Sedimentation, stratigraphy, and petroleum potential of Krishna-Godavari basin, East Coast of IndiaG. N. Rao AUTHOR G. N. Rao Oil and Natural Gas Corporation, 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 at the Indian School of Mines for an M.Tech. degree in petroleum exploration during 1984. He has experience in analyzing hydrocarbon prospectivity in all the eastern divergent margin basins of India. His interests include global tectonics in relation to basin evolution for petroleum exploration and genesis of abnormal formation pressures. Rao has associated with Soviet specialists in assessing the hydrocarbon resources of India and with the Institute Francais du Petrole Paris team for ¸ ´ thrust-pach modeling for fold belts of the northeastern convergent margin of India. ACKNOWLEDGEMENTS I thank ONGC Ltd. for providing the data and permitting to publish the same. I also thank S. N. Talukdar, former member (exploration), and P. K. Chandra, former vice-chairman of ONGC, for critically reviewing the manuscript. The guidance provided by K. Satyanarayana of ONGC and C. Kasipathi of Andhra University and help from James Peters of ONGC in shaping the work is gratefully acknowledged. I thank the AAPG reviewers Bob Reynolds, Ben Law, and Mauren Wan for their constructive criticism of the manuscript. Thanks to Firoz Dhotiwala of Kesava Deva Malvya Institute of Petroleum Exploration for providing expert technical help in presenting the Landsat images. Finally I wish to express my deep sense of gratitude to A. W. Balley of Rice University, Houston, Texas, for his encouragement and valuable guidance in reshaping the text. ABSTRACT The Krishna-Godavari basin is located in the central part of the eastern passive continental margin of India. The structural grain of the basin is northeast-southwest. Exposures of Upper Cretaceous sedimentary rocks demarcate the basin margin toward the northwest, where the northwest-southeast–trending Pranahita-Godavari graben abuts the basin. The basin contains thick sequences of sediments with several cycles of deposition ranging in age from Late Carboniferous to Holocene. A major delta with a thick, argillaceous facies that has prograded seaward since the Late Cretaceous is a hydrocarbon exploration target. Magnetic and gravity data predicted the basin architecture, which was subsequently confirmed by a multichannel seismic survey. The basin is divided into subbasins by fault-controlled ridges. Sediments accumulated in subbasins more than 5 km thick. Above the basement ridges, thin sediments are found. Until the Jurassic period, sediments were deposited in the rift valley and in topographic lows. This sequence is completely overlain by a Lower Cretaceous, transgressive sedimentary wedge. Later, continued delta progradation characterized basin sedimentation. With an areal extent of approximately 45,000 km2, this proven petroliferous basin has potential reservoirs ranging in age from the Permian to the Pliocene. Exploratory drilling of more than 350 wells in more than 160 structures has resulted in the discovery of 42 oil and gas bearing structures. Good source rocks are known from sequences ranging in age from Permian–Carboniferous to early Miocene. Because the reservoir sand bodies have limited lateral variation, understanding the stratigraphy and depositional subenvironments in different sequences is essential to decipher the favorable 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 November 9, 2000. AAPG Bulletin, v. 85, no. 9 (September 2001), pp. 1623–1643 1623 INTRODUCTION The Krishna-Godavari basin, a pericratonic basin, is located along the East Coast of the Indian peninsula. It includes the deltaic plains of the Krishna and Godavari rivers and the interdeltaic regions. Geographically, the basin lies between Kakinada in the northeast and Ongole in the southwest. Archean crystalline basement and Upper Cretaceous sedimentary outcrops demarcate the northwest basin margin. The basin extends southeast into the deep water of the Bay of Bengal. A significant part of the onshore basinal area is covered by Quarternary alluvium (Figure 1). Geologists in the Geological Survey of India such as Blandford et al. (1856) and King (1881) were the first to study outcrops in the Krishna-Godavari basinmargin area. Later, Vasudeva Rao and Krishna Rao (1977) described these outcrops in detail and interpreted tectonic settings and depositional environments for the basin. Murty and Ramakrishna (1980) used geophysics to describe the subsurface geology. Venkatarangan and Ray (1993) recognized exploration targets and the petroleum system in the basin. Scientists from the National Institute of Oceanography (Murty et al., 1995) studied the geodynamic aspects of the offshore Krishna-Godavari basin and identified the offshore extension of the northwest-southeast–trending pre-Cretaceous rift graben along the cross-trends. I also contributed to previous hydrocarbon exploration efforts, summarizing the geological evolution of the basin, proposing depositional models for hydrocarbon reservoirs (Rao, 1991), and formulating a new lithostratigraphic nomenclature for the area (Rao, 1993a, b, c). In this article I interpret the depositional system of the Krishna-Godavari basin within a sequence stratigraphic framework. The basic data for this study include lithologic and electric logs of deep wells, seismic sections, and analyses of sedimentologic features in conventional cores and in basin-margin outcrops. METHODS OF STUDY Outcrops were examined for detailed lithofacies variations, deposition cycles, and sedimentary structures. Conventional cores were examined for sedimentary structures to identify depositional processes. Crossplots of core data (Visher, 1969) were used to differentiate depositional environments. Depositional patterns were presented in the form of isopach maps. Where conventional cores were absent, interpretations were made on the basis of logs obtained from 26 wells, which were designated names from the English alphabet (A to Z). As a whole, the wells cover the entire basin, and a few penetrate down into the Archean Figure 1. Location map of the study area, Krishna-Godavari basin. To the northwest the sedimentary exposures include (1) Lower Triassic–Upper Permian Chintalapudi Sandstone; (2) Lower Cretaceous–Upper Jurassic Gollapalli and Budavada sandstones; (3) Lower Cretaceous Raghavapuram and Vemavaram shales; (4) Upper Cretaceous Tirupati Sandstone; (5) Deccan traps with Upper Cretaceous–lower Paleocene intertrappean beds; (6) Miocene– Pliocene Rajahmundry Sandstone. 1624 Petroleum Potential of Krishna-Godavari Basin basement. To understand the depositional environment, good quality seismic sections were tied to the well data. Age markers identified in samples of the deep wells were calibrated using lithomarkers identified in the time sections. This method allowed for the accurate identification of depositional patterns within the marked sequences. REGIONAL GEOLOGICAL SETTING Plate Tectonic Model The basin was a major intracratonic rift within Gondwanaland until the Early Jurassic. When Gondwanaland rifted apart, the eastern margin of the Indian peninsula was positioned at latitude 50 S and was oriented in an east-west direction (Chatterjee and Hotton, 1986). Since the Cretaceous, the Indian plate has moved northward, and the eastern continental passive margin rotated 20 in a counterclockwise direction (Gordon et al., 1990) until it collided with Eurasia in the late Eocene (Srivastava and Chowhan, 1987). The triple junction between the Indian peninsula, Australia, and Antarctica is located at Masulipatnam Bay (Figure 1) (Thompson, 1976). The northwest-southeast– trending Pranahita-Godavari graben (Figure 2) formed a failed arm of the triple junction (Burke and Dewey, 1973). Since the Cretaceous, the basin has become a pericratonic basin. Its thick, fluvial sediment load was associated with the faulting of basement blocks due to the reactivation of northeast-southwest–trending Precambrian faults (Biswas, 1992). These differential vertical block movements allowed magma to rise through and facilitated emplacement of the Deccan traps. Figure 2. Bouguer gravity map of the Krishna-Godavari basin, showing tectonic elements (Shenoi and Rao, 1982). KB Krishna subbasin, 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. Rao 1625 During the Tertiary, the deltaic system generally prograded to the southeast, although some deltaic lobes have shifted in direction in response to changing rates of sediment influx and growth faulting (Rangaraju, 1987). Basin Architecture Based on Bouguer gravity data, Murty and Ramakrishna (1980) have identified three subbasins separated by two basement horsts. From the southwest, these are the Krishna, West Godavari, and East Godavari subbasins separated by the Bapatla and Tanuku horsts, respectively (Figure 2). The West Godavari subbasin is further subdivided into the Gudivada and Bantumilli grabens, which are separated by the Kaza-Kaikaluru horst (Kumar, 1983) (Figure 3). The Kommugudem and Mandapeta troughs are situated on either side of the Tanuku horst (Figure 2). The Krishna subbasin contains 1560 m of Cretaceous and older sediments above the Archean basement. Bapatla horst lies between the Krishna and the West Godavari subbasins. Many lower Mesozoic sequences are thin over the Bapatla horst and lie disconformably with erosional contact, which suggests that the Bapatla horst was uplifted during the Cretaceous. The Bapatla horst is discontinuous in the northeastern part of the basin. In the West Godavari subbasin, Cretaceous sediments are thin over the Kaza-Kaikaluru horst, compared to the thick section on either side of the horst in the Bantumilli and Gudivada grabens (Figure 4a, c). The data suggest that the Kaza-Kaikaluru horst remained uplifted during the Early Cretaceous. Tanuku horst lies between the West Godavari and East Godavari subbasins. The sedimentary cover over the Tanuku horst is about 2500 m in the southeastern flank of 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, litholog correlations, 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. 1626 Petroleum Potential of Krishna-Godavari Basin This trend may indicate that the horst plunges southwesterly into Masulipatnam Bay. In contrast, Lower Cretaceous strata dip monoclinally toward the southeast over the Tanuku horst, suggesting the absence of any tectonic uplift of the horst since the Cretaceous. Deep drilling to the southeast of the Tanuku horst has documented the presence of a thick sand-shale sequence with coal beds, characteristic of the Barakar Formation of the Carboniferous to Permian. In the East Godavari subbasin, the sedimentary fill ranges from 2900 m over the preexisting basement horsts, to more than 5000 m in the deep basin area in the southeast (Prabhakar and Zutshi, 1993). The 2000 m of argillaceous sediments of the Cretaceous in the southeast might have been excessive enough to enhance tectonic subsidence and down-faulting of sequences and form a series of en echelon faults in the East Godavari subbasin during the Late Cretaceous– early Paleocene (Figure 4b). Subsidence in the southeastern parts of the East Godavari subbasin may have also contributed to the formation of a steep step-fault zone in early Paleocene basalts. This fault zone is known as the Matsyapuri-Palakollu fault zone (MPFZ). Tectonic readjustment has resulted in terraces and tilted fault blocks in the northeastern area of the basin. The discontinuity of the Bapatla horst in the northeastern part of the basin is associated with the presence of a northwesterly orientation of gravity contours and strongly negative gravity anomalies. This suggests the possible continuation of the northwest-southeast–trending Pranahita-Godavari graben beneath the northeast-southwest–trending KrishnaGodavari basin. The bounding faults of the Pranahita-Godavari Gondwana graben are known by the extents of the older sedimentary outcrops in the basin margin area. These fault trends can be traced up to the Tanuku horst, based on available seismic data. These northwest-southeast–trending bounding faults of the earlier rift valley are termed the Chintalapudi and Pithapuram cross-trends in the northeast-southwest–trending Krishna-Godavari basin (Figure 2). Based on offshore magnetic data, these cross-trends can be identified up to the deep-water area of the Krishna-Godavari basin, and the ocean-continent boundary (OCB) was marked accordingly (Murty et al., 1995). The OCB defines the southeastern termination of the northwest-southeast–trending Pranahita-Godavari graben (Figure 3). Both the Kommugudem and Mandapeta troughs are deep pre-Callovian downwarps (Figure 2). In these troughs, Upper Carboniferous–Lower Triassic sediments were deposited in varied depositional environments ranging from glacial to marginal marine conditions. The two troughs can be differentiated in that the Mandapeta trough experienced Late Jurassic– Cretaceous downwarping, but the Kommugudem trough did not. SEDIMENTATION Outcrop Stratigraphy Lower Permian–Upper Carboniferous outcrops exist farther to the northwest, within the PranahitaGodavari graben. They include the Talchir beds comprising greenish sand-shale alternations, which are overlain by the Barakar Formation with its characteristic coal-sand-shale sequences (Raiverman et al., 1986). Outcrops in the basin margin area of the Krishna-Godavari basin include the Permian Chintalapudi Sandstone, which consists of cross-bedded, loosely cemented, variegated shales (Figure 5). The Chintalapudi Sandstone is overlain by the Maleris Formation, a red arenaceous facies (Lakshminarayana and Murty, 1990). Cretaceous outcrops in the northeastern margin of the basin include the Gollapalli Sandstone, Raghavapuram Shale, and Tirupati Sandstone. Equivalent outcrops in the southwestern margin are the Budavada Sandstone, the Vemavaram Shale, and the Pavaluru Sandstone, respectively (Figure 1). The Gollapalli sandstones are ferruginous and micaceous, with a paleocurrent direction of 10–20 to the northwest (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 found in the Raghavapuram Shale (Bhalla, 1967; Radhakrishna, 1977). The Tirupati sandstones are feldspathic toward the top and ferruginous toward the bottom. Lenticular clay beds, petrified wood, and cross-bedding characterize the sandstone, which has a paleocurrent direction of 5–10 dip toward the southeast. Basalt lava flows with Upper Cretaceous–lower Paleocene intertrappean beds are overlain by the Miocene–Pliocene Rajahmundry Sandstone. The Rajahmundry Sandstone is red, feldspathic, ferruginous, laterized, cross-bedded, and conglomeratic and directly overlies the Deccan basalts (Figure 5). Rao 1627 Figure 4. Litholog correlations of drilled wells in the Krishna-Godavari basin. Single letters are well names. (a) Southwest-northeast profile in strike direction of the basin showing sediment fill in subbasins and basement horst blocks. (b) Northwest-southeast–trending dip profile across East Godavari subbasin with thick pre-Cretaceous sediments toward the northwest and a thick Tertiary section toward the southeast of the MPFZ. Continued. 1628 Petroleum Potential of Krishna-Godavari Basin Figure 4. Continued. (c) Northwest-southeast– trending dip profile across West Godavari subbasin with thick argillaceous Tertiary sediments toward the southeast of the MPFZ. ROCK STRATIGRAPHIC NOMENCLATURE Lithomarkers and Age Boundaries Wherever faunal age details were not available, litholog variations were considered to differentiate the formation boundaries. Distinctive lithomarkers in the subsurface of the basin include the Archean basement, pre-Cretaceous red bed, lower Paleocene basalts, and middle Eocene limestone. These lithomarkers, as observed in well data, were tabulated (Table 1). The Archean basement consists of mainly gneisses and granites. Locally named the Khondalites, the Archean basement gneiss is composed of quartz, mica, feldspar, and garnet. Locally named the Charnockites, the Archean granite consists of quartz, feldspar, biotite, and hypersthene. Within the fluvial preCretaceous section, the presence of thick coal beds, along with sand-shale sequences, characterizes the Permian–Carboniferous strata underlying the homogeneous Permian–Triassic feldspathic sandstone. Faunal data, along with the red bed, delineate marine Cretaceous sequences from pre-Cretaceous nonmarine sections. Detailed paleontology of cuttings established the age of basalt flows including intertrap sediments as ranging from 68 to 61 Ma (Raju et al., 1994). Govindan (1980) identified the Paleocene–Eocene boundary, and Vijayalakshmi (1988) used faunal studies to demarcate the base of the Miocene boundary. A prominent transgressive clay bed is chosen as a lithostratigraphic marker to demarcate the top of the Miocene. Although rock stratigraphic nomenclature is used in outcrop descriptions as illustrated in Figure 5, the subsurface data generated from the exploratory drilling needed codification. Lithofacies variations in wells are referred by geological age and lithology. The comprehensive rock stratigraphic nomenclature, as I suggested (G. N. Rao, 1990, unpublished data) for the major lithofacies variations drilled in the subsurface, follows the guidelines provided by the North American Stratigraphic Code (1983). The lithofacies variations of a given geological time, that is, the proximal arenaceous and basinal argillaceous sequences, were classified separately. The suggested rock stratigraphic nomenclature with subsequent minor modifications (R. Venkatarangan et al., 1993, unpublished data) is shown in Figure 6. The pre-Cretaceous Chintalapudi Sandstone is a recognizable, distinct unit in outcrop. In the subsurface, however, identifying the Chintalapudi Sandstone Rao 1629 Figure 5. Lithostratigraphic nomenclature used at the basin-margin exposures and tectonic phases and sea level changes in Krishna-Godavari basin. See text for descriptions of seismic sequences. from seismic data alone is not as easy. Consequently, different rock stratigraphic nomenclature is suggested for the Chintalapudi Sandstone, along with the Talchir and Barakar formations: the three lower Gondwana lithofacies. Sediments equivalent to the Talchir Formation are found in well C and have been named the Draksharama Shale. The shale is greenish black and underlies a coal-sand-shale section. The maximum thickness of sediments equivalent to the Barakar Formation was found in well A. This facies is named the Kommugudem Formation and consists of thick coalsand-shale beds. The major arenaceous facies overlying 1630 Petroleum Potential of Krishna-Godavari Basin the Kommugudem Formation are named the Mandapeta Sandstone, and the type section is in well B. The arenaceous facies, which has a distinct red claystone bed, overlies the Mandapeta Sandstone and is discontinuous in the basin. The red claystone bed separating marine and nonmarine facies in the basin is named the Red Bed. In well A, Cretaceous formations are known to correlate with outcrops. Hence, separate nomenclature is not suggested for the three subsurface lithofacies of the Cretaceous. The Tirupati Sandstone, however, laterally changes into a massive claystone toward the Table 1. Lithostratigraphic Correlation of Wells Considered for the Study Well Name Shown in Figures A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Name of the Structure Kommugudem Mandapeta Draksharama GS-17 Bhimanapalli Amalapuram Ravva Tanuku Jonnalanka Pasarlapudi GS-20 G-5 Palakollu Razole Chintalapalli Mori GS-19 G-13 Gajulapadu Kaikaluru Bantumilli GS-11 Nimmakurru Mantripalem Bobbarlanka GS-38 Drilled Depth (m) 4508 4302 3145 4020 3007 4003 3172 3145 3504 3902 2500 3208 4501 4501 4500 3200 2442 4110 3811 1972 3650 4625 3071 2850 4258 3517 Well Bottomed in (geological age) Carboniferous Archean Archean Late Cretaceous Early Paleocene Late Cretaceous Early Paleocene Archean Late Cretaceous Late Cretaceous Early Paleocene Miocene Late Cretaceous Late Cretaceous Late Cretaceous Paleocene Miocene Paleocene Pre-Cretaceous Archean Archean Late Cretaceous Archean Archean Archean Eocene Top of Archean Basement (m) – 4246 2892 – – – – 3047 – – – – – – – – – – – 1935 3575 – 2980 2720 4187 – Top of Basalt (m) 90 738 966 – 3000 – – 745 2802 3575 – – 2576 3363 3568 – – – 725 965 1484 – 885 1001 – – Top of Limestone (m) – – 684 – – 2798 – – 1224 1555 – – – 1683 1667 1518 1518 – – – – – – – – – southeast part of the MPFZ (Figure 4). In well O, the thickest section of the claystone was penetrated and is termed the Chintalapalli Claystone. Basalt flows with intertrappean beds are named the Razole Formation. The postbasalt section located in the northwestern section of the MPFZ is mainly arenaceous and cannot be subdivided into lithofacies because of a lack of recognizable marker beds. These Paleogene arenites are termed the Nimmakuru Sandstone. Between the MPFZ and the Krishna Amalapuram high trend (KAHT) passing through wells F and Z, the Paleocene and Eocene sections have distinct lithomarkers (Figures 2, 3). The Paleocene section is mainly shale and is named the Palakollu Shale. The Eocene section can be divided into a lower silty facies (Pasarlapudi Formation), middle limestones (Bhimanapalli Limestone), and upper arenites (Matsyapuri Sand- stone) (Figure 7). Southeast of the KAHT, the Eocene section is argillaceous and is named the Vadaparru Shale. Although the Oligocene sequence is very thin, it is easily identified and can be used to mark the base of the Neogene sediments. The Oligocene sequence consists mainly of claystone with limestone and sandstone beds and is termed the Narasapur Claystone. The Miocene–Pliocene section is known as the Rajahmundry Sandstone in outcrops and in the subsurface in the proximal part of the basin. In the distal subcrop, it changes into a major argillaceous facies with sandstone beds, from which oil and gas are being produced in the Ravva field. Hence, it is termed the Ravva Formation, which is equivalent to the Rajahmundry Sandstone (Figure 7). The Quarternary–Holocene alluvial cover in the continental part of the basin is called the Andhra Rao 1631 alluvium, and the offshore clay sequence overlying the Ravva Formation is known as the Godavari clay. SEQUENCE STRATIGRAPHY Using the geological age markers, the sedimentary fill of the basin was divided into five sections: preCretaceous, Cretaceous, Paleocene, Eocene, and Miocene and above. To identify depositional styles within each section, geological age boundaries of lithocolumns were transferred onto seismic sections (Vail et al., 1977). Sea level fluctuations in the basin, along with sequence boundaries, are shown in Figure 5. The pre-Cretaceous section is divisible into two units: a lower sand-shale sequence with thick coal beds (PC-I) and an upper sandy unit (PC-II). The Archean basement forms the base of the lower unit whereas the top of the unit is an unconformity surface (Figure 8a.). A red claystone separates the fluvial preCretaceous and the marine Cretaceous sections. The Red Bed is an unconformity and represents the breakup of Gondwanaland. The strong acoustic impedance in the lower sequence generated strong seismic reflections in the coal beds. The divergent reflectors indicate the subsidence of the basin floor during deposition. Because PC-I and PC-II were deposited in a half graben, the divergence of depositional events suggests synsedimentary reactivation of a bounding fault in the graben. The hummocky clinoform reflections suggest a fluvial origin for the sequences. The strong, discontinuous, parallel reflection patterns within PC-II indicate stable conditions of the basin floor during deposition. The top of the Cretaceous is marked by basalt lava flows. Well data indicate that the Cretaceous section consists of bottom sandstone, middle shale, and upper sand-shale units. These three lithologic units are demarcated as seismic sequences C-I, C-II, and C-III, respectively, to clarify depositional conditions. The lowest sequence, C-I, is very thin and underlies a thick marine transgressive shale. This is the Gollapalli Sandstone, which was deposited in geomorphic lows prior to the major transgression in the basin. This sequence is absent over the Precambrian highs, such as the Draksharama high toward the northeast part of the basin and the Kaza-Kaikaluru horst toward the southwest. The data show that these horsts were elevated during deposition of C-I. Reflections within C-I toward the northwest are either weak or absent, suggesting the sequence consists of fluvial sand. In contrast, in the 1632 Petroleum Potential of Krishna-Godavari Basin Figure 6. Lithostratigraphic nomenclature of sedimentary sequences penetrated in the basin. Wherever the continuity of the outcrop lithologic unit with the subsurface could be established, the same name is retained; otherwise, new rock stratigraphic nomenclature is suggested. southeastern part of the basin, reflections are strong and continuous, indicating that C-I may have been deposited in fluviomarine conditions. The top of C-II represents a regressive surface over which the downlap of the overlying sequence is observed. The reflection character is strong and contin- Figure 7. Structural cross section across the East Godavari subbasin showing thick Tertiary sediments toward the southeast of the MPFZ as seen in well G in the offshore part of the basin; toward the northwestern basin margin area at well A, thick pre-Cretaceous sediments were encountered. uous within the sequence, which indicates marine depositional conditions. The C-II reflection pattern is parallel up to wells B and C, which suggests stable depositional conditions. Farther to the southeast, the divergent C-II pattern is a response to passive margin subsidence. Nearer to the basin margin area, the C-II reflections are weak or absent, suggesting that the sequence consists of sand that was deposited in marginal marine conditions. This sequence is equivalent in time to the Raghavapuram Shale. C-III represents Upper Cretaceous deposition. In the northwestern basin margin area, it is a major arenaceous unit; reflections are weak or absent, and marginal marine to fluvial depositional conditions may have existed. C-III becomes a thick (more than 2000 m) claystone in the southeastern part of the East Godavari subbasin, where the reflection character is strong and has a divergent pattern, suggestive of synsedimentary subsidence (Figure 8b). The P seismic sequence represents Paleocene deposition. The top of the sequence has a strong reflection character and is directly below the base of the Eocene section. The unit is thin and absent in the northwestern part of the basin, where wells B, H, T, and W were drilled. The shelf edge is interpreted to be southeast of wells U and H. Strong reflections within the P sequence may represent sandshale alternations. Shale diapirism may also be seen near well F (Figure 8c). The Oligocene–Eocene section is represented by the EO seismic sequence, although the thin Oligocene sediments (40–100 m) could not be identified in seismic sections. The EO sequence onlaps over the Paleocene sequence, suggesting a marine transgression. The Eocene shelf edge is demarcated southeast of well F. The divergence reflection pattern in the southeastern part of the basin may be related to passive margin subsidence. The reflection-free areas within the carbonate section indicate possible reefal buildups in the EO sequence. Toward the basin, shale diapirism is observed in the EO sequence. An erosional surface marks the top of the Miocene–Pliocene (MP) sequence in the offshore part of the basin. Seismic signatures within the sequence may indicate channels, growth faults, and rollover structures of a deltaic system (Rangaraju and Yalamarty, 1984). Reflections in the proximal part of the basin are weak, whereas they are strong and divergent in the distal part, suggesting a basinal subsidence. DEPOSITIONAL ENVIRONMENTS Pre-Cretaceous Isopachs of PC-I and PC-II show a maximum thickness in well A (Figure 9). PC-I and PC-II are two distinct lithofacies: the lower sand-shale-coal facies and the upper feldspathic sandstone. In well A, PC-I is 2000 m thick (Rao et al., 1993). In wells drilled outside the Pranahita-Godavari graben (wells S, X, and Y), PC-II is absent, and the sandy facies directly overlie the Archean basement. The electrolog patterns suggest mainly coarsening-upward sands containing some fining-upward sequences. PC-I and PC-II comprise nonmarine deposits. Observed log patterns indicate fluvial channels with point bars (Figure 10). To understand depositional environments, granulometric studies of samples taken from conventional cores were carried out using the methods of Visher (1969) and Rao 1633 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 seismic sequences. (b) Southeastern part of Tanuku horst (Mandapeta trough) showing older rift fill sequences are superimposed with Cretaceous sequences. (c) Thick Tertiary sequences toward the southeast of the MPFZ. Figure 9. Isopach map of pre-Cretaceous sediments of the Krishna-Godavari basin. The sequence comprises three distinctive lithologic units: A lower coal-bearing unit (Barakar/Kommugudem formations); B middle arenaceous unit (Chintalapudi/Mandapeta sandstones); and C upper red beds that represent a breakup unconformity. A thick Paleozoic graben is seen at well A toward the northwestern part of Tanuku horst. Folk and Ward (1957). The core data indicate that deposition occurred in inland river channels. Petrographically, the sandstones were classified as quartzarenites (Dott, 1964), with a grain composition of more than 70% quartz, 10% feldspars, and an unspecified amount of lithic fragments. The matrix is clay rich and siliceous in places. Cretaceous The isopach map of Cretaceous sediments indicates that the unit is thicker toward the southeast (Figure 11). In outcrop and in the subsurface, Cretaceous sediments can be divided into three distinctive sequences: C-I is the lower sandy facies, C-II is composed of mainly shales, and C-III is the upper sandy facies. Electrologs show a coarsening-upward pattern in the lower sandy unit, a predominantly fining-upward pattern in the middle shale unit, and both patterns in the upper sandy unit. Petrographically, the rock types are lithic arenite, mudstones, and quartzwackes. The matrix is generally clay rich and micaceous. The dominant clay minerals are kaolinite and illite. Other minerals in Cretaceous sediments include pyrites, tourmaline, and garnets. Grain-size parameters indicate the deposition of Cretaceous sediments in low-energy conditions within a fluvial system. Two prominent basement highs, one at KazaKaikaluru and the other at Draksharama, remained positive during the Early Cretaceous, resulting in thin deposition over these highs (Figure 2). This correlates to the absence of the lower sandy unit (Mohinuddin et al., 1993). Both seismic sections and electrolog correlations show that a peneplanation surface existed over paleohighs at the base of the middle argillaceous unit (Raghavapuram Shale). Crossplots of K2O and F2O3 suggest that the upper sandy unit was deposited in fluvial conditions in the proximal part of the basin. The upper unit laterally becomes argillaceous toward the southeast under marine depositional conditions. Paleocene Basalt forms the basin floor for Paleocene sediments. The unit is not outcropped anywhere in the basin. In the subsurface, the P sequence is dominantly sandy up to the northeast-southwest–trending MPFZ (Figure 3). In contrast, the P sequence is mainly argillaceous southeast of the fault zone. Isopach maps indicate a depocenter in the area of wells O and P (Figure 12). Electrologs depict a blocky SP log pattern and a predominantly coarsening-upward sequence (Rao et al., 1996). Statistical parameters derived from granulometric studies indicate a marginal marine depositional environment. Paleocene sedimentary rocks consist mainly of quartzwackes. Laths of muscovite, glauconite, secondary calcite, pyrites, and zircons have been observed. The presence of limestone beds in wells I, Rao 1635 ditions. Electrolog patterns indicate a coarseningupward sequence. The thick middle limestone may indicate a shelf edge carbonate facies during the middle Eocene. The granulometric data show that fluvial conditions may have existed for the lower argillaceous unit, and nearshore conditions may have existed for the upper arenaceous unit. This would indicate that a transgressive phase occurred in the late Eocene due to minor fluctuations in sea level. In the thin section, the forams (nummulites) are found to be embedded in micrite, and recystallization of spar is seen. Trace element analysis shows elevated concentrations of CaO and MnO, which indicate that marine conditions may have existed in the area of wells I and E. The Oligocene sequence is very thin, having a maximum thickness of 150 m. Sedimentary rocks consist of sandstone, siltstone, claystone, and occasional limestone. O sequence sediments can be identified by wavy SP patterns in electrologs. The Oligocene apparently experienced a major regression event associated with upwarp, as evidenced by thin sedimentation. Miocene Isopachs of the Miocene sequence show a depositional center around the Ravva field offshore (well G). Another depositional lobe is identified near the Krishna River mouth (Figure 14). The data suggest that the Krishna and Godavari rivers were separated during the Miocene (Satyanarayana et al., 1996). Thin section petrography indicates that Miocene sedimentary rocks consist of quartzwackes with glauconite and benthonic/planktonic forams. Electrolog patterns indicate a coarsening-upward pattern. Trace element data (Ni/Co) correlated with granulometric data show that the lower part of the M sequence may have been deposited in nearshore conditions. In contrast, trace element data suggest that marine conditions existed in the upper part of the M sequence. This suggests marine transgression occurred sometime during the late Miocene–early Pliocene. Figure 10. Electrolog correlation of pre-Cretaceous sequences (profile location in Figure 3). Lithologic units identified by log motifs are indicated. At well B, a downthrow of unit 2 is seen, whereas because of uplift followed by erosion it is absent at well C (Draksharama high). Older electrofacies 3, 4, and 5 indicate postdepositional uplift of areas around wells A and C. N, and Z suggests a shelf edge environment during the Paleocene. Eocene Isopachs of the Eocene sequence show a northnortheast–south-southwest–aligned depositional center, where well O has the thickest section (Figure 13). The depositional center is found to be shifted toward the northeast compared to the Paleocene depositional center. The Eocene sequence in the subsurface is a thin sandy unit near the MPFZ. Between wells M and P in the East Godavari subbasin, the Eocene sequence is thicker and can be divided into the lower argillaceous, middle carbonate, and upper arenaceous units. In the lower argillaceous unit, thin sands show ripple marks in the top layers, whereas the lower layers contain claystone pebbles and exhibit burrowing and channel filling, which may indicate nearshore depositional con1636 Petroleum Potential of Krishna-Godavari Basin PETROLEUM POTENTIAL Hydrocarbon Occurrence Initial successful hydrocarbon exploration in the Krishna-Godavari basin was in thin Upper Cretaceous reservoirs in the Narasapur structure of the East Godavari subbasin. Exploration efforts since 1978 have Figure 11. Isopach map of the Cretaceous sequence of the Krishna-Godavari basin. The sequence uniformly thickens toward the southeast. The unit is not completely drilled toward the southeastern part of the basin. Three distinctive lithologic units are identified: lower arenaceous (Gollapalli Sandstone), middle argillaceous (Raghavapuram Shale), and upper arenaceous (Tirpupati Sandstone), which varies to argillaceous facies toward the southeast of the MPFZ (Chintalapalli Claystone). Single letters are well names. Figure 12. Isopach map of the Paleocene sequence showing a depocenter at well V, Masulipatnam Bay. Up to the MPFZ the unit is sandy, and toward the southeast it is mainly argillaceous. Single letters are well names. established oil and gas reservoirs ranging in age from Late Permian to Pliocene (Rao, 1991). Sikka (1990) used the probabilistic model to estimate the undiscovered hydrocarbon potential of Cretaceous and Tertiary plays in the Krishna-Godavari basin to be 726 million tons. The estimate took into consideration basin analysis, chance of success of individual plays, risk analysis, and Monte Carlo simulations. Petroleum System Analysis By analyzing the hydrocarbon occurrence in the basin in relation to petroleum system classification, four systems can be identified. They are the KommugudemMandapeta–Red Bed, Raghavapuram-Gollapalli-Razole, Palakollu-Pasarlapudi-Bhimanapalli, and the RavvaGodavari petroleum systems. Locations of oil and gas Rao 1637 Figure 13. Isopach map of the Eocene sequence with a major depocenter at the well P coastal tracts of the Godavari river. Also note the shift of the depocenter toward the northeast, compared to the Paleocene depocenter. Single letters are well names. Figure 14. Isopach map of Miocene and younger sequences. Two distinctive depocenters lie at the mouths of the Godavari and Krishna rivers. Single letters are well names. fields are shown in Figure 15. Geological ages of reservoirs are indicated in Table 2. The geologically oldest petroleum system is the pre-Cretaceous Kommugudem-Mandapeta–Red Bed (Rao, 1994). This system is confined to the northwestsoutheast–trending rift valley extending beneath the East Godavari subbasin. The hydrocarbon potential of the system is estimated to be 330 million tons. The 1638 Petroleum Potential of Krishna-Godavari Basin source rocks yield mainly gas. The system is associated with many erosional unconformities. To date, only 20 million tons in reserves have been established. The apparent lack of high-amplitude anticlinal closures and permeability barriers currently impede new exploration efforts. The Cretaceous petroleum system is named the Raghavapuram-Gollapalli-Razole system. In the West Figure 15. Hydrocarbon bearing prospects of the Krishna-Godavari basin. For geological age of the reservoirs and field names refer to Table 2. Godavari subbasin, thin sands and limestones within source facies are reservoirs. In the East Godavari subbasin, lenses of sands in the Chintalapalli Claystone produce hydrocarbons. Southeast of the MPFZ, the system was undercompacted during the Eocene. The petroleum potential has been estimated to be 230 million tons, of which so far only 15 million tons in reserves have been established. The Paleogene Palakollu-Pasarlapudi-Bhimanapalli petroleum system is the most prolific system in the Krishna-Godavari basin. Located southeast of the MPFZ in the East Godavari subbasin, the system contains abnormally pressured source rocks and normally pressured reservoirs (Rao and Mani, 1993). Anticlinal closures serve to entrap hydrocarbons. The estimated hydrocarbon resources are estimated to be 300 million tons. To date, about 80 million tons in reserves have been established. The Neogene petroleum system is called the Ravva-Godavari System. The most promising area for commercial hydrocarbon production is offshore. The abnormally high geothermal gradient has caused lower Miocene sediments to mature and generate hydrocarbons. The estimated hydrocarbon resources are estimated to be 200 million tons. Nearly 70 million tons in reserves have been established thus far. Source Rocks About 200 m of source rocks were identified in Permian–Carboniferous coal-shale sediments. The source rock quality ranges from poor in well C to very good in well B, where the source rock was in the oil generative window during the Permian and is presently in the metagenetic stage (Brahmajirao et al., 1991). The Lower Cretaceous shales are oil prone. This facies is 800 m thick in well A. Based on the time temperature index (TTI), source rocks in the area around well B have been capable of generating liquid hydrocarbons since the late Eocene. Upper Cretaceous argillaceous rocks located in the southeastern onshore basin have also matured to generate gaseous hydrocarbons since the Eocene and are also good source rocks. Paleocene shales of the East Godavari subbasin contain dominantly type III kerogen with 2–3% organic matter and have fair to good source potential. The level of maturation is in the early to peak oil phase of generation. Gas and subordinate oil-prone facies have been identified in Paleocene sediments. Lower Eocene shales located southeast of well F contain high organic matter concentrations (TOC 3– 4%). Geochemical studies reveal that they are in the early phases of maturation. The quality of organic Rao 1639 Table 2. Geological Age of Hydrocarbon Fields in Krishna Godavari Basin Onland Geological Age Pliocene Miocene Oil Gas Oil 1. G-1 2. G-2 3. Ravva 4. GS-15. 5. GS-23 6. GS-29 7. GS-38 Oligocene Eocene 10. Mori 8. Adavipalem 9. Kesavadasupalem 11. Elamanchili 12. Tatipaka 13. Pasarlapudi 14. Ponnamanda 15. Mulikipalli 16. Kadali 17. Magatapalli 18. Medapadu 19. Kesanapalli 20. Bandamurlanka 21. Rangapuram 22. Lankapalem 23. Mummidivaram 24. Achanta 25. Razole 26. Palakollu 28. Enugupalli 29. Narasapur 30. Penumadam 31. Chintalapalli 32. Kavitam 35. Kaza 36. Vadali 37. Mahadevapatnam 38. Gokarnapuram 39. Bantumilli 40. Nandigama 41. Endamuru 42. Mandapeta 1650 1800 2400–2700 Offshore Gas Average Depth (m) of HC Bearing Zone 1700** 900–2560 Paleocene 27. GS-8 2000–2500 Late Cretaceous Early Cretaceous 33. Kaikaluru 34. Lingala 2400 3400 3400 4450 4380 1800–2000 3300 4000 Late Jurassic Late Permian 1700 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. These source rocks have a tendency to generate gas and subordinate oil and have generated hydrocarbons since the early Miocene (Neeraja et al., 1997). Offshore, lower Miocene–Oligocene source rocks are identified in the Ravva area, where well G was 1640 Petroleum Potential of Krishna-Godavari Basin 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 facies, however, is rated very poor for generating commercial quantities of liquid hydrocarbons (Philip et al., 1991a, b). Reservoirs Potential Krishna-Godavari basin reservoirs range in age from Early Permian to Miocene–Pliocene. The oldest reservoirs are thick prerift Permian sandstones that overlie Permian–Carboniferous source beds. Thin sandstone beds in Lower and Upper Cretaceous source shale beds have proven to be reservoirs capable of producing both oil and gas. Lower Eocene sandstones that overlie Paleocene source beds have proven reserves. Miocene and Pliocene deltaic sand beds in the Ravva area are good oil producers possibly because hydrocarbons have migrated from a deeper source. Trap Styles and Play Types Seismic surveys and geological mapping indicate that although structural traps do exist in the KrishnaGodavari basin, most of them are only small to medium in size. In contrast, updip pinch-outs, unconformity surfaces, and permeability barriers all play an important role in the entrapment of hydrocarbons. The oldest gas-producing reservoir is the Permian Mandapeta Sandstone. The thickness of the reservoir is more than 2000 m. Fault-controlled structures are common, but simple amplitude reversals are uncom- mon. The dominant factor for entrapment is the permeability barrier, as evidenced in the presence of quartz overgrowths observed in thin-section petrography; however, the red bed overlying rift-fill sediments acts as regional seal. Upper Jurassic–Lower Cretaceous sandstones are truncated against preexisting basement highs. The Raghavapuram Shale, which is the overlying Lower Cretaceous transgressive shale, may exist as a regional seal for these reservoirs. Thin limestone and sandstone beds deposited within the Raghavapuram Shale are reservoir rocks. Thin sandstone beds within thick Upper Cretaceous claystone beds are potential reservoirs in the East Godavari subbasin. Stratigraphic plays include turbidite fans located southeast of the MPFZ. Basalt may form the regional seal for all the Cretaceous sediments. Anticlinal closures serve as traps for lower Eocene reservoirs. Interbedded shales may serve as local seals for these reservoirs. The thick middle Eocene carbonate may also act as a regional seal. Sandstone beds in the Miocene–Pliocene sediments are interspersed with clay beds, which may act as local seals for entrapment; however, growth faults and associated rollover anticlines are more effective traps. The Miocene–Pliocene erosional unconformity Figure 16. Conceptual hydrocarbon play types in the Krishna-Godavari basin. Play concepts include (1) permeability barriers; (2) fault closures in Upper Permian reservoirs; (3) clastic wedges and associated unconformity traps in Upper Jurassic (Gollapalli Sandstone) fluviomarine sediments; (4) sand lenses within Cretaceous and Paleocene argillaceous facies; (5) anticlinal closures in lower Eocene nearshore clastic reservoirs; (6) growth faults, erosional cut, and rollover anticlinal accumulations in Miocene–Pliocene clastics in the shallow marine area of the basin. Regional seals: I red beds, II Raghavapuram shales, III Deccan basalts, IV middle Eocene carbonates, V Pliocene clays. Rao 1641 surface, prominent in seismic sections, is overlain by thick Pliocene unconsolidated clay beds, which form a regional seal (Figure 16). CONCLUSIONS The subsurface data reveal the presence of a thick sediment fill in the basin and the extension of the northwest-southeast–trending Pranahita-Godavari graben underneath the northeast-southwest–trending Krishna-Godavari basin between two major crosstrends. The continuation of the basin is visualized to reach the OCB, located in the deep-water area of the basin. The southeastern part of the basin is a major Tertiary depositional center because of a series of down-to-the-basin faulting during the early Paleocene. Because of change in the gradient, major delta progradation is not seen in the area during the Paleocene and early Eocene. The rapid sediment fill in the low has facilitated smooth progradation of the delta toward the southeast since the middle Eocene. Seismic facies analysis of different sequences suggests typical passive margin subsidence. The positions of shelf edges through the geological ages have been marked. The Neogene depositional system has bifurcated into two separate depocenters in the basin since the Miocene. The lithofacies variations within marked sequences were discussed in detail and a lithostratigraphic nomenclature is proposed for the entire sedimentary column of the basin. In the basin, good source rocks are identified in the sequence ranging from Permian–Carboniferous to lower Miocene. The envisaged depositional patterns of the basin through geological ages help to identify locales of good reservoir facies in the vicinity of source facies. The basin has good hydrocarbon potential, and exploratory efforts to locate oil and gas pools through seismicstratigraphic analysis may enhance the discovery rate. REFERENCES CITED Bhalla, S. N., 1967, Foraminifera from the intertrappean beds (lower Eocene) of the Pangidi area, India: Micropalaeontology, v. 23, p. 351–368. Biswas, S. 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