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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 field of Krishna-Godavari Basin, India, provided impetus for intensified exploration in Mandapeta and the adjoining Kommugudem, Draksharama and Endamuru fields. 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 Manda- peta 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 Kommu- gudem 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 configurations. The present day maturity level of the Permian sediments in the Mandapeta field is 1.2% Ro or greater, capable of gen- erating gas dominantly. The Raghavapuram shale in the Mandapeta area is adequately mature and has good hydro- carbon potential for oil generation. The probability of finding 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 field; Petroleum generation; Maturation and isomerisation 1. Introduction The discovery of gaseous hydrocarbons in commercial quantities from Gondwanan sediments (Golapalli For- mation of Early Cretaceous and Mandapeta Formation of Triassic age) in the Mandapeta area of Krishna-God- avari onland basin provided impetus for intensified 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). 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- fied 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 sub- sequent 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 Organic Geochemistry 31 (2000) 1495–1507 www.elsevier.nl/locate/orggeochem * Corresponding author. Tel.: +91-3772-23560.
Transcript
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Generation and hydrocarbon entrapment withinGondwanan 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 adjoiningKommugudem, 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 Manda-

peta formation have sourced the oils from the Mandapeta Sandstone reservoir (Triassic). The oils being produced fromGolapalli Sandstone reservoir (Early Cretaceous) are relatively less mature and have been sourced by the underlyingshales in the Mandapeta Formation at a maturity level of 0.80±0.85% Ro. The source and maturity data precludeliquid hydrocarbon sourcing from the Kommugudem (Permian) sequence. Permian coals and shales of the Kommu-

gudem Formation are the major source rocks for gaseous hydrocarbons in this area. The hydrocarbon generationstarted in Early Cretaceous in the Kommugudem Formation, but the intermittent tectonic activity (with associatedstructural 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 gen-erating gas dominantly. The Raghavapuram shale in the Mandapeta area is adequately mature and has good hydro-carbon 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 newexploration 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 commercialquantities from Gondwanan sediments (Golapalli For-

mation of Early Cretaceous and Mandapeta Formationof Triassic age) in the Mandapeta area of Krishna-God-avari onland basin provided impetus for intensi®ed

exploration in Mandapeta (MDP) and adjoining areas.The discovery well Mandapeta-1 (MDP-A, Fig. 1) wasdrilled to a depth of 4302 m and encountered the top of

gneissic basement at a depth of 4263 m. It penetratedPermian to Recent sediments and produced gas fromGondwanan sediments in the interval 2804±2740 m(Mandapeta Sandstone).

To date, more than 16 wells have been drilled on the

Mandapeta structure (Fig. 1). Out of these about sevenare hydrocarbon bearing in Mandapeta Sandstonereservoir and three are hydrocarbon bearing in Golapalli

Sandstone reservoir (Fig. 2). As Kommugudem (KMG),Draksharama (DRK) and Endamuru (END) structureslike Mandapeta (MDP) structure have also been identi-

®ed to possess Gondwanan sediments, they are consideredtogether for this study.Seven wells KMG-A, MDP-A, -D, -L, -O, DRK-A

and END-A have been studied along with oil samplesfrom MDP-A, -C, -I, -L and-O wells for molecular levelcharacterisation (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 sub-sequent migration and accumulation of hydrocarbons inrelation to the geological framework of the area.

0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PI I : S0146-6380(00 )00132-7

Organic Geochemistry 31 (2000) 1495±1507

www.elsevier.nl/locate/orggeochem

* Corresponding author. Tel.: +91-3772-23560.

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2. Tectonics and stratigraphy

Mandapeta graben, lying in the East Godavari sub-basin, was formed during Early Palaeozoic rift phase.This extensive track of Lower Gondwanan deposition is

collinear with Pranhita Godavari Graben, which hoststhe complete sequence of Gondwanan Super Groupranging in age from Permian to Lower Cretaceous. The

generalised stratigraphic succession of the Mandapetaarea based on interpreted lithology and electrolog cor-relation is given in Fig. 2.In the Mandapeta area Kommugudem formations of

Lower Gondwanan comprising coal/shale/sandstone weredeposited in the lower deltaic to lacustrine environmentover an Archean basement. A major erosional unconfor-

mity has been found at the top of the KommugudemFormation over which Mandapeta Fluvial Sandstone ofTriassic age were deposited. The top of this sandstone

represents a hiatus overlain by Red Bed interval ofTriassic age comprising reddish to dark claystone.The Mandapeta Sandstone together with the Red

Beds belongs to the Lower Gondwanan. At this junc-ture, the NW dip of the basin was reversed to SEbecause of thermal doming associated with rifting. Thisled to the erosion of elevated areas and deposition of

eroded material mainly as alluvial fans in adjacent lows.Subsequent cooling and simultaneous crustal subsidencepart heralded the ®rst transgression where paralic to

marine sediments of Cretaceous age (Golapalli Sand-stone, Raghavapuram Shale and Tirupati Sandstone

formations) were deposited. The end of Cretaceoussedimentation is marked by marine regression. This wasfollowed by widespread volcanic activity where basaltic

¯ows with interbedded sediments like limestone, sand-stone 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 RajahmundrySandstone of Mio-Pliocene age followed by Pliestoceneto Recent sediments.

3. Experimental

Rock samples were crushed in a shatterbox at roomtemperature. The crushed samples were soxhlet extracted

for 48 h with chloroform. The solvent was removed in arotary evaporator and the extract separated by columnchromatography using alumina and silica gel into three

fractions: alkane hydrocarbons, aromatic hydrocarbonsand polar compounds. The oils were separated into theirrespective fractions (alkanes, aromatics and polar com-pounds) by column chromatography following the same

procedure. Fractionation of aromatic hydrocarbons intomono-, di- and triaromatic was carried out on Waters840 HPLC system in a normal phase isocratic mode

Fig. 1. Location map of wells considered in present study.

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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, 0.25mm 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 temperature280�C.

The triaromatic hydrocarbon fractions were analysed

on a Shimadzu GC 9A system using a 60 m long, 0.25mm i.d. SE-54, fused silica column with helium as thecarrier 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).

Fig. 2. The generalised lithostratigraphy. The gas/liquid hydrocarbon occurrences have been highlighted by asterisks.

M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507 1497

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4. Results and discussion

4.1. Occurrence of hydrocarbons

Hydrocarbon accumulations have been found inMandapeta Formation (Triassic) and Golapalli Forma-tion (Early Cretaceous) in Mandapeta area. Regional

cap rock for Golapalli Sandstone reservoir is providedby Raghavapuram shale and the regional cap rock forMandapeta 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. Thepresence of oil has been indicated in MDP-A, -C (Man-dapeta Sandstone) and MDP-I, -L and -O (Golapalli

Sandstone). There is no prominent hydrocarbon ®nd inDraksharama or Kommugudem area.

4.2. Hydrocarbon type and characteristics

The oils from MDP-A and MDP-C (produced from

Mandapeta Sandstone) are light coloured with mediumAPI 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, -Lare dark coloured while the oil from MDP-O is lightcoloured. All liquid hydrocarbons are para�nic in nat-ure. The oils found in MDP-A, -C and -O, though light

in colour, have high yields of 300�C+ residue (around40%) and substantial amounts of wax (more than 8%).These types of oils may result from deasphalting.

The oils from Mandapeta Sandstone have values ofpristane/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

these oils from terrestrial organic matter deposited inoxic environment.Two aromatic biomarker ratios (the ratio of the con-

centration of 1-methyl phenanthrene to that of 9-methyl

phenanthrene and the ratio of the concentration of 1,7-dimethylphenanthrene to that of a peak labelled x, whichis due to an unresolved mixture of 1,3-dimethylphenan-

threne, 3,9-DMP, 2,10-DMP and 3,10-DMP have beencalculated which provide information about the biologicalorigins (Alexander et al., 1992). Alexander et al. (1992),

have termed them as age speci®c biomarkers and havereported that these ratios are helpful for correlation stud-ies at moderate maturities (the signi®cance of the values of

these ratios has been discussed in the following text undersub-title `source rocks'). The values of these ratios for oilsamples are shown in Fig. 3. Both the ratios are very muchsimilar for oils from the Mandapeta reservoir as well as

the Golapalli reservoir, indicating that the source sequen-ces which have given rise to these ¯uids received the same/similar type of organic matter (Fig. 3).

4.3. Maturity

The maturity of the oils has been assessed using aro-matic compounds. The aromatic based maturity para-meters are considered quite reliable as these compounds

are present in substantial concentration. The assessmentis based on the fact that during geological times thethermodynamic less stable methyl phenanthrene isomersare converted into more stable isomers. The Rc was

calibrated with Ro by plotting the MPI of rock extractsagainst the observed Ro. It was found that the equationof Radke and Welte (1983) is valid in this area. The

maturity derived from calibrated Rc also matches wellwith other maturity parameters.The oils found in the Mandapeta reservoir are more

mature than the oils found in the Golapalli Reservoir. Thematurity of Mandapeta and Golapalli reservoired oils is0.95±1.0% Rc and 0.80±0.85% Rc, respectively (Fig. 3).

Table 1

Characteristics of oils of the Mandapeta area

Well no.

MDP-A MDP-C MDP-I MDP-L MDP-O

Depth (m) 2804±2795 2835±2831 2271±2275 2249±2246 2373±2359

Formation Mandapeta Mandapeta Golapalli Golapalli Golapalli

Reservoir age Triassic Triassic Cretaceous Cretaceous Cretaceous

Density (15�C) 0.7814 0.8068 0.8019 0.8204 0.7742

API gravity 49.5 43.8 44.9 41.0 51.2

Gross comp.

Saturates% 83.54 81.24 75.25 74.34 76.62

Aromatics% 12.61 13.61 15.92 18.41 16.91

NSO% 3.85 5.15 8.83 7.25 6.47

Sat/arom 6.62 5.97 4.73 4.04 4.53

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4.4. Source rocks

Fair to good, locally rich source rocks occurthroughout the Permian, Triassic and Cretaceous units.

Source rock pyrolysis logs, maturation, extract data andmolecular level parameters have been compiled frommany wells throughout the sub-basin. Fig. 4 shows the

correlation of di�erent stratigraphic units of the KMG-MDP-DRK-END belt.

4.5. Permian source sequence (KommugudemFormation)

The Permian Gondwanan coal/coaly shale sediments

were deposited in ¯uvial-lower deltaic- lacustrine envir-onment. The total organic carbon of these sediments ishigh 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.

M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507 1499

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of Permian section (Kommugudem Formation) areshown in Figs. 5 and 6, respectively.

In the Permian Gondwanan sediments of this basin, thevalues of the ratios of 1 MP/9MP and 1,7 DMP/x are lessthan 0.65 and 0.35 respectively, which show the major

contribution of Glossopteris Pteridosperms. This is alsosupported by palynological information (Prasad et al.,1995).

Alexander et al. (1992) have shown that in Cooper/Eromanga Basin, low values of 1MP/9MP and 1,7 DMP/x ratios (less than 0.65 and 0.35, respectively) in oils androck extracts are indicator of Permian ¯ora (mainly Pter-

idosperms). The values of these ratios increase remarkablyas the ¯ora changes.The Permian Gondwanan coals/coaly shales formed

from relatively simple Glossopteris pteridosperms ¯orain a cold to cool temperate climate. The Glossopteris arepteridosperms gymnosperms (Gould and Shibaoka,

1980; Prasad et al., 1995), the preserved components ofwhich are rich in cellulose and lignin, which form thehydrogen-poor, woody parts of trees (Cooper and

Murchison, 1969). Permian ¯ora, itself poor in exinite,was also highly susceptible to dessication and oxidationduring the peat forming process. Hence the organicmatter, which was mainly hydrogen de®cient initially,

was subjected to further degradation in unfavourablesites of deposition (Thomas, 1982). Since the Glassop-teris ¯ora does not seem to have been rich in cuticle or

resin, major oil accumulations of land plant origin are notconsidered likely. The organic character of the Permian is,

therefore, mainly gas prone, possibly with low yields inareas where the primary inertinite content is high.

4.6. Triassic sediments (Mandapeta Formation)

This sequence is sand dominated with little sig-

ni®cance as a source rock. These sediments however,contain many thin, non-coal, locally rich source rockintervals. This sequence was deposited in a ¯uvial envi-ronment. The intraformational shales in the Kommugu-

dem area are thick and of good quality as compared to theMandapeta area, there being a great variation in sourcerock 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. Thesource parameters (1 MP/9 MP and I,7-DMP/x) based

on aromatic biomarkers are di�erent from those of thePermian. The explanation is that the ¯ora had changed(Figs. 6 and 7). The palynological data indicates that theDicroidium Pteridosperms (Pteridospermous gymno-

sperms) had become dominant (along with some earlyconifers) over Glossopteris ¯ora (Prasad et al., 1995).The Triassic sequence contains some exinite fraction

Fig. 4. Source rock potential of the Kommugudem, Mandapeta, Draksharama and Endamuru belt (given values are ranges over

depths).

1500 M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507

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which might have been derived from thick cutinite of

Dicroidium (Cook and Taylor, 1963; Cook, 1975) andsome 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 Enda-muru (Fig. 4). The quality of organic matter deterioratesfrom Kommugudem to Mandapeta area, while the

organic matter in Mandapeta and Draksharama areas isalmost similar in quality. Angiosperms appeared in theCretaceous. 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 organicmatter which has a high potential for liquid yield. Thissequence was deposited in a shallow marine environ-

ment. The aromatic biomarkers are di�erent (1MP/

9MP>1.2 and 1,7-DMP/x>0.65) from the Triassic andPermian sequences. (Figs. 6 and 7). During this periodthe ¯ora had the dominance of conifers (Prasad et al.,

1995).

4.8. Organic maturity

The degree of maturity of source rocks has beendetermined from Rc, spore discoloration, Rock Eval

Tmax. Solvent extracts of source rocks provide additionaldata on chemical maturity as well as providing a means ofoil-source correlation. All these methods have practicalde®ciencies and integrated approach to maturity estima-

tion has been adopted (Fig. 8). As discussed above MPIof the rock extracts has been calibrated with observedRo to check validity of these parameters in this area.

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).

M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507 1501

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According to Tissot and Welte (1984), the onset of oilgeneration occurs between 0.5 and 0.7% Ro dependingon the organic type. The transformation ratio derivedfrom hydrous pyrolysis of immature rock sample of this

area indicates that the onset of oil generation hasoccurred at around 0.65% Ro.The Permian sequence is post oil mature in the

Kommugudem area. The maturity increases towards

Fig. 7. GC traces of aromatic fractions of typical oil and typical Cretaceous, Triassic, and Permian rock samples.

Fig. 6. Variation of source speci®c parameters in the study area.

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Fig. 8. Maturity levels at di�erent 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.

M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507 1503

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Mandapeta. The maturity decreases towards Drakshar-ama and Endamuru. In the Triassic sequence, maturityincreases from Kommugudem to Mandapeta. Around

Kommugudem, the sequence is moderately mature tomature and around Mandapeta the sequence is mature.Signi®cant maturation commenced around 2000 m inCretaceous sediments in Kommugudem and Manda-

peta. The Cretaceous sections are marginal to moderatelymature in the studied area. The maturity increases fromKommugudem to Mandapeta and decreases towards

Draksharama and Endamuru.

4.9. Oil±source rock correlation

Fig. 3 shows values of the aromatic hydrocarbonbased maturity indicators on oils recovered from the

Mandapeta ®eld. The striking feature of this data is theprogressive increase in maturity of oils with increasingage (depth) of the reservoir formation based on Rc frommethyl phenanthrenes. The maturity indicators show

that the oils in the reservoir have similar maturities tothose of indigenous hydrocarbons contained in shales insimilar stratigraphic locations (Figs. 3 and 8).

The oils found in the Mandapeta and Golapallireservoirs have been derived from a similar type ofsource organics based on aromatic biomarker ratios (1

MP/9 MP and 1,7-DMP/x). The oils from the Golapallireservoir are slightly less mature than the oils from theMandapeta 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 moderatematurity, have sourced the oils reservoired in the Gola-

palli sandstone based on source and maturity parametersshown in Fig. 9.Mandapeta sandstone oils (from wells A and C) have

been generated by underlying and comparatively moremature source rocks at the depth interval 2850±3150 m inthe Mandapeta Formation as evident from the close

values of pristane/phytane ratio and aromatic parameters(1MP/9MP and 1,7-DMP/x) ofMandapeta reservoir oilsand probable rock extracts shown in Figs. 10 and 11. Thematurity of ¯uids is around 0.95±1.0% Rc which matches

well with the maturity of basal shales in Mandapeta For-mation (Figs. 10 and 11). The origin of these oils fromunderlying Permian sequence is ruled out on the basis of

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.

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maturity as well as aromatic biomarker di�erences (Figs. 6and 7). Moreover, Permian sequence is post oil matureand hydrogen-de®cient which can generate only gaseous

hydrocarbons. The ¯ora available during Permian timewas of poor quality (as discussed earlier) and must havegenerated only gaseous hydrocarbons during peak oil

generation as evident from a number of gas occurrencesin the sands of Kommugudem (Permian) Formation.

4.10. Gas composition and source

The composition of gases recovered from the Gola-

palli and Mandapeta sandstone reservoirs is given inTable 2 and Fig. 3. The simple composition of gases andfactors such as multiple source, maturity and fractiona-

tion during migration make it di�cult to correlate themwith their source rocks. The gas generating potential of

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.

Table 2

Characteristics of gases of the Mandapeta area

Well no.

MDP-A MDP-C MDP-G MDP-L

Depth (m) 2804±2795 2835±2777 2925±2895 2249±2246

Formation Mandapeta Mandapeta Mandapeta Golapalli

Age Triassic Triassic Triassic Cretaceous

d 13C1 ÿ32.6 ÿ32.6 ÿ32.5 ÿ38.9d 13C2 ÿ25.6 ÿ24.1 ÿ25.0 ÿ29.0C1 (vol. %) 89.13 85.18 85.2 53.13

C2+ (vol. %) 10.3 9.32 11.1 41.15

C1/C2+C3 9.66 9.68 8.67 1.77

C2/C3+ 2.42 3.63 2.17 0.39

iC4/nC4 0.80 0.81 1.12 0.49

C1/� Cn 0.90 0.90 0.88 0.56

M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507 1505

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Permian sediments in the Mandapeta sub-basin has longbeen 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 amixed maturity source (catagenetic and metagenetic) witha methane stable carbon isotope ratio of ÿ32.5 to

ÿ32.6%. The bulk of the gas is from the metageneticstage and appears to be sourced by KommugudemFormation. The maturity of these gases is equivalent to

1.2% Ro.Gas reservoired in the overlying Golapalli Formation

is di�erent from the Mandapeta Sandstone gas, in that

it is wetter and isotopically lighter, the gas compositionsuggests the gas to be a typical oil-associated gas.

4.11. Origin, migration and entrapment of hydrocarbons

The Triassic reservoir (Mandapeta sandstone) is over-lying the Permian coal-shale sequence (Kommugudem

Formation), resulting in extremely favourable entrap-ment conditions with the Red Bed being the regionalcap rock. Vertical and lateral migration is facilitated by

communication of the predominantly channel point barsandstone and local syndepositional faulting of theKommugudem Formation.

The Lower Cretaceous (Golapalli) reservoir is overlyingthe Red Bed. The observed migration of hydrocarbonsfrom underlying source rocks will have occurred due toerosion or non-deposition of the regional seal over these

trends, together with faulting.

The basin has experienced (Fig. 2) continuous sub-sidence interrupted by episodes of non-deposition(example of heating with a constant rise in temperaturepunctuated by isothermal heating). The two commonly

used methods of maturity modeling, namely TTImethod and Easy Ro method, have been applied in thisbasin. It has been found that Waples TTI method

(Waples, 1980) overestimates maturation while the Rovalues calculated from Easy Ro of Sweeney and Burn-ham (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 andmaturity level of di�erent layers in the KMG-MDP-

DRK-END belt. It has been argued that the Permiancoals and shales of the Kommugudem Formation arethe major source rocks for gas in this area. The hydro-carbon generation started in Early Cretaceous in the

Kommugudem Formation as estimated by Easy Ro ofSweeney and Burnham (1990). The traps were availableduring this time, but the intermittent tectonic activity

has resulted into reorientation and redistribution of theoriginal trap geometries. The present day maturity levelof the Permian in the Mandapeta area is Ro 1.2% or

greater, which is consistent with the maturity of gasesencountered in this area. They are thermogenic (mixedi.e. derived at late catagenetic as well as early metagen-

etic stages) in origin.Localised shales in the Mandapeta Formation have

sourced the Mandapeta and the Golapalli oils. The oilsbeing 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.

1506 M.S. Raza Khan et al. / Organic Geochemistry 31 (2000) 1495±1507

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4.12. Hydrocarbon occurrence and fault blocks

The fault blocks in the Mandapeta area are the resultof tectonic activity, which was probably initiated prior

to the Permian and has continued intermittently. Thishas in¯uenced the generation and entrapment of hydro-carbons by:

1. Providing conditions conducive to the depositionof 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 upfaults to reservoirs.

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 marineenvironment has substantial contribution of marine

organic matter. The Permian section is post mature for oilgeneration in the Kommugudem and Mandapeta areaof the belt. The Triassic section is oil mature and theCretaceous section is early to moderately mature.

Hydrocarbon accumulations aremainly gas and gas/oil.Gas in the Golapalli and Mandapeta reservoirs has beengenerated from both the Triassic shales (Mandapeta For-

mation) and Permian coals/coaly shales (KommugudemFormation), although there is evidence that the Permianis the principal source.

The associated small accumulations of oils encounteredin parts of this belt are attributed to the oil-prone shales inthe Mandapeta Formation. Most of the oils discoveredare para�nic in nature and have mature character. The oil

to source correlation and the basin con®guration suggestvertical and short distance migration.The study indicates that Raghavapuram shale in the

Mandapeta area has adequate maturity and hydro-carbon potential for oil generation. Oil has been dis-covered in the interbedded sands of one well recently.

The probability of ®nding hydrocarbon reserves insands of Raghavapuram shale and other suitable trapsis high. Modern seismic information together with geo-

logic models can give new exploration leads.

Acknowledgements

The authors are grateful to Director (Exploration) ShriT.K.N. Gopalaswami, for according permission to pub-

lish this work. Profound thanks are due to Shri KuldeepChandra, Executive Director and Head, KDMIPE, forhis valuable guidance and suggestions during the pre-paration of this manuscript. Thanks are due to Shri

K.N. Misra, G.M. (GRG), KDMIPE, Dehradun, forhis valuable guidance and encouragement. The authorsare also grateful to Shri Lehamber Singh, G.M. (Exp),

ERBC, and Dr. B.K. Sharma, G.M. (Chem), ERBC, forproviding valuable guidance and a wonderful environ-ment to complete this work. The authors are also grateful

to Dr. Rajiv Sharma, Sr. Chemist, for his contributionin the preparation of this report. We thank the review-ers, Drs. R.G. Schaefer and C. Clayton, for their valu-

able criticism and suggestions.

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