Sedimentation, Stratigraphy And Petroleum Potential of Krishna-Godavari Basin,East Coast of India

AAPG Bulletin, v. 85, no. 9 (September 2001), pp. 1623–1643 1623

Sedimentation, stratigraphy,and petroleum potential ofKrishna-Godavari basin,East Coast of IndiaG. N. Rao

ABSTRACT

The Krishna-Godavari basin is located in the central part of theeastern passive continental margin of India. The structural grain ofthe basin is northeast-southwest. Exposures of Upper Cretaceoussedimentary rocks demarcate the basin margin toward the north-west, where the northwest-southeast–trending Pranahita-Godavarigraben abuts the basin. The basin contains thick sequences of sed-iments with several cycles of deposition ranging in age from LateCarboniferous to Holocene. A major delta with a thick, argillaceousfacies that has prograded seaward since the Late Cretaceous is ahydrocarbon exploration target.

Magnetic and gravity data predicted the basin architecture,which was subsequently confirmed by a multichannel seismic sur-vey. The basin is divided into subbasins by fault-controlled ridges.Sediments accumulated in subbasins more than 5 km thick. Abovethe basement ridges, thin sediments are found. Until the Jurassicperiod, sediments were deposited in the rift valley and in topo-graphic lows. This sequence is completely overlain by a Lower Cre-taceous, transgressive sedimentary wedge. Later, continued deltaprogradation characterized basin sedimentation.

With an areal extent of approximately 45,000 km2, this provenpetroliferous basin has potential reservoirs ranging in age from thePermian to the Pliocene. Exploratory drilling of more than 350wells in more than 160 structures has resulted in the discovery of42 oil and gas bearing structures. Good source rocks are knownfrom sequences ranging in age from Permian–Carboniferous to earlyMiocene. Because the reservoir sand bodies have limited lateralvariation, understanding the stratigraphy and depositional sub-environments in different sequences is essential to decipher the fa-vorable locales for reservoir sands. Tilted fault blocks, growth faults,and related rollover anticlines provide the structural traps.

Copyright �2001. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received June 3, 1997; revised manuscript received May 19, 1998; final acceptance November9, 2000.

AUTHOR

G. N. Rao � Oil and Natural GasCorporation, India, EXCOM Section SRBC,X(W) CMDA Towers, 8 Gandhi-Irwin Road,Egmore, Chennai-600 008, India;[email protected]

G. N. Rao works as deputy general manager(geology) at Oil and Natural Gas Corporation(ONGC), India. He received his M.Sc. (tech.)degree in applied geology in 1975 and Ph.D.in 1994 from Andhra University. He studied atthe Indian School of Mines for an M.Tech.degree in petroleum exploration during 1984.He has experience in analyzing hydrocarbonprospectivity in all the eastern divergentmargin basins of India. His interests includeglobal tectonics in relation to basin evolutionfor petroleum exploration and genesis ofabnormal formation pressures. Rao hasassociated with Soviet specialists in assessingthe hydrocarbon resources of India and withthe Institute Francais du Petrole Paris team forthrust-pach modeling for fold belts of thenortheastern convergent margin of India.

ACKNOWLEDGEMENTS

I thank ONGC Ltd. for providing the data andpermitting to publish the same. I also thankS. N. Talukdar, former member (exploration),and P. K. Chandra, former vice-chairman ofONGC, for critically reviewing the manuscript.The guidance provided by K. Satyanarayanaof ONGC and C. Kasipathi of Andhra Univer-sity and help from James Peters of ONGC inshaping the work is gratefully acknowledged. Ithank the AAPG reviewers Bob Reynolds, BenLaw, and Mauren Wan for their constructivecriticism of the manuscript. Thanks to FirozDhotiwala of Kesava Deva Malvya Institute ofPetroleum Exploration for providing experttechnical help in presenting the Landsat im-ages. Finally I wish to express my deep senseof gratitude to A. W. Balley of Rice University,Houston, Texas, for his encouragement andvaluable guidance in reshaping the text.

1624 Petroleum Potential of Krishna-Godavari Basin

Figure 1. Location map of thestudy area, Krishna-Godavaribasin. To the northwest thesedimentary exposures include(1) Lower Triassic–Upper Per-mian Chintalapudi Sandstone;(2) Lower Cretaceous–UpperJurassic Gollapalli and Buda-vada sandstones; (3) LowerCretaceous Raghavapuram andVemavaram shales; (4) UpperCretaceous Tirupati Sandstone;(5) Deccan traps with UpperCretaceous–lower Paleocene in-tertrappean beds; (6) Miocene–Pliocene RajahmundrySandstone.

INTRODUCTION

The Krishna-Godavari basin, a pericratonic basin, is lo-cated along the East Coast of the Indian peninsula. Itincludes the deltaic plains of the Krishna and Godavaririvers and the interdeltaic regions. Geographically, thebasin lies between Kakinada in the northeast and On-gole in the southwest. Archean crystalline basementand Upper Cretaceous sedimentary outcrops demar-cate the northwest basin margin. The basin extendssoutheast into the deep water of the Bay of Bengal. Asignificant part of the onshore basinal area is coveredby Quarternary alluvium (Figure 1).

Geologists in the Geological Survey of India suchas Blandford et al. (1856) and King (1881) were thefirst to study outcrops in the Krishna-Godavari basin-margin area. Later, Vasudeva Rao and Krishna Rao(1977) described these outcrops in detail and inter-preted tectonic settings and depositional environmentsfor the basin. Murty and Ramakrishna (1980) usedgeophysics to describe the subsurface geology. Ven-katarangan and Ray (1993) recognized exploration tar-gets and the petroleum system in the basin. Scientistsfrom the National Institute of Oceanography (Murtyet al., 1995) studied the geodynamic aspects of the off-shore Krishna-Godavari basin and identified the off-shore extension of the northwest-southeast–trendingpre-Cretaceous rift graben along the cross-trends. I also

contributed to previous hydrocarbon exploration ef-forts, summarizing the geological evolution of the ba-sin, proposing depositional models for hydrocarbonreservoirs (Rao, 1991), and formulating a new litho-stratigraphic nomenclature for the area (Rao, 1993a,b, c).

In this article I interpret the depositional system ofthe Krishna-Godavari basin within a sequence strati-graphic framework. The basic data for this study in-clude lithologic and electric logs of deep wells, seismicsections, and analyses of sedimentologic features inconventional cores and in basin-margin outcrops.

METHODS OF STUDY

Outcrops were examined for detailed lithofacies vari-ations, deposition cycles, and sedimentary structures.Conventional cores were examined for sedimentarystructures to identify depositional processes. Cross-plots of core data (Visher, 1969) were used to differ-entiate depositional environments. Depositional pat-terns were presented in the form of isopach maps.Where conventional cores were absent, interpretationswere made on the basis of logs obtained from 26 wells,which were designated names from the English alpha-bet (A to Z). As a whole, the wells cover the entirebasin, and a few penetrate down into the Archean

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basement. To understand the depositional environ-ment, good quality seismic sections were tied to thewell data. Age markers identified in samples of thedeep wells were calibrated using lithomarkers identi-fied in the time sections. This method allowed for theaccurate identification of depositional patterns withinthe marked sequences.

REGIONAL GEOLOGICAL SETT ING

Plate Tectonic Model

The basin was a major intracratonic rift within Gond-wanaland until the Early Jurassic. When Gondwana-land rifted apart, the eastern margin of the Indian pen-insula was positioned at latitude 50� S andwas orientedin an east-west direction (Chatterjee and Hotton,

1986). Since the Cretaceous, the Indian plate hasmoved northward, and the eastern continental passivemargin rotated 20� in a counterclockwise direction(Gordon et al., 1990) until it collided with Eurasia inthe late Eocene (Srivastava and Chowhan, 1987). Thetriple junction between the Indian peninsula, Austra-lia, and Antarctica is located at MasulipatnamBay (Fig-ure 1) (Thompson, 1976). The northwest-southeast–trending Pranahita-Godavari graben (Figure 2) formeda failed arm of the triple junction (Burke and Dewey,1973).

Since the Cretaceous, the basin has become a per-icratonic basin. Its thick, fluvial sediment load was as-sociated with the faulting of basement blocks due tothe reactivation of northeast-southwest–trending Pre-cambrian faults (Biswas, 1992). These differential ver-tical block movements allowed magma to rise throughand facilitated emplacement of the Deccan traps.

Figure 2. Bouguer gravity map of the Krishna-Godavari basin, showing tectonic elements (Shenoi and Rao, 1982). KB � Krishnasubbasin, BH � Bapatla horst, WG � West Godavari subbasin, EG � East Godavari subbasin, PG � Pranahita-Godavari graben,CCT � Chintalapudi cross-trend, PCT � Pithapuram cross-trend, KT � Kommugudem trough, KKT � Kakinada terrace, DH �Draksharama high, EH � Endamuru high, TH � Tanuku high, MT � Mandapeta trough, KAHT� Krishna Amalapuram high trend,MPFZ � Matsyapuri-Palakollu fault zone.


During the Tertiary, the deltaic system generally pro-graded to the southeast, although some deltaic lobeshave shifted in direction in response to changing ratesof sediment influx and growth faulting (Rangaraju,1987).

Basin Architecture

Based on Bouguer gravity data, Murty and Ramak-rishna (1980) have identified three subbasins separatedby two basement horsts. From the southwest, these arethe Krishna, West Godavari, and East Godavari sub-basins separated by the Bapatla and Tanuku horsts, re-spectively (Figure 2). The West Godavari subbasin isfurther subdivided into the Gudivada and Bantumilligrabens, which are separated by the Kaza-Kaikaluruhorst (Kumar, 1983) (Figure 3). The Kommugudemand Mandapeta troughs are situated on either side ofthe Tanuku horst (Figure 2).

The Krishna subbasin contains 1560 m of Creta-ceous and older sediments above the Archean base-ment. Bapatla horst lies between the Krishna and theWest Godavari subbasins. Many lower Mesozoic se-quences are thin over the Bapatla horst and lie discon-formably with erosional contact, which suggests thatthe Bapatla horst was uplifted during the Cretaceous.The Bapatla horst is discontinuous in the northeasternpart of the basin.

In the West Godavari subbasin, Cretaceous sedi-ments are thin over the Kaza-Kaikaluru horst, com-pared to the thick section on either side of the horst inthe Bantumilli and Gudivada grabens (Figure 4a, c).The data suggest that the Kaza-Kaikaluru horst re-mained uplifted during the Early Cretaceous. Tanukuhorst lies between the West Godavari and East Go-davari subbasins. The sedimentary cover over the Ta-nuku horst is about 2500 m in the southeastern flankof the horst and increases to 3500 m in the southwest.

Figure 3. Tectonic map of the Krishna-Godavari basin with deep wells considered for this study using seismic sections, lithologcorrelations, and electrolog profiles. Subsurface tectonic elements: BNTG � Bantumilli graben, GDVG � Gudivada graben, MPFZ�Matsyapuri-Palakollu fault zone, KAHT � Krishna Amalapuram high trend, BTH � Bantumilli high, KKH � Kaza-Kaikaluru high.Single letters are well names.

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This trend may indicate that the horst plunges south-westerly into Masulipatnam Bay. In contrast, LowerCretaceous strata dip monoclinally toward the south-east over the Tanuku horst, suggesting the absence ofany tectonic uplift of the horst since the Cretaceous.Deep drilling to the southeast of the Tanuku horst hasdocumented the presence of a thick sand-shale se-quence with coal beds, characteristic of the BarakarFormation of the Carboniferous to Permian.

In the East Godavari subbasin, the sedimentary fillranges from 2900 m over the preexisting basementhorsts, to more than 5000 m in the deep basin areain the southeast (Prabhakar and Zutshi, 1993). The2000 m of argillaceous sediments of the Cretaceousin the southeast might have been excessive enough toenhance tectonic subsidence and down-faulting of se-quences and form a series of en echelon faults in theEast Godavari subbasin during the Late Cretaceous–early Paleocene (Figure 4b). Subsidence in the south-eastern parts of the East Godavari subbasin may havealso contributed to the formation of a steep step-faultzone in early Paleocene basalts. This fault zone isknown as the Matsyapuri-Palakollu fault zone(MPFZ). Tectonic readjustment has resulted in ter-races and tilted fault blocks in the northeastern areaof the basin.

The discontinuity of the Bapatla horst in thenortheastern part of the basin is associated with thepresence of a northwesterly orientation of gravitycontours and strongly negative gravity anomalies.This suggests the possible continuation of the north-west-southeast–trending Pranahita-Godavari grabenbeneath the northeast-southwest–trending Krishna-Godavari basin. The bounding faults of the Prana-hita-Godavari Gondwana graben are known by theextents of the older sedimentary outcrops in the ba-sin margin area. These fault trends can be traced upto the Tanuku horst, based on available seismic data.These northwest-southeast–trending bounding faultsof the earlier rift valley are termed the Chintalapudiand Pithapuram cross-trends in the northeast-south-west–trending Krishna-Godavari basin (Figure 2).Based on offshore magnetic data, these cross-trendscan be identified up to the deep-water area of theKrishna-Godavari basin, and the ocean-continentboundary (OCB) was marked accordingly (Murty etal., 1995). The OCB defines the southeastern ter-mination of the northwest-southeast–trending Prana-hita-Godavari graben (Figure 3).

Both the Kommugudem and Mandapeta troughsare deep pre-Callovian downwarps (Figure 2). In

these troughs, Upper Carboniferous–Lower Triassicsediments were deposited in varied depositional en-vironments ranging from glacial to marginal marineconditions. The two troughs can be differentiated inthat the Mandapeta trough experienced Late Jurassic–Cretaceous downwarping, but the Kommugudemtrough did not.

SEDIMENTATION

Outcrop Stratigraphy

Lower Permian–Upper Carboniferous outcrops existfarther to the northwest, within the Pranahita-Godavari graben. They include the Talchir beds com-prising greenish sand-shale alternations, which areoverlain by the Barakar Formation with its character-istic coal-sand-shale sequences (Raiverman et al.,1986). Outcrops in the basin margin area of theKrishna-Godavari basin include the Permian Chinta-lapudi Sandstone, which consists of cross-bedded,loosely cemented, variegated shales (Figure 5). TheChintalapudi Sandstone is overlain by the Maleris For-mation, a red arenaceous facies (Lakshminarayana andMurty, 1990).

Cretaceous outcrops in the northeastern marginof the basin include the Gollapalli Sandstone, Rag-havapuram Shale, and Tirupati Sandstone. Equiva-lent outcrops in the southwestern margin are theBudavada Sandstone, the Vemavaram Shale, and thePavaluru Sandstone, respectively (Figure 1). TheGollapalli sandstones are ferruginous and micaceous,with a paleocurrent direction of 10–20� to thenorthwest (Vasudeva Rao and Krishna Rao, 1977).The Raghavapuram shales are mainly white, buff,and lilac clays, underlain by fine-grained sandstones.Abundant Lower Cretaceous plant fossils are foundin the Raghavapuram Shale (Bhalla, 1967; Radhak-rishna, 1977). The Tirupati sandstones are feld-spathic toward the top and ferruginous toward thebottom. Lenticular clay beds, petrified wood, andcross-bedding characterize the sandstone, which hasa paleocurrent direction of 5–10� dip toward thesoutheast.

Basalt lava flows with Upper Cretaceous–lowerPaleocene intertrappean beds are overlain by theMiocene–Pliocene Rajahmundry Sandstone. The Ra-jahmundry Sandstone is red, feldspathic, ferruginous,laterized, cross-bedded, and conglomeratic and di-rectly overlies the Deccan basalts (Figure 5).


Figure 4. Litholog correlations of drilled wells in the Krishna-Godavari basin. Single letters are well names. (a) Southwest-northeastprofile in strike direction of the basin showing sediment fill in subbasins and basement horst blocks. (b) Northwest-southeast–trendingdip profile across East Godavari subbasin with thick pre-Cretaceous sediments toward the northwest and a thick Tertiary sectiontoward the southeast of the MPFZ. Continued.

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Figure 4. Continued.(c) Northwest-southeast–trending dip profile across WestGodavari subbasin with thickargillaceous Tertiary sedimentstoward the southeast of theMPFZ.

ROCK STRATIGRAPHIC NOMENCLATURE

Lithomarkers and Age Boundaries

Wherever faunal age details were not available, lithologvariations were considered to differentiate the forma-tion boundaries. Distinctive lithomarkers in the sub-surface of the basin include the Archean basement,pre-Cretaceous red bed, lower Paleocene basalts, andmiddle Eocene limestone. These lithomarkers, as ob-served in well data, were tabulated (Table 1).

The Archean basement consists of mainly gneissesand granites. Locally named the Khondalites, the Ar-chean basement gneiss is composed of quartz, mica,feldspar, and garnet. Locally named the Charnockites,the Archean granite consists of quartz, feldspar, bio-tite, and hypersthene. Within the fluvial pre-Cretaceous section, the presence of thick coal beds,along with sand-shale sequences, characterizes thePermian–Carboniferous strata underlying the homo-geneous Permian–Triassic feldspathic sandstone. Fau-nal data, along with the red bed, delineate marine Cre-taceous sequences from pre-Cretaceous nonmarinesections. Detailed paleontology of cuttings establishedthe age of basalt flows including intertrap sediments as

ranging from 68 to 61 Ma (Raju et al., 1994). Govin-dan (1980) identified the Paleocene–Eocene boundary,and Vijayalakshmi (1988) used faunal studies to de-marcate the base of the Miocene boundary. A promi-nent transgressive clay bed is chosen as a lithostrati-graphic marker to demarcate the top of the Miocene.

Although rock stratigraphic nomenclature is usedin outcrop descriptions as illustrated in Figure 5, thesubsurface data generated from the exploratory drillingneeded codification. Lithofacies variations in wells arereferred by geological age and lithology. The compre-hensive rock stratigraphic nomenclature, as I suggested(G. N. Rao, 1990, unpublished data) for themajor lith-ofacies variations drilled in the subsurface, follows theguidelines provided by the North American Strati-graphic Code (1983). The lithofacies variations of agiven geological time, that is, the proximal arenaceousand basinal argillaceous sequences, were classified sep-arately. The suggested rock stratigraphic nomenclaturewith subsequent minor modifications (R. Venkatar-angan et al., 1993, unpublished data) is shown in Fig-ure 6.

The pre-Cretaceous Chintalapudi Sandstone is arecognizable, distinct unit in outcrop. In the subsur-face, however, identifying the Chintalapudi Sandstone


Figure 5. Lithostratigraphicnomenclature used at thebasin-margin exposures andtectonic phases and sea levelchanges in Krishna-Godavaribasin. See text for descriptionsof seismic sequences.

from seismic data alone is not as easy. Consequently,different rock stratigraphic nomenclature is suggestedfor the Chintalapudi Sandstone, along with the Talchirand Barakar formations: the three lower Gondwanalithofacies. Sediments equivalent to the Talchir For-mation are found in well C and have been named theDraksharama Shale. The shale is greenish black andunderlies a coal-sand-shale section. The maximumthickness of sediments equivalent to the Barakar For-mation was found in well A. This facies is named theKommugudem Formation and consists of thick coal-sand-shale beds. The major arenaceous facies overlying

the Kommugudem Formation are named the Manda-peta Sandstone, and the type section is in well B. Thearenaceous facies, which has a distinct red claystonebed, overlies the Mandapeta Sandstone and is discon-tinuous in the basin. The red claystone bed separatingmarine and nonmarine facies in the basin is named theRed Bed.

In well A, Cretaceous formations are known tocorrelate with outcrops. Hence, separate nomenclatureis not suggested for the three subsurface lithofacies ofthe Cretaceous. The Tirupati Sandstone, however, lat-erally changes into a massive claystone toward the

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Table 1. Lithostratigraphic Correlation of Wells Considered for the Study

Well NameShown in Figures

Name ofthe Structure

Drilled Depth(m)

Well Bottomed in(geological age)

Top of ArcheanBasement (m)

Top of Basalt(m)

Top of Limestone(m)

A Kommugudem 4508 Carboniferous – 90 –B Mandapeta 4302 Archean 4246 738 –C Draksharama 3145 Archean 2892 966 684D GS-17 4020 Late Cretaceous – – –E Bhimanapalli 3007 Early Paleocene – 3000 –F Amalapuram 4003 Late Cretaceous – – 2798G Ravva 3172 Early Paleocene – – –H Tanuku 3145 Archean 3047 745 –I Jonnalanka 3504 Late Cretaceous – 2802 1224J Pasarlapudi 3902 Late Cretaceous – 3575 1555K GS-20 2500 Early Paleocene – – –L G-5 3208 Miocene – – –M Palakollu 4501 Late Cretaceous – 2576 –N Razole 4501 Late Cretaceous – 3363 1683O Chintalapalli 4500 Late Cretaceous – 3568 1667P Mori 3200 Paleocene – – 1518Q GS-19 2442 Miocene – – 1518R G-13 4110 Paleocene – – –S Gajulapadu 3811 Pre-Cretaceous – 725 –T Kaikaluru 1972 Archean 1935 965 –U Bantumilli 3650 Archean 3575 1484 –V GS-11 4625 Late Cretaceous – – –W Nimmakurru 3071 Archean 2980 885 –X Mantripalem 2850 Archean 2720 1001 –Y Bobbarlanka 4258 Archean 4187 – –Z GS-38 3517 Eocene – – –

southeast part of the MPFZ (Figure 4). In well O, thethickest section of the claystone was penetrated and istermed the Chintalapalli Claystone.

Basalt flows with intertrappean beds are named theRazole Formation. The postbasalt section located inthe northwestern section of the MPFZ is mainlyarenaceous and cannot be subdivided into lithofaciesbecause of a lack of recognizable marker beds. ThesePaleogene arenites are termed the NimmakuruSandstone.

Between the MPFZ and the Krishna Amalapuramhigh trend (KAHT) passing through wells F and Z, thePaleocene and Eocene sections have distinct litho-markers (Figures 2, 3). The Paleocene section is mainlyshale and is named the Palakollu Shale. The Eocenesection can be divided into a lower silty facies (Pasar-lapudi Formation), middle limestones (BhimanapalliLimestone), and upper arenites (Matsyapuri Sand-

stone) (Figure 7). Southeast of the KAHT, the Eocenesection is argillaceous and is named the VadaparruShale.

Although the Oligocene sequence is very thin, itis easily identified and can be used to mark the base ofthe Neogene sediments. The Oligocene sequence con-sists mainly of claystone with limestone and sandstonebeds and is termed the Narasapur Claystone.

The Miocene–Pliocene section is known as the Ra-jahmundry Sandstone in outcrops and in the subsur-face in the proximal part of the basin. In the distalsubcrop, it changes into amajor argillaceous facieswithsandstone beds, from which oil and gas are being pro-duced in the Ravva field. Hence, it is termed the RavvaFormation, which is equivalent to the RajahmundrySandstone (Figure 7).

The Quarternary–Holocene alluvial cover in thecontinental part of the basin is called the Andhra


Figure 6. Lithostratigraphic nomenclature of sedimentary se-quences penetrated in the basin. Wherever the continuity of theoutcrop lithologic unit with the subsurface could be established,the same name is retained; otherwise, new rock stratigraphicnomenclature is suggested.

alluvium, and the offshore clay sequence overlying theRavva Formation is known as the Godavari clay.

SEQUENCE STRATIGRAPHY

Using the geological age markers, the sedimentary fillof the basin was divided into five sections: pre-Cretaceous, Cretaceous, Paleocene, Eocene, and Mio-cene and above. To identify depositional styles withineach section, geological age boundaries of lithocolumnswere transferred onto seismic sections (Vail et al.,1977). Sea level fluctuations in the basin, along withsequence boundaries, are shown in Figure 5.

The pre-Cretaceous section is divisible into twounits: a lower sand-shale sequence with thick coal beds(PC-I) and an upper sandy unit (PC-II). The Archeanbasement forms the base of the lower unit whereas thetop of the unit is an unconformity surface (Figure 8a.).

A red claystone separates the fluvial pre-Cretaceous and the marine Cretaceous sections. TheRed Bed is an unconformity and represents thebreakup of Gondwanaland. The strong acoustic im-pedance in the lower sequence generated strong seis-mic reflections in the coal beds. The divergent reflec-tors indicate the subsidence of the basin floor duringdeposition. Because PC-I and PC-II were deposited ina half graben, the divergence of depositional eventssuggests synsedimentary reactivation of a boundingfault in the graben. The hummocky clinoform reflec-tions suggest a fluvial origin for the sequences. Thestrong, discontinuous, parallel reflection patternswithin PC-II indicate stable conditions of the basinfloor during deposition.

The top of the Cretaceous is marked by basalt lavaflows. Well data indicate that the Cretaceous sectionconsists of bottom sandstone, middle shale, and uppersand-shale units. These three lithologic units are de-marcated as seismic sequences C-I, C-II, and C-III, re-spectively, to clarify depositional conditions.

The lowest sequence, C-I, is very thin and under-lies a thick marine transgressive shale. This is the Gol-lapalli Sandstone, which was deposited in geomorphiclows prior to the major transgression in the basin. Thissequence is absent over the Precambrian highs, such asthe Draksharama high toward the northeast part of thebasin and the Kaza-Kaikaluru horst toward the south-west. The data show that these horsts were elevatedduring deposition of C-I. Reflections within C-I towardthe northwest are either weak or absent, suggesting thesequence consists of fluvial sand. In contrast, in the

southeastern part of the basin, reflections are strongand continuous, indicating that C-I may have been de-posited in fluviomarine conditions.

The top of C-II represents a regressive surface overwhich the downlap of the overlying sequence is ob-served. The reflection character is strong and contin-

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Figure 7. Structural cross section across the East Godavari subbasin showing thick Tertiary sediments toward the southeast of theMPFZ as seen in well G in the offshore part of the basin; toward the northwestern basin margin area at well A, thick pre-Cretaceoussediments were encountered.

uous within the sequence, which indicates marine dep-ositional conditions. The C-II reflection pattern isparallel up to wells B and C, which suggests stable dep-ositional conditions. Farther to the southeast, the di-vergent C-II pattern is a response to passive marginsubsidence. Nearer to the basin margin area, the C-IIreflections are weak or absent, suggesting that the se-quence consists of sand that was deposited in marginalmarine conditions. This sequence is equivalent in timeto the Raghavapuram Shale.

C-III represents Upper Cretaceous deposition. Inthe northwestern basin margin area, it is a major are-naceous unit; reflections are weak or absent, and mar-ginal marine to fluvial depositional conditions mayhave existed. C-III becomes a thick (more than 2000m) claystone in the southeastern part of the East Go-davari subbasin, where the reflection character isstrong and has a divergent pattern, suggestive of syn-sedimentary subsidence (Figure 8b).

The P seismic sequence represents Paleocenedeposition. The top of the sequence has a strong re-flection character and is directly below the base ofthe Eocene section. The unit is thin and absent inthe northwestern part of the basin, where wells B,H, T, and W were drilled. The shelf edge is inter-preted to be southeast of wells U and H. Strong re-flections within the P sequence may represent sand-shale alternations. Shale diapirism may also be seennear well F (Figure 8c).

The Oligocene–Eocene section is represented bythe EO seismic sequence, although the thin Oligocenesediments (40–100 m) could not be identified in seis-mic sections. The EO sequence onlaps over the Paleo-cene sequence, suggesting a marine transgression. TheEocene shelf edge is demarcated southeast of well F.

The divergence reflection pattern in the southeasternpart of the basin may be related to passive margin sub-sidence. The reflection-free areas within the carbonatesection indicate possible reefal buildups in the EO se-quence. Toward the basin, shale diapirism is observedin the EO sequence.

An erosional surface marks the top of theMiocene–Pliocene (MP) sequence in the offshore partof the basin. Seismic signatures within the sequencemay indicate channels, growth faults, and rolloverstructures of a deltaic system (Rangaraju and Yala-marty, 1984). Reflections in the proximal part of thebasin are weak, whereas they are strong and divergentin the distal part, suggesting a basinal subsidence.

DEPOSIT IONAL ENVIRONMENTS

Pre-Cretaceous

Isopachs of PC-I and PC-II show a maximum thicknessin well A (Figure 9). PC-I and PC-II are two distinctlithofacies: the lower sand-shale-coal facies and the up-per feldspathic sandstone. In well A, PC-I is 2000 mthick (Rao et al., 1993). In wells drilled outside thePranahita-Godavari graben (wells S, X, and Y), PC-IIis absent, and the sandy facies directly overlie the Ar-chean basement. The electrolog patterns suggestmainly coarsening-upward sands containing somefining-upward sequences. PC-I and PC-II comprisenonmarine deposits. Observed log patterns indicatefluvial channels with point bars (Figure 10). To under-stand depositional environments, granulometric stud-ies of samples taken from conventional cores werecarried out using the methods of Visher (1969) and

Figure 8. Seismic line across the East Godavari subbasin in parts (see Figure 3 for line location) and geological interpretation.(a) Northwestern part of Tanuku horst (Kommugudem trough) showing pre-Cretaceous rift fill sedimentation indicating seismicsequences. (b) Southeastern part of Tanuku horst (Mandapeta trough) showing older rift fill sequences are superimposed withCretaceous sequences. (c) Thick Tertiary sequences toward the southeast of the MPFZ.

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Figure 9. Isopach map ofpre-Cretaceous sediments ofthe Krishna-Godavari basin. Thesequence comprises three dis-tinctive lithologic units: A �lower coal-bearing unit(Barakar/Kommugudem forma-tions); B � middle arenaceousunit (Chintalapudi/Mandapetasandstones); and C � upperred beds that represent abreakup unconformity. A thickPaleozoic graben is seen at wellA toward the northwestern partof Tanuku horst.

Folk and Ward (1957). The core data indicate thatdeposition occurred in inland river channels. Petro-graphically, the sandstones were classified as quartza-renites (Dott, 1964), with a grain composition of morethan 70% quartz, 10% feldspars, and an unspecifiedamount of lithic fragments. The matrix is clay rich andsiliceous in places.

Cretaceous

The isopach map of Cretaceous sediments indicatesthat the unit is thicker toward the southeast (Figure11). In outcrop and in the subsurface, Cretaceous sed-iments can be divided into three distinctive sequences:C-I is the lower sandy facies, C-II is composed ofmainly shales, and C-III is the upper sandy facies. Elec-trologs show a coarsening-upward pattern in the lowersandy unit, a predominantly fining-upward pattern inthe middle shale unit, and both patterns in the uppersandy unit. Petrographically, the rock types are lithicarenite, mudstones, and quartzwackes. The matrix isgenerally clay rich and micaceous. The dominant clayminerals are kaolinite and illite. Other minerals in Cre-taceous sediments include pyrites, tourmaline, and gar-nets. Grain-size parameters indicate the deposition ofCretaceous sediments in low-energy conditions withina fluvial system.

Two prominent basement highs, one at Kaza-Kaikaluru and the other at Draksharama, remained

positive during the Early Cretaceous, resulting in thindeposition over these highs (Figure 2). This correlatesto the absence of the lower sandy unit (Mohinuddin etal., 1993). Both seismic sections and electrolog corre-lations show that a peneplanation surface existed overpaleohighs at the base of the middle argillaceous unit(Raghavapuram Shale). Crossplots of K2O and F2O3

suggest that the upper sandy unit was deposited in flu-vial conditions in the proximal part of the basin. Theupper unit laterally becomes argillaceous toward thesoutheast under marine depositional conditions.

Paleocene

Basalt forms the basin floor for Paleocene sediments.The unit is not outcropped anywhere in the basin. Inthe subsurface, the P sequence is dominantly sandy upto the northeast-southwest–trending MPFZ (Figure 3).In contrast, the P sequence is mainly argillaceoussoutheast of the fault zone. Isopach maps indicate adepocenter in the area of wells O and P (Figure 12).Electrologs depict a blocky SP log pattern and a pre-dominantly coarsening-upward sequence (Rao et al.,1996). Statistical parameters derived from granulo-metric studies indicate a marginal marine depositionalenvironment. Paleocene sedimentary rocks consistmainly of quartzwackes. Laths of muscovite, glauco-nite, secondary calcite, pyrites, and zircons have beenobserved. The presence of limestone beds in wells I,


Figure 10. Electrolog correlation of pre-Cretaceous sequences(profile location in Figure 3). Lithologic units identified by logmotifs are indicated. At well B, a downthrow of unit 2 is seen,whereas because of uplift followed by erosion it is absent atwell C (Draksharama high). Older electrofacies 3, 4, and 5 in-dicate postdepositional uplift of areas around wells A and C.

N, and Z suggests a shelf edge environment during thePaleocene.

Eocene

Isopachs of the Eocene sequence show a north-northeast–south-southwest–aligned depositional cen-ter, where well O has the thickest section (Figure 13).The depositional center is found to be shifted towardthe northeast compared to the Paleocene depositionalcenter. The Eocene sequence in the subsurface is a thinsandy unit near the MPFZ. Between wells M and P inthe East Godavari subbasin, the Eocene sequence isthicker and can be divided into the lower argillaceous,middle carbonate, and upper arenaceous units. In thelower argillaceous unit, thin sands show ripple marksin the top layers, whereas the lower layers containclaystone pebbles and exhibit burrowing and channelfilling, which may indicate nearshore depositional con-

ditions. Electrolog patterns indicate a coarsening-upward sequence. The thick middle limestone may in-dicate a shelf edge carbonate facies during the middleEocene. The granulometric data show that fluvial con-ditions may have existed for the lower argillaceousunit, and nearshore conditions may have existed for theupper arenaceous unit. This would indicate that atransgressive phase occurred in the late Eocene due tominor fluctuations in sea level. In the thin section, theforams (nummulites) are found to be embedded in mi-crite, and recystallization of spar is seen. Trace elementanalysis shows elevated concentrations of CaO andMnO, which indicate that marine conditions may haveexisted in the area of wells I and E.

The Oligocene sequence is very thin, having amaximum thickness of 150 m. Sedimentary rocks con-sist of sandstone, siltstone, claystone, and occasionallimestone. O sequence sediments can be identified bywavy SP patterns in electrologs. The Oligocene appar-ently experienced a major regression event associatedwith upwarp, as evidenced by thin sedimentation.

Miocene

Isopachs of the Miocene sequence show a deposi-tional center around the Ravva field offshore (wellG). Another depositional lobe is identified near theKrishna River mouth (Figure 14). The data suggestthat the Krishna and Godavari rivers were separatedduring the Miocene (Satyanarayana et al., 1996).Thin section petrography indicates that Miocenesedimentary rocks consist of quartzwackes with glau-conite and benthonic/planktonic forams. Electrologpatterns indicate a coarsening-upward pattern. Traceelement data (Ni/Co) correlated with granulometricdata show that the lower part of the M sequence mayhave been deposited in nearshore conditions. In con-trast, trace element data suggest that marine condi-tions existed in the upper part of the M sequence.This suggests marine transgression occurred some-time during the late Miocene–early Pliocene.

PETROLEUM POTENTIAL

Hydrocarbon Occurrence

Initial successful hydrocarbon exploration in theKrishna-Godavari basin was in thin Upper Cretaceousreservoirs in the Narasapur structure of the East Go-davari subbasin. Exploration efforts since 1978 have

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Figure 11. Isopach map ofthe Cretaceous sequence of theKrishna-Godavari basin. The se-quence uniformly thickens to-ward the southeast. The unit isnot completely drilled towardthe southeastern part of the ba-sin. Three distinctive lithologicunits are identified: lower are-naceous (Gollapalli Sandstone),middle argillaceous (Raghava-puram Shale), and upper are-naceous (Tirpupati Sandstone),which varies to argillaceous fa-cies toward the southeast of theMPFZ (Chintalapalli Claystone).Single letters are well names.

Figure 12. Isopach map ofthe Paleocene sequence show-ing a depocenter at well V, Ma-sulipatnam Bay. Up to theMPFZ the unit is sandy, and to-ward the southeast it is mainlyargillaceous. Single letters arewell names.

established oil and gas reservoirs ranging in age fromLate Permian to Pliocene (Rao, 1991).

Sikka (1990) used the probabilistic model to esti-mate the undiscovered hydrocarbon potential of Cre-taceous and Tertiary plays in the Krishna-Godavari ba-sin to be 726 million tons. The estimate took intoconsideration basin analysis, chance of success of indi-vidual plays, risk analysis, andMonteCarlo simulations.

Petroleum System Analysis

By analyzing the hydrocarbon occurrence in the basinin relation to petroleum system classification, four sys-tems can be identified. They are the Kommugudem-Mandapeta–RedBed,Raghavapuram-Gollapalli-Razole,Palakollu-Pasarlapudi-Bhimanapalli, and the Ravva-Godavari petroleum systems. Locations of oil and gas


Figure 13. Isopach map ofthe Eocene sequence with amajor depocenter at the well Pcoastal tracts of the Godavaririver. Also note the shift of thedepocenter toward the north-east, compared to the Paleo-cene depocenter. Single lettersare well names.

Figure 14. Isopach map ofMiocene and younger se-quences. Two distinctive depo-centers lie at the mouths of theGodavari and Krishna rivers.Single letters are well names.

fields are shown in Figure 15. Geological ages of res-ervoirs are indicated in Table 2.

The geologically oldest petroleum system is thepre-Cretaceous Kommugudem-Mandapeta–Red Bed(Rao, 1994). This system is confined to the northwest-southeast–trending rift valley extending beneath theEast Godavari subbasin. The hydrocarbon potential ofthe system is estimated to be 330 million tons. The

source rocks yield mainly gas. The system is associatedwith many erosional unconformities. To date, only 20million tons in reserves have been established. The ap-parent lack of high-amplitude anticlinal closures andpermeability barriers currently impede new explora-tion efforts.

The Cretaceous petroleum system is named theRaghavapuram-Gollapalli-Razole system. In the West

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Figure 15. Hydrocarbonbearing prospects of theKrishna-Godavari basin. Forgeological age of the reservoirsand field names refer toTable 2.

Godavari subbasin, thin sands and limestones withinsource facies are reservoirs. In the East Godavari sub-basin, lenses of sands in the Chintalapalli Claystoneproduce hydrocarbons. Southeast of the MPFZ, thesystem was undercompacted during the Eocene. Thepetroleum potential has been estimated to be 230 mil-lion tons, of which so far only 15 million tons in re-serves have been established.

The Paleogene Palakollu-Pasarlapudi-Bhimana-palli petroleum system is the most prolific system inthe Krishna-Godavari basin. Located southeast of theMPFZ in the East Godavari subbasin, the system con-tains abnormally pressured source rocks and normallypressured reservoirs (Rao and Mani, 1993). Anticlinalclosures serve to entrap hydrocarbons. The estimatedhydrocarbon resources are estimated to be 300 milliontons. To date, about 80 million tons in reserves havebeen established.

The Neogene petroleum system is called theRavva-Godavari System. The most promising area forcommercial hydrocarbon production is offshore. Theabnormally high geothermal gradient has caused lowerMiocene sediments to mature and generate hydrocar-bons. The estimated hydrocarbon resources are esti-mated to be 200 million tons. Nearly 70 million tonsin reserves have been established thus far.

Source Rocks

About 200 m of source rocks were identified inPermian–Carboniferous coal-shale sediments. Thesource rock quality ranges from poor in well C to verygood in well B, where the source rock was in the oilgenerative window during the Permian and is presentlyin the metagenetic stage (Brahmajirao et al., 1991).

The Lower Cretaceous shales are oil prone. Thisfacies is 800 m thick in well A. Based on the time tem-perature index (TTI), source rocks in the area aroundwell B have been capable of generating liquid hydro-carbons since the late Eocene. Upper Cretaceous ar-gillaceous rocks located in the southeastern onshorebasin have also matured to generate gaseous hydrocar-bons since the Eocene and are also good source rocks.

Paleocene shales of the East Godavari subbasincontain dominantly type III kerogen with 2–3% organicmatter and have fair to good source potential. The levelof maturation is in the early to peak oil phase of gen-eration. Gas and subordinate oil-prone facies havebeen identified in Paleocene sediments.

Lower Eocene shales located southeast of well Fcontain high organic matter concentrations (TOC 3–4%). Geochemical studies reveal that they are in theearly phases of maturation. The quality of organic


Table 2. Geological Age of Hydrocarbon Fields in Krishna Godavari Basin

Onland Offshore Average Depth (m)of HC Bearing ZoneGeological Age Oil Gas Oil Gas

Pliocene 1. G-12. G-2

1700**

Miocene 3. Ravva4. GS-15.5. GS-236. GS-29

900–2560

7. GS-38Oligocene 8. Adavipalem

9. Kesavadasupalem16501800

Eocene 10. Mori 11. Elamanchili12. Tatipaka13. Pasarlapudi14. Ponnamanda15. Mulikipalli16. Kadali17. Magatapalli18. Medapadu19. Kesanapalli20. Bandamurlanka21. Rangapuram22. Lankapalem

2400–2700

Paleocene 23. Mummidivaram24. Achanta25. Razole26. Palakollu

27. GS-8 2000–2500

Late Cretaceous 28. Enugupalli29. Narasapur30. Penumadam31. Chintalapalli32. Kavitam

24003400340044504380

Early Cretaceous 33. Kaikaluru34. Lingala

35. Kaza36. Vadali37. Mahadevapatnam38. Gokarnapuram39. Bantumilli40. Nandigama

1800–200033004000

Late Jurassic 41. Endamuru 1700Late Permian 42. Mandapeta 2700

**In many fields the producing zones are multilayered.*Use the number that is shown to locate the field in the figure.

matter is mainly type III and minor type II. Thesesource rocks have a tendency to generate gas and sub-ordinate oil and have generated hydrocarbons since theearly Miocene (Neeraja et al., 1997).

Offshore, lower Miocene–Oligocene source rocksare identified in the Ravva area, where well G was

drilled. The thickness of source beds is about 400 m,and they have a high organic matter concentration(TOC 1.49–1.86%). The quality of the source rock fa-cies, however, is rated very poor for generating com-mercial quantities of liquid hydrocarbons (Philip et al.,1991a, b).

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Reservoirs

Potential Krishna-Godavari basin reservoirs range inage from Early Permian to Miocene–Pliocene. The old-est reservoirs are thick prerift Permian sandstones thatoverlie Permian–Carboniferous source beds. Thinsandstone beds in Lower and Upper Cretaceous sourceshale beds have proven to be reservoirs capable of pro-ducing both oil and gas. Lower Eocene sandstones thatoverlie Paleocene source beds have proven reserves.Miocene and Pliocene deltaic sand beds in the Ravvaarea are good oil producers possibly because hydrocar-bons have migrated from a deeper source.

Trap Styles and Play Types

Seismic surveys and geological mapping indicate thatalthough structural traps do exist in the Krishna-Godavari basin, most of them are only small to me-dium in size. In contrast, updip pinch-outs, unconfor-mity surfaces, and permeability barriers all play animportant role in the entrapment of hydrocarbons.

The oldest gas-producing reservoir is the PermianMandapeta Sandstone. The thickness of the reservoiris more than 2000 m. Fault-controlled structures arecommon, but simple amplitude reversals are uncom-

mon. The dominant factor for entrapment is the per-meability barrier, as evidenced in the presence ofquartz overgrowths observed in thin-section petrogra-phy; however, the red bed overlying rift-fill sedimentsacts as regional seal.

Upper Jurassic–Lower Cretaceous sandstones aretruncated against preexisting basement highs. TheRag-havapuram Shale, which is the overlying Lower Cre-taceous transgressive shale, may exist as a regional sealfor these reservoirs. Thin limestone and sandstone bedsdeposited within the Raghavapuram Shale are reser-voir rocks.

Thin sandstone beds within thick Upper Creta-ceous claystone beds are potential reservoirs in the EastGodavari subbasin. Stratigraphic plays include turbi-dite fans located southeast of the MPFZ. Basalt mayform the regional seal for all the Cretaceous sediments.

Anticlinal closures serve as traps for lower Eocenereservoirs. Interbedded shales may serve as local sealsfor these reservoirs. The thick middle Eocene carbon-ate may also act as a regional seal.

Sandstone beds in the Miocene–Pliocene sedi-ments are interspersed with clay beds, which may actas local seals for entrapment; however, growth faultsand associated rollover anticlines are more effectivetraps. The Miocene–Pliocene erosional unconformity

Figure 16. Conceptual hydro-carbon play types in theKrishna-Godavari basin. Playconcepts include (1) permeabil-ity barriers; (2) fault closures inUpper Permian reservoirs;(3) clastic wedges and associ-ated unconformity traps in Up-per Jurassic (Gollapalli Sand-stone) fluviomarine sediments;(4) sand lenses within Creta-ceous and Paleocene argilla-ceous facies; (5) anticlinalclosures in lower Eocene near-shore clastic reservoirs;(6) growth faults, erosional cut,and rollover anticlinal accumu-lations in Miocene–Plioceneclastics in the shallow marinearea of the basin. Regionalseals: I � red beds, II � Rag-havapuram shales, III � Dec-can basalts, IV � middle Eo-cene carbonates, V � Plioceneclays.


surface, prominent in seismic sections, is overlain bythick Pliocene unconsolidated clay beds, which form aregional seal (Figure 16).

CONCLUSIONS

The subsurface data reveal the presence of a thick sed-iment fill in the basin and the extension of thenorthwest-southeast–trending Pranahita-Godavari gra-ben underneath the northeast-southwest–trendingKrishna-Godavari basin between two major cross-trends. The continuation of the basin is visualized toreach the OCB, located in the deep-water area of thebasin.

The southeastern part of the basin is a majorTertiary depositional center because of a series ofdown-to-the-basin faulting during the early Paleocene.Because of change in the gradient, major delta progra-dation is not seen in the area during the Paleocene andearly Eocene. The rapid sediment fill in the low hasfacilitated smooth progradation of the delta toward thesoutheast since the middle Eocene.

Seismic facies analysis of different sequences sug-gests typical passive margin subsidence. The positionsof shelf edges through the geological ages have beenmarked. The Neogene depositional system has bifur-cated into two separate depocenters in the basin sincethe Miocene.

The lithofacies variations within marked se-quences were discussed in detail and a lithostrati-graphic nomenclature is proposed for the entire sedi-mentary column of the basin. In the basin, good sourcerocks are identified in the sequence ranging fromPermian–Carboniferous to lower Miocene.

The envisaged depositional patterns of the basinthrough geological ages help to identify locales of goodreservoir facies in the vicinity of source facies. The ba-sin has good hydrocarbon potential, and exploratoryefforts to locate oil and gas pools through seismic-stratigraphic analysis may enhance the discovery rate.

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Sedimentation, Stratigraphy And Petroleum Potential of Krishna-Godavari Basin,East Coast of India