions with the Red Bed being the regional
cap rock. Vertical and lateral migration is facilitated by
communication of the predominantly channel point bar
sandstone and local syndepositional faulting of the
Kommugudem Formation.
The Lower Cretaceous (Golapalli) reservoir is overlying
the Red Bed. The observed migration of hydrocarbons
from underlying source rocks will have occurred due to
erosion or non-deposition of the regional seal over these
trends, together with faulting.
The basin has experienced (Fig. 2) continuous subsidence interrupted by episodes of non-deposition
(example of heating with a constant rise in temperature
punctuated by isothermal heating). The two commonly
used methods of maturity modeling, namely TTI
method and Easy Ro method, have been applied in this
basin. It has been found that Waples TTI method
(Waples, 1980) overestimates maturation while the Ro
values calculated from Easy Ro of Sweeney and Burnham (1990) are in close agreement with the observed
ones in marginal to high maturity range (0.5±1.35%
Ro). Fig. 12 depicts the source rock potential and
maturity level of dierent layers in the KMG-MDPDRK-END belt. It has been argued that the Permian
coals and shales of the Kommugudem Formation are
the major source rocks for gas in this area. The hydrocarbon generation started in Early Cretaceous in the
Kommugudem Formation as estimated by Easy Ro of
Sweeney and Burnham (1990). The traps were available
during this time, but the intermittent tectonic activity
has resulted into reorientation and redistribution of the
original trap geometries. The present day maturity level
of the Permian in the Mandapeta area is Ro 1.2% or
greater, which is consistent with the maturity of gases
encountered in this area. They are thermogenic (mixed
i.e. derived at late catagenetic as well as early metagenetic stages) in origin.
Localised shales in the Mandapeta Formation have
sourced the Mandapeta and the Golapalli oils. The oils
being produced from Golapalli Formation have been
generated at lower maturity levels.
Fig. 12. Source rock potential (along A±A0 ) based on organofacies and its maturity level.
M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507
4.12. Hydrocarbon occurrence and fault blocks
The fault blocks in the Mandapeta area are the result
of tectonic activity, which was probably initiated prior
to the Permian and has continued intermittently. This
has in¯uenced the generation and entrapment of hydrocarbons by:
1. Providing conditions conducive to the deposition
of potential Permian, Triassic and Cretaceous
source rocks, sometimes in considerable thickness,
in close proximity to potential reservoir/seal pairs.
2. The formation of a signi®cant number of proven
structural traps and probably stratigraphic traps.
3. A favourable timing of trap development relative
to hydrocarbon expulsion.
4. Possibly allowing hydrocarbons to migrate up
faults to reservoirs.
1507
lish this work. Profound thanks are due to Shri Kuldeep
Chandra, Executive Director and Head, KDMIPE, for
his valuable guidance and suggestions during the preparation of this manuscript. Thanks are due to Shri
K.N. Misra, G.M. (GRG), KDMIPE, Dehradun, for
his valuable guidance and encouragement. The authors
are also grateful to Shri Lehamber Singh, G.M. (Exp),
ERBC, and Dr. B.K. Sharma, G.M. (Chem), ERBC, for
providing valuable guidance and a wonderful environment to complete this work. The authors are also grateful
to Dr. Rajiv Sharma, Sr. Chemist, for his contribution
in the preparation of this report. We thank the reviewers, Drs. R.G. Schaefer and C. Clayton, for their valuable criticism and suggestions.
References
5. Conclusions
Land plant rich source rocks are widely distributed
throughout the Permian and Triassic sections of the basin.
The Cretaceous sequence deposited in shallow marine
environment has substantial contribution of marine
organic matter. The Permian section is post mature for oil
generation in the Kommugudem and Mandapeta area
of the belt. The Triassic section is oil mature and the
Cretaceous section is early to moderately mature.
Hydrocarbon accumulations are mainly gas and gas/oil.
Gas in the Golapalli and Mandapeta reservoirs has been
generated from both the Triassic shales (Mandapeta Formation) and Permian coals/coaly shales (Kommugudem
Formation), although there is evidence that the Permian
is the principal source.
The associated small accumulations of oils encountered
in parts of this belt are attributed to the oil-prone shales in
the Mandapeta Formation. Most of the oils discovered
are paranic in nature and have mature character. The oil
to source correlation and the basin con®guration suggest
vertical and short distance migration.
The study indicates that Raghavapuram shale in the
Mandapeta area has adequate maturity and hydrocarbon potential for oil generation. Oil has been discovered in the interbedded sands of one well recently.
The probability of ®nding hydrocarbon reserves in
sands of Raghavapuram shale and other suitable traps
is high. Modern seismic information together with geologic models can give new exploration leads.
Acknowledgements
The authors are grateful to Director (Exploration) Shri
T.K.N. Gopalaswami, for according permission to pub-
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