Krishna Godavari Basin is a peri-cratonic passive marginbasin on the east coast of India (Figure 1). The onland partconsists of 28 000 km2 and is mostly alluvium covered.Krishna and Godavari are the two major river systems whichdrain the area and discharge in the Bay of Bengal. The off-shore basinal area covers 24 000 km2 to the isobath of 200 m.However, the basin extends into deeper water and covers amuch larger area. The basins characteristic feature is its en-echelon horst and graben system which is filled with a thickpile of sediments of Permian-to-Recent age and emergingas one of Indias most promising petroliferous areas. Com-mercial accumulation of hydrocarbons occurs in sedimentsfrom the Permian to as young as the Pliocene.

Krishna Godavari Basin is orthogonally juxtaposed toNW-SE trending Pranhita Godavari Gondwana graben in thenorth (Figure 2). The NE-SW basin margin is the most exten-sive fault trend over the area (Figure 3). It takes a bow-likeswing and comes to the coast near Kakinada graben in thenortheast and near Palar-Pennar graben in the southwest.

The onland basinal area is characterized by en-echelonand arcuate horsts and grabens associated with major crosstrends. In addition to the basin margin fault, three moreregional faults developed further basinward: the onlandMatsyapuri-Palakollu fault, a Miocene structure buildingfault in shallow water close to the coast, and a Pliocenestructure building fault in deeper water. The arcuate horstsand the four regional arcuate faults are more or less paral-lel. Offshore, the sediments are mostly influenced by growth-related tectonics.

Stratigraphy. The sedimentary sequence of KrishnaGodavari Basin ranges from Permian-to-Recent. ThePrecambrian metamorphic basement consists of gneisses,quartzites, charnokite, and khondalite. Subsurface knowl-edge is limited due to alluvium cover. However, isolated out-crops of Permian, Cretaceous, Paleocene, and Mio-Pliocenerocks are present near the basin edge. Figure 4 shows thegeologic map of Krishna Godavari Basin and Figure 5 thegeneralized stratigraphy.

Sedimentation in Gondwana Basin was initiated duringthe Early Permian over the crystalline basement and isknown as Draksharama/Kommugudem Formation. Sed-iments of Triassic age are conspicuous by their absence.Golapalli sandstone of Early Cretaceous age lies uncon-formably over Mandapeta sandstone. The top of theGolapalli sandstone is a basinwide regional unconformityand is in turn covered by thick Late Cretaceous sedimentsand relatively thin Tertiary sediments. The areas of Gudivadagraben and Krishna graben constitute the Jurassic rift basin.The Jurassic and Early Cretaceous sediments are the mainfill for the Jurassic Basin. The overlying Tertiary and Recentsediments are relatively thin and generally undifferenti-ated. Bantumilli graben and Nizampatnam graben werecreated during the Cretaceous and therefore named theCretaceous Basin. The Cretaceous sedimentation over theGondwana, Jurassic, and Cretaceous basins began simulta-neously during Aptian/Albian time. These sediments arecalled the Golapalli sandstone in Mandapeta graben andBhimadolu graben, the Nandigama Formation in Bantumilligraben, and Gajulapadu shale/Kanukollu sandstone in

Gudivada graben and Krishna graben. The overlyingRaghavapuram shale and Tirupati sandstone of LateCretaceous age, followed by the relatively thin Tertiary sed-iment, is widely distributed. Therefore it can be surmisedthat Krishna Godavari Basin was initially made up of

Basin architecture and petroleum system of Krishna Godavari Basin, east coast of IndiaS. K. GUPTA, Oil and Natural Gas Corporation, Dehradun, India

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Figure 1. Location of Krishna Godavari Basin.

Figure 2. Tectonic map of Krishna Godavari Basin.

Gondwana Basin, Jurassic Basin, and Cretaceous Basin, andthey remained a unified basin until the Late Cretaceous. TheGodavari graben area to the south of the Eocene growth fault(Matsyapuri-Palakollu fault) is the main depocenter forTertiary sediments. Further basinward, Tertiary sedimenta-tion was successively influenced by the Miocene growth faultand the Pliocene growth fault. Each kicked off a series ofsubsidiary growth faults which locally guided the sedi-mentation pattern.

Lithostratigraphy. Kommugudem Formation (EarlyPermian is the oldest sediment deposited over the Archeanbasement although, at places, argillite (DraksharamaFormation) is present underneath the Kommugudem.Kommugudem Formation is dominantly a shale sequencewith interbedded coal and sand. The coal beds are gener-ally 16 m in thickness. The environment of deposition isfluvial-to-lagoonal.

Mandapeta sandstone (Late Permian) is a thick nonma-rine feldspathick and micaceous sandstone deposited in afluvial environment. The presence of relatively thick inter-vening shales suggests cyclic flood plain conditions.

Bapatla sandstone (Late Jurassic) is a nonmarine sand-stone, clay, and shale section. It rests directly on the Archeanbasement. The basal section is claystone with thin sandstonelayers interbedded. The overlying section is dominantlyarenaceous with thin intercalations of shale and claystone.

Gajulapadu shale and overlying Kanukollu sandstone

(Aptian/Albian) rest unconformably over the Bapatla sand-stone in the Gudivada graben. The Gajulapadu shale,deposited in a lacustrine environment, is highly carbona-ceous and rich in organic matter. A few interbedded sand-stone layers are also present. The overlying Kanukollusandstone is predominantly sandy and fairly consistent indistribution. Deposition took place in a marginal marineenvironment.

Nandigama Formation (Aptian/Albian) is primarilymarine shale with thin sandstone beds. The basal part iscoarser clastics with thin sandstone interbedded.

Golapalli sandstone (Aptian/Albian) is red claystone,overlain by sandstone, which rests unconformably over theMandapeta sandstone as fill sediment. The depositionalenvironment was shallow marine.

Raghavapuram shale (Cenomanian to Early Maas-trichtian) can be subdivided into lower and upper units. Thehigh-resistivity lower unit is rich in organic matter. Theupper unit has thin interbedded layers of lenticular sandand shale. Sedimentation took place under shallow marineconditions.

Tirupati sandstone (Early to Late Maastrichtian) uncon-formably overlies the Raghavapuram shale. It was depositedduring the retreating Cretaceous sea and is predominantlysandstone with minor claystone. It is progressively shaly bas-inward and is called the Chintalapalli shale.

Razole Formation (Early Paleocene) consists of wide-spread volcanic flows over the Tirupati sandstone. There are

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Figure 3. Time structure map on top of basement depicting tectonic elements of Krishna Godavari Basin.

at least 23 submarine trap flows. The traps at times developfractures, and the top surface is altered and weathered intovariegated clay.

Palakollu shale (Middle to Late Paleocene) was depositedin an outer neretic to bathyal environment south of theMatsyapuri-Palakollu fault. The thickness of this lithounitincreases basinward.

Pasarlapudi Formation (Early Eocene) overlies thePalakollu shale and consists of alternating sand and shalelayers with some limestone. Toward the south and east, thePasarlapudi sands grade into shale and are called theVadaparru shale. The depositional environment was outerneretic to bathyal.

Bhimanapalli limestone (Middle Eocene) is an algal dolo-stone with abundant dolomite crystal and red algal frag-ments. Interbedded sandstones within thick carbonates arecommon. The depositional environment is outer neretic tobathyal.

Matsyapuri sandstone (Oligocene-Miocene) is a thicksandstone sequence with interbedded claystone, betweenthe Matsyapuri-Palakollu fault and the Miocene growthfault close to the coast. Sedimentation occurred in middleshelf conditions.

Ravva Formation (Miocene) consists of thick and coarseclastics deposited south of the Miocene growth fault in aninner-to-middle shelf environment. Sediments were sub-

jected to push ups and rotation because of the tectonicsof the underlying Vadaparru shale. The sands are separatedby interbedded clay.

Narsapur claystone (Pliocene) is a monotonous clay-stone section with a minor amount of intervening sand andsilt. This unit thickens basinward where it is called theGodavari clay. The depositional environment was shallowinner-to-middle shelf.

Vadaparru shale (Miocene-Late Eocene) is a thick marineclay section deposited under outer shelf conditions. It alsorepresents the basinward clay facies of the PasarlapudiFormation, Bhimanapalli limestone, and Matsyapuri sand-stone.

Godavari Formation (Plio-Pleistocene) sediments aremainly clay with minor silt layers deposited south of theMiocene growth fault. However, the depocenter is farthersouth of the Pliocene growth fault where thick coarser clas-tics, brought in by a deepwater river-fan system, accumu-lated. The depositional environment was inner-to-middleshelf.

Evolutionary history of Krishna Godavari Basin. The basinappears to have gone through eight stages prior to assum-ing its present form.

Rift Stage I: Prior to drifting, Gondwanaland comprisedthe continents of Africa, South America, Antarctica, Aus-

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Figure 4. Geologic map of Krishna Godavari Basin.

tralia, and the Indian subcontinent. The Upper Carbon-iferous-to-Jurassic sediments deposited selectively in lineartroughs traversing the unified continents. The PranhitaGodavari graben on the east coast of India belongs to suchrifted troughs with possible rift continuity to eastern Ant-arctia. During this period, Kommugudem Formation (EarlyPermian) and the Mandapeta sandstone (Late Permian)were deposited. The absence of Triassic sediment is proba-bly related to the breaking up and fragmentation of Gond-wanaland during the Jurassic when the Permian and theTriassic sediments were subjected to thermal upwelling. Asa consequence, the uplifted mass of Triassic sediments overthe emerging east coast of India were subjected to deep ero-sion in Gondwana graben. The Bhimadolu, Mandapeta, andKakinada grabens form part of Gondwana Basin. Duringthe fragmentation of India, a major NE-SW Jurassic riftbasin was created which accommodated thick Late Jurassicsediments. The newly emerged Jurassic rift basin cuts acrossthe Krishna Godavari Gondwana graben orthogonally andis named the Trans Godavari graben. A series of concentricand parallel to subparallel weak planes also emerged. The

Jurassic Basin deposited thick fluvial and lacustrine sedi-ments (Bapatla sandstone) during the synrift stage. TheJurassic Basin is represented by Krishna graben andGudivada graben. The main source of sediment was from

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Figure 5. Generalized stratigraphy.

Figure 6. Conceptual model of Cretaceous deposition during Early DriftStage I.

Figure 7. Conceptual model showing breakup of Trans Godavari horstand Trans Godavari graben into smaller en-echelon horst and grabensystems during Early Drift Stage II.

Figure 8. Tertiary depositional model showing influence of growth tecton-ics over the sedimentation during Late Drift Stage IV.

the surrounding granitic country rock.Rift Stage II: Intense tectonic activity took place at the

end of the Jurassic. During this period, the Trans Godavarigraben acquired half-graben configuration as a result ofconsiderable accentuation of the fault on the grabens west-ern margin. This fault ultimately transformed into a basinmargin fault for the Krishna Godavari Basin. Simultaneously,the eastern margin of Trans Godavari graben was subjectedto reactivation of the faults and emergence of a long andlinear horst, named the Trans Godavari Horst. This mega-horst was due to basement uplift and collapse of the flanks.At this time, Bantumilli graben also emerged as an associ-ated low southeast of the Trans Godavari Horst.

Early Drift Stage I: India and Antarctica started movingapart during the Neocomian, creating the ocean floor for theemerging Bay of Bengal. The period witnessed the initialdeposition of shale (Gajulapadu shale) in lacustrine envi-ronment in the southern part of the landlocked Trans Go-davari graben (represented today by Krishna and Gudivadagraben). At a later stage because of rising sea level, the Ka-nukollu sandstone (Aptian/Albian) was deposited under

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Figure 9. Prospect map of Krishna Godavari Basin.

Figure 10. Geographic distribution of petroleum systems in KrishnaGodavari Basin.

marginal marine environment. The northeastern part of theTrans Godavari graben is represented at present by theBhimadolu, Mandapeta, and Kakinada grabens. Thesegrabens were deposited as thick coarser clastics (Golapallisandstone) due to its proximity to the basin margin.Sedimentation took place under a marginal marine environ-

ment. During the same period, theBantumilli graben, exposed on thesouth to the open sea, depositedmarine shale (Nandigama Forma-tion). All these lithounits, althoughtime equivalent (Aptian/Albian), haddifferent depositional settings. Duringthe period, the Trans Godavari grabenfilled up and achieved peniplaination.The sediment supply was mostlyfrom the newly emerged horst and theexposed areas north of the basin mar-gin (Figure 6).

Early Drift Stage II: The TransGodavari horst and graben systememerged transverse to the NW-SEPaleozoic Gondwana trends. As a re-sult, the newly emerged megahorstand graben were under great stress.Subsequent to the sediment fill a crit-ical tolerance level was exceeded(Early Eocene), and the stress wasreleased through the breaking up ofthe Trans Godavari horst and grabenat different places and then dislocat-ing them in the direction of olderPaleozoic trends oriented NW-SE.Five such cross trends are identifiedover the basin. This geologic phe-nomenon therefore resulted in theformation of series of smaller en-ech-elon horst and graben systems fromthe single megahorst and graben(Figure 7).

Late Drift Stage I: The top of theEarly Cretaceous sediment was sub-jected to wide erosion and penepla-nation when most horsts ceased to bea positive area. During the period, astrong southeasterly basinal tiltoccurred with the basin margin faultacting as the hinge. This was followedby Cretaceous sea transgression, andflooding of the entire basinal areas.Now, for the first time, almost theentire area underwent deposition asa single basinal unit. It is at this stagethat the present form of KrishnaGodavari Basin took shape. Duringthe regional transgression, Late Cre-taceous marine sediments (Raghava-puram shale) were deposited over theEarly Cretaceous sediment fill with awell marked regional unconformity(Raghavapuram regional unconfor-mity). The basinal tilt toward thesoutheast increased considerably dur-ing the Maastrichtian. As a conse-quence, coarser clastics (Tirupatisandstone) were deposited in thenorthern part of the basin.

Late Drift Stage II: During the Paleocene, the subductionof the Indian Plate below the Tibet Plate intensified. Thistriggered volcanic eruptions (Razole Formation) over mostof Krishna Godavari Basin. The postvolcanic period wit-nessed regression/emergence of the Krishna and theGodavari rivers. The continued tilting caused a fresh sup-

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Figure 11. Geologic section depicting source, reservoirs, and traps in the Palakollu-Pasarlapudisystem.

Figure 12. Seismogeologic section of Gudivada Graben showing source, reservoirs, and structure.

Figure 13. Seismogeologic section of Bhimadolu graben showing rotated fault blocks with sourceand reservoirs.

ply of clastics, accompanied by rapid loading. The contin-uation of this process produced a major Eocene growth faultwhich is arcuate in nature, more or less parallel to the basinmargin, and is called the Matsyapuri-Palakollu fault. Thisalso marked the birth of Godavari graben, which formedthe depocenter for Tertiary sediments. The Palakollu shaleand Pasarlapudi Formation were deposited during thisperiod.

Late Drift Stage III: Sea level lowered during the Oligoceneperiod and deposition of sediments near the coastal areas,which were at places subjected to erosion, was minor. Thearea of sediment source in the north was continuously accen-tuated due to basin tilt. As a result, the Godavari Riverstarted prograding basinward and built the Miocene delta.Rapid loading of sediments resulted in a well developedgrowth fault, arcuate in shape, more or less parallel to theMatsyapuri-Palakollu fault, and called the Miocene growthfault. During the period, Miocene sediments were depositedunder the growth fault regime and were influenced by shaletectonics.

Late Drift Stage IV: During the Pliocene, another spurt ofclastic input triggered formation of the Pliocene growth fault.The underlying Vadaparru shale was subjected to wide shaletectonism. As a consequence, large anticlinal structures andnumerous fault closures formed. It is interesting that all threemajor growth faults described above are parallel to the LateJurassic basin margin fault. Further basinward, the Pliocenelimit is marked by the pronounced Pliocene/Pleistocene toethrust. The subthrust and the area beyond it are dominatedby the Bengal fan system (Figure 8).

Petroleum systems. The presence of hydrocarbons inKrishna Godavari Basin was established in 1979. The initialcommercial discovery was made in 1980 by offshore prospectG-1. In 198186, many small gas fields were discoveredonshore. Two major discoveries occurred in 1987, Mioceneoil in the offshore Ravva Field and Eocene gas onshore inPasarlapudi Field. This success has continued. Some sub-sequently discovered oil and gas fields are (onshore)Mandapeta, Endamuru, Kesanapalli west, and Mori and(offshore) GS-29, GS-15, GS-23, G-4, G-1-12, and Dhirubhai(Figure 9). The sustained exploration effort in KrishnaGodavari Basin has generated a rich knowledge base withrespect to hydrocarbon generation, migration, and entrap-ment. A systematic study that integrated the available infor-mation established the presence of five petroleum systemsin Krishna Godavari Basin (Gupta et al., 2000). Figure 10shows their geographical distribution.

The Vadaparru-Ravva/Godavariclay system, the youngest Tertiary pet-roleum system in Krishna GodavariBasin, includes a large offshore areaand a narrow coastal strip of Godavarigraben. Ravva, GS-15, GS-23, and GS-29 are Miocene clastic reservoirs. G-1, G-4, and Dhirubhai are structuraland stratistructural traps of Plioceneclastic reservoirs. For both Mioceneand Pliocene reservoirs, Vadaparruacted as the main source rock. TheMiocene reservoirs of Ravva Forma-tion are fine- to medium-grainedsands with interbedded clay. The Plio-Pleistocene reservoirs were depositedunder a deepwater channel and fancomplex. The sands are fine- to med-ium-grained and at times extensive

and substantially thick. The regional cap to the Miocene reser-voirs is the widespread Pliocene clay sequence (Godavariclay), and the cap for the Pliocene reservoirs is the overlyingPleistocene clay. The Miocene and Pliocene growth faultscaused large anticlinal structures by triggering shale tecton-ics in the underlying Vadaparru shale. Rotated fault blocks,geomorphic highs, and unconformity-related trapping con-ditions are common. In addition, the numerous channel andfan geobodies generated stratigraphic or stratistructural traps.

Palakollu-Pasarlapudi system includes the PasarlapudiFormation of Late Paleocene-to-Early Eocene age, a mostimportant producer in Krishna Godavari Basin. The impor-tant finds are Pasarlapudi, Tatipaka, Rangapuram, andEllamanchilli gas fields and Mori oil field. WidespreadPalakollu shales are the main source rock and the overly-ing Pasarlapudi sandstone forms the reservoir. Bhimanapallilimestone overlying the Pasarlapudi reservoirs provides aneffective regional cap (Figure 11). At times intervening shalesalso act as local seals for the reservoirs. The shale tectonicsover the area caused a series of parallel-to-subparallel NE-SW arcuate faults. These faults provide an excellent updipseal for the reservoirs. The anticlinal structures, formed asa result of bulging of the underlying shales, are the best trapsfor the hydrocarbons.

Raghavapuram-Tirupati is the dominant system in thewest of the Krishna Godavari Basin. Exploratory drillingover Gudivada graben and Bantumilli graben confirmed oil,gas, and condensate from the Kaikalur, Lingala, Bantumilli,Nandigama, Mahadevapatnam, Gokarnapuram prospects.The Raghavapuram shale is the main source rock. Alter-nating sand and silt within Raghavapuram shale forms thereservoir. The sands within Raghavapuram shales are sealedby overlying and underlying shales. Tirupati sandstonereservoirs are capped by the overlying Razole Formation.Traps are mainly over the flanks of the horst. Wedge-outsand fault closures are also common. The sands within themounded geobodies are also favorable locales for hydro-carbon accumulation.

The Gajulapadu-Kanukollu system is restricted toGudivada graben. Discovered pools are few and small. TheGajulapadu shale is the source for the overlying Kanukollusandstone reservoir. The shales are moderately hard, poorlyfissile, and compact. The consistently distributed Kanukollusandstone is dominantly sandstone with minor shales. Sandsare fine- to medium-grained, subangular-to-subrounded,and fairly sorted. The Raghavapuram shale provides anefficient cap to the underlying Kanukollu sandstone reser-voirs (Figure 12). The reservoir shows updip wedgings

836 THE LEADING EDGE JULY 2006

Figure 14. Seismogeologic section of Mandapeta graben depicting structural entrapment of gas.

against the rising flanks of the Kaza and Kaikalur horsts,creating a stratistructural trap. The basement faults cutacross the reservoir section to create fault closures.

Kommugudem-Mandapeta/Golapalli is the oldest pet-roleum system of Krishna Godavari Basin. The importantgas fields are Mandapeta, Mandapeta West, and Endamuru.The thick and widespread Kommugudem coal/shale se-quence is a proven source. The overlying Golapalli andMandapeta sandstones are the main reservoir sequences. Thesandstones are feldspathic and micaceous with thin inter-calations of shale and claystone. The Raghavapuram shaleoverlying the Golapalli sandstone reservoir, widespreadand thick, provides a good cap to the reservoir (Figure 13).The Mandapeta sandstone is overlain by thick red claystonebelonging to the lower part of the Golapalli sandstone.Entrapment is due to fault closures and anticlinal structures(Figure 14).

Conclusion. The evolution of Krishna Godavari Basin beganduring the Permian when the linear-trending (NE-SW)Gondwana graben was formed. A major, long and linear(NE-SW), rifted graben and horst system was created dur-ing the Jurassic and named the Trans Godavari graben andTrans Godavari horst. The end of the Early Cretaceous wasmarked by dislocation of this horst and graben along fivemajor cross trends. A series of smaller en-echelon horst andgraben systems were formed from the existing singlemegasystem. The Late Cretaceous witnessed widespreadmarine transgression when, for the first time, the entire basi-nal area was under deposition. The Paleocene was charac-

terized by subaquous lava flow. The postvolcanic period wit-nessed active Tertiary sedimentation and, during this period,Eocene, Miocene, and Pliocene regional growth faults wereformed with a corresponding depocenter. Further basin-ward, the limit of Tertiary deposition is marked by a Plio-Pleistocene toe thrust.

Known hydrocarbon areas are classified into five petro-leum systems. The major source rocks are Early Permian,Cretaceous, Paleocene, and Eocene. Favorably placed clas-tic reservoirs are from Late Permian, Cretaceous, Eocene,Miocene, and Plio-Pleistocene. Both structural and strati-graphic traps are common in these systems.

Suggested reading. Genesis of petroleum systems in KrishnaGodavari Basin by Gupta et al. (AAPG 2000 InternationalConference). Pre rift, syn rift sedimentation and hydrocarbonpotentials of Krishna Godavari Basin by Gupta et al. (AAPG1997 International Conference). Geology and hydrocarbonprospects of Krishna Godavari and Cauvery Basin by Kumar(in Petroliferous Basins of India, ONGC, 1983). Krishna GodavariBasin Stratigraphy, Petroleum Geochemistry and PetroleumGeology by Robertson Research Group (ONGC report, 1987).Lithostratigraphy of Indian Petroleum Basin Document VIII, KrishnaGodavari Basin (ONGC Publication, 1993). TLE

Acknowledgment: The author is thankful to Oil and Natural GasCorporation, India.

Corresponding author: drgupta333@yahoo.com

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