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Reconstructing Sedimentary Depositional Environment with Borehole Imaging and Core: A Case Study from Eastern Offshore India Chandramani Shrivastva and Sanchita Ganguly, Schlumberger; and Zuber Khan, GSPC

Copyright 2008, International Petroleum Technology Conference This paper was prepared for presentation at the International Petroleum Technology Conference held in Kuala Lumpur, Malaysia, 3–5 December 2008. This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, IPTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax +1-972-952-9435.

Abstract Establishing the depositional sedimentary environment is the most important task for exploration geologists to model the reservoir heterogeneities. Interpretation of borehole images has been the key to better understanding of the sedimentary environment in the study area in Krishna-Godavari basin (KG basin) along the east coast of India. The present study aims at reconstructing sedimentary depositional environment with the help of image logs and cores and other available data set. Data analysis and integration of borehole images in 9 wells of the study area present a detailed insight into the different architectural elements of the sedimentary environment envisaged. This study helps in static modeling of the reservoir with better understanding of process sedimentology that controls the reservoir properties of sands.

The study area has been interpreted from Lower to Upper Cretaceous. The major lithofacies identified are sandstone (massive, laminated and cross-bedded), shale (thin laminated and slumped), siltstone (laminated) and heterolithics (thin alternation of sand/silt and shale). The vertical association of these facies in conjunction with azimuthal variation in dip patterns and image texture led to identification of different architectural elements of the system. The sediment paleotransport direction is governed by the rifting episodes that have subdivided the KG Basin into a complex array of horst and graben structures. The lower cretaceous formations in the study area have been interpreted to be of fluvio-deltaic setting with good sands development in channels and delta distributaries. The upper cretaceous formations are more of shallow marine setting with sand developments mostly in tidal channels, bars and sandy flats.

The study helped in understanding the heterogeneities in the petrophysical properties of different sand bodies encountered in the study area. The control of sedimentary depositional environment in spatial distribution of sand bodies and their geometries is better understood in the study area with the help of image logs.

Introduction The eastern offshore of Indian peninsula (Figure 1) has become a major exploration target after substantial hydrocarbon discoveries of late. The study area of this work is offshore Krishna – Godavari basin (KG Basin) which has taken the centre stage after major oil and gas discoveries in thick clastic successions from Mesozoic to Cenozoic in various depositional setting. 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. The basin contains thick sequences of sediments with several cycles of deposition ranging in age from Late Carboniferous to Holocene, stretching from onland to offshore to deepwater. The basin is divided into sub basins by fault-controlled ridges. Sediments accumulated in sub basins more than 5 km thick.

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Figure 1: Location of KG Basin and study area This stretch of sedimentary tract contains a vast range of geologic settings, such as costal basin, delta, shelf-slope apron, deep-sea channel, and deepwater fan complex (Bastia, 2001). A proper understanding of the depositional environment becomes very important for a static reservoir modeling. The reservoir sands distribution pattern and their extent are largely governed by the sedimentological processes for their deposition and the prevailing structural grains. These factors do exercise their controls on the variability in petrophysical properties of the sand units. To model these heterogeneities in petrophysical behavior, the facies model must honor the sub surface geology. High resolution borehole images and core data acquired in the study area provide insight into different sub-environments of deposition for various sand units encountered. Background Geology: A decent overview of the background geology is imperative before attempting high resolution interpretation of borehole data. The KG basin was a major intracratonic rift within Gondwanaland until Early Jurassic (Rao, 2001). It evolved as a composite of rifted horsts and grabens, beginning in Late Jurassic, and formed a part of the development of the east coast divergent margin. The horsts and grabens were separated by vertical or steeply dipping faults. Since the Cretaceous, it has become a pericratonic rift basin. The initial rifting-drifting phase during this time generated fluvio-lacustrine sediments all over the basin. The rift phase terminated by the end of Turonian in most parts of the basin, and subsequently, the post-rift sedimentary sequences prograded to the east with development of a continental shelf-slope system. The shelf areas received deposits of clastics and carbonate sediments while the slope and deepwater basinal part registered submarine fan sediments. This setting, during which progradation was dominant, persisted throughout the Tertiary. The Paleocene and Eocene, in part, are considered to have been deposited during sea-level low stands, thus forming fan complexes. From Oligocene onward, sea level began to rise, and more accommodation was available. The deepwater area became structurally deformed by numerous sets of growth faults and related features. In most areas, a major décollment surface is present near or at the top of the Eocene and marks a major tectonic event (Bastia, 2001). A series of growth fault systems progressively developed, with increased sediment influx during the Oligocene through Miocene time. The onset of the Pliocene is marked by major sea-level fall and a prominent erosional surface. These low stand conditions prevailed into the Pleistocene. Regional Stratigraphy: The regional stratigraphy and the petroleum systems based on the previous works can be summarized in Figure 2. Many discoveries have been made in KG Basin onland in Permo-Triassic and Cretaceous sequences. The regional stratigraphy is well understood in these parts. Seaward in the KG basin the nomenclature of different stratigraphic unit varies with the operating companies. Reservoir potential of the barrier bars, tidal bars and deepwater channel-fan complexes is already established in the offshore KG basin by many workers. The stratigraphy as revealed in this study area after drilling and logging exploration wells is also summarized towards the right hand side in Figure 2. The zone of interest lies from lower to upper Cretaceous.

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Figure2: Regional and study area stratigraphy for KG Basin. The N-S seismic section in Figure 3 shows the structures and the reflectors in the field. The well trajectories have been marked for a few exploration wells. The horst and graben structures of the rift basin are aptly marked.

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igure 3: Structural framework of the study area on a N-S seismic line

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Methodology and Data Used: The borehole micro-resistivity images and core data are the pivotal dataset used in the study in conjunction with the conventional open hole logs of resistivity, density, gamma ray etc and master log. The borehole images in the conductive mud systems provided a vertical resolution of 0.2 inches whereas the borehole images in oil based mud systems provided a vertical resolution of 1.2 inches. This high resolution of images helps in identification of small-scale sedimentary features and their interpretation based on their association with lithofacies. The methodology applied to derive results from the borehole images can be put in the form of a flow-diagram as shown in Figure 4. The borehole images are displayed in the shades of black to yellow corresponding to the micro-resistivity distribution from conductive to resistive. A green curve running on the image log is gamma-ray and is displayed from left to right increasing. A dip track is associated with the image logs where the tadpoles describe the azimuth and magnitude of the dipping plane. Figure 4 also illustrates how a plane is visualized on a 2-D unwrapped borehole image presentation, the format used in this study. The borehole images and core data complement each other in the interpretation, since coring and imaging was not done in every well. Therefore concepts developed with the core data have been applied to the image interpretation and vice versa. In fact, borehole images could not be acquired in many high temperatures, high pressure wells in greater depths. The most important contribution of the image data has been the understanding of dip directions in sand bodies. Many architectural elements of the system have been interpreted with the classical dipmeter interpretation techniques.

Figure 4: Methodology for Image log presentation and interpretation Broad Lithofacies in the study area Six lithofacies were identified with the help of image logs and the core.

1. Massive sandstone: massive sandstone facies are identified on the basis of absence pf any sedimentary features. They could be a structure less high energy dumping of sands or thoroughly bioturbated sequence where the internal sedimentary structures have been destroyed. The image log shows a very resistive, bright yellow response in the massive sands with no internal structure whatsoever and a blocky gamma-ray profile. Towards the top of the interval a few dips could be picked which suggest that the sands were deposited dipping south-easterly. This interval is devoid of any bioturbation and is interpreted as high energy sand dump.

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Figure 5: Massive sandstone, as seen on core and borehole image.

2. Laminated sandstone: These sandstones are differentiated on the basis of thin sands lamination, well to very well sorted, fine to medium, sub-angular to sub-rounded, parallel laminated to lowangle cross-laminated sandstones. Minor carbonaceous flecks are common throughout. Sometimes the laminae appear wavy, associated with slight soft sediment laminated sandstones are indicative of deposition via weak traction currents. The image log exhibits overall a resistive appearance of sandstone, with slightly reddish planes marking the lamination boundary. The sands dip southerly with a dip amount of 6-10 degrees.

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igure 6: Laminated sandstone, as seen on core and borehole image

3. Cross-bedded Sandstone: These sandstones are identified with the help of tabular or trough cross-bedding in the formation. Planar cross-beeded sandstone is characterized by medium to thickly bedded, angular to sub-rounded, moderately sorted coarse to medium sandstone with steeply dipping foresets. Bed boundaries are also highly inclined. Some of the bed bases contain granule- to pebblesize lags. Local aligned granule- and pebble-size grains result in a characteristic bimodal grain size. Varying amounts of mudstone rip-up clasts, wood fragments and carbonaceous fragments also occur. This lithofacies represents the migration of 2-D dunes under upper flow regime conditions. Sand is transported up the flack of the bedform by traction and deposited at the crest. Upper and lower boundaries are typically flat, with little evidence of scouring.

Trough cross-bedded sandstone is characterized by thinly to thickly bedded, moderately to moderately well sorted, fine to coarse, sub-angular to sub-rounded sandstones with small and large scale trough cross-beds (Figure 7). Lags occur locally and contain carbonaceous detritus, clasts up to medium pebble size and granule lags. Carbonaceous specks, micas, rip-up clasts and plant detritus are also common locally. Some highly slumped beds associated with soft sediment deformation. Dewatering structures also occur locally, particularly in the more fine to medium sandstones, with fluid-escape pipes and dish structures observed locally. Trough cross-bedding is formed by migrating 3-D dunes in shallow water settings reflecting upper flow regime conditions. Soft sediment deformation suggests rapid sedimentation. The borehole image shows high-angled (10-20 degrees) forsets dipping towards south-east in a probable braid bar.

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Figure 7: Cross-bedded sandstone, as seen on core and borehole image

4. Siltstone: the siltstone lithofacies is found to be both laminated and massive. This lithofacies comprises of carbonaceous material and sand grains floating. The image log shows siltstone facies as a primarily argillaceous facies with a considerable sand amount that makes the image appear in shades of yellow between black and bright yellow.

Figur

e 8: Siltstone, as seen on core and borehole image

5. Heterolithics: Heterolithics are thin alternations of sand/shale or sand/silt or silt/shale. They are found extensively throughout the field. Mud content varies, with a combination of muddy heterolithics Clay drapes are often reworked into elongate mudclasts. Lenticular bedding is locally abundant in argillaceous units. Pinstripe lamination and soft sediment deformation structures including load casts also occur. Soft-sediment deformation is evident throughout, as is extensive injection of sandstones into adjacent siltstones and claystones Deposition of alternating sandstone and mudstone indicate variable flow regimes and sediment supply locally.

Figure 9: Heterolithics, as seen on core and borehole image

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6. Shale: Shale is the most predominant facies throughout the field and represents non-reservoir facies. This represents argillaceous facies of both continental and shallow marine settings. The continental shale is represented by abandoned channel fill and inter-distributary fines. Marine shale is found in the suspension settling of pro-delta muds. They are both massive and laminated, and soft sedimentary deformation is common.

Figure 10: Shale, as seen on core and borehole image

Association of lithofacies Fluvial and Deltaic facies were identified for sands deposition in the core and borehole images. Shallow marine settings for tidal channels and bars were also interpreted in a couple of wells on the image log. Two main types of fluvial channel fill deposits are interpreted for KG wells in Cretaceous, aggradational fluvial channel fill and fining up sequences. Aggradational fluvial channel fill This lithofacies association is characterized by dominantly medium to coarse grained, thick to very thickly bedded, primarily trough cross-bedded sandstone. Horizontally bedded and massive bedded sandstones form a secondary component to this association. In general laminated sandstones and siltstones are rare to absent. Examples of a series or aggradational channel fill deposits are illustrated in Figure 11. Many of the sandstones are highly granular and pebbly throughout, with numerous granule- and pebbly-rich lags (Figure 11). Trough cross-bedding is the most common structure, with thicknesses of sets decreasing upwards, from 30-100cm thick, to 3-10cm thick (cut and fill structures). These deposits are characterized by a lower erosive surface, often overlain by a conglomeratic lag or zones with mudstone rip-up clasts, and overlain by a fining-upward unit of medium to coarse sandstones. Cemented and uncemented mudclasts and rock fragments are common, some up to cobble size. Fluid-escape pipes and injected sandstones are generally absent within these strata. These deposits are thought to represent the deposits of an unconfined sandy –braided fluvial system. The coarse grain sizes, such as gravels and coarse sand, form the dominant load. The borehole image in Figure shows three bright yellow instances of high resistivity where gamma ray gets cleaner. These are interpreted as channel sands which fine upwards. The lowermost sand unit shows a high angle tadpole at the bottom of the interval which marks an erosional base. The sequence is a typical channel sequence where argillaceous fine overlie towards the channel top. The sands in this channelised sequence dip south-easterly with a dip amount of 5-10 degrees. The upper two sands are stacked 1-m scale channel sands with southerly dipping beds.

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Figure 11: Stacking of channel facies, as interpreted on core and borehole images

Fining-Upward Channel Deposits This lithofacies association is characterized by medium to thickly bedded trough cross-bedded and horizontally stratified fine to medium sandstones (usually bimodal), alternating with thinly bedded laminated very fine to fine sandstones. Individual packages display fining-upward trends. Beds often display evidence of intense soft-sediment deformation including over steepened/folded bedding fabrics, sometimes totally overturned, and dish structures locally. Typically, individual packages display a large-scale fining-up stacking pattern, with increasing horizontally stratified and parallel laminated sandstones dominant towards the top. This lithofacies association is interlayered with aggradational channel fill deposits in many intervals. They typically alternate with thin and very thick suspension deposits, many highly mottled. A typical section of fining upwards channel fill is illustrated in Figure 12. These deposits are thought to represent the deposits of a more confined fluvial system than that described previously. The stacked channel successions represent repeated cut and fill within the channel. The main depositional element is the point bar, which builds laterally and downstream across the flood plain. The highly erosive bases of the channel sandstone units reflect currents with initial erosive power (at peak flood) and the capacity to transport a relatively coarse sand population as a series of migrating, decimetre-scale dunes under unidirectional flows as bedload. Upward-fining grain-size profiles, combined with corresponding upward decreases in the scale of trough cross bedding and abundance of ripple stratification, are interpreted to reflect a decrease in equilibrium flow velocities through time. This can result from either lateral migration of the point bar, or more commonly, from channel abandonment. The upper borehole image in the figure on right hand sideshows a channelised sequence of 10-m scale with very important information on dip azimuth. Westerly to south-westerly dipping beds indicate the current direction, whereas the north-westerly dipping sands are interpreted as the lateral accretionary surfaces of a point bar. Towards the top of the interval, southerly to south-easterly dipping horizontal to sub-horizontal beddings were interpreted as levee element. The lower borehole image shows two 1-m scale channel sands with south-easterly to easterly dipping beds. The upper channelised sand is deformed towards the top as indicated by the dip trend.

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Figure 12: Channel sands, interpreted on core and borehole images Delta front lithofacies association Proximal delta front facies association comprises a series of stacked, sandy heterolithics comprising parallel, hummocky and ripple-laminated sandstones alternating with laminated mudstones, representing deposition below fair-weather wave base in a proximal delta front setting (Figure 13). The more wavy and hummocky laminae are thought to represent possible storm generated Hummocky Cross Stratification type bed forms in sandstone. The lack of bioturbation in the system suggests a substantial freshwater influx, a feature which is common at the front of deltas, as opposed to an equivalent inner shelf setting, with no deltaic influence. The substantial soft sediment deformation is typical of instability associated wit rapid sedimentation at the front of a delta. Distal delta front lithofacies association comprises dominantly muddy heterolithics, dominated by siltstones and claystones with thin parallel- and ripple laminated sandstone units and thin mm-scale stringers. As for the proximal delta front strata, soft-sediment deformation is also a common feature of the more distal strata, with faulting and folded/over steepened laminae associated with rapid sedimentation and instability at the front of a delta. Borehole image shows the onset of a prograding bar, where a coarsening upward sequence is identified on gamma-ray and image. This represents a 10-m scale sand body with south-easterly dipping sands of distributaries and mouth bar amalgamated in the sequence.

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Figure 13: Deltaic facies, interpreted on core and borehole images Shallow marine lithofacies association The shallow marine lithofacies association in this context is interpreted with the help of image logs in conjunction with the broad understanding of the setting developed. The snapshot on the left in Figure 14 illustrates a tidal bar development with sands dipping northwards, whereas the snapshot on the right is an example of mixed tidal flat. Extensive sandy tidal flats could also be prospective reservoirs, but scarcity of data limits the interpretation.

Figure 14: Tidal facies on the image log. Depositional Model An attempt has been made to understand the process sedimentology and architectural elements of the sand bodies encountered in the study. Neither the core, nor the borehole imaging was available in every interval. Therefore the different results were weaved through to prepare the conceptual depositional model for the sand intervals. Overall the fluvial sequences of Early Cretaceous represent the deposition of a low-sinuosity sandy braided system, with the section characterized by a dominance of stacked channel deposits, with little to no argillaceous facies. Inter-bedded argillaceous and sandstone lithofacies is thought to represent the shift to a more meandering or moderate sinuosity fluvial system. Within sandy meandering systems the channel and bar sediments are dominantly sand, although some intraformational conglomerates are present as channel-floor lag deposits. Within the channels some sandstone sheets are made up of thin, overlapping channel-fill bundles or accretionary wedges separated by scours, indicating considerable channel instability. Not every element of point bar or lateral accretions could be identified separately for the lack of data. The shallow marine deposition of Late Cretaceosu is represented within a proximal to distal tidally influenced deltaic system.

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Tidal channel and bar sands have been interpreted in the borehole images. A detailed analysis of trends observed in dips data (Figure 15) in conjunction with image texture revealed that the predominant direction of sand transport is south-easterly, with a wide variance caused by different architectural elements of the depositional setting. The paleo-shoreline, thus could be interpreted to be broadly east-west trending. The cross-beddings identified on the image log strongly suggest a southerly transport direction, both in fluvial channels and delta progradation. The northerly dipping sands are interpreted in many places as tidal channel sands, though splay elements of meandering fluvial systems were also observed in the study area.

Figure 15: Dip azimuth rosette and magnitude histogram plot for KG-wells. The different sand units were thus interpreted for their depositional environment. The overall depositional model is envisaged from fluvio-deltaic to shallow marine in the Cretaceosu sequences in the study area in KG Basin. The image log and dips on a compressed scale are shown for two wells in Figure 16 where a broad diposition of facies is illustrated along the logged interval. The image log on left hand side shows a fluvio-deltaic setting broadly whereas the image log on right shows more of a shallow marine setting.

Figure 16: Image log and dips compressed in two well-sections to illustrate disposition of facies.

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Conclusion: The integration of core and borehole images has been instrumental in developing an understanding of the depositional sedimentary environment in the KG basin of Eastern Offshore India. Core and images complement each other and compensate for each other where either of the two is not available. The conceptual model, thus prepared can be applied to the study area to understand the reservoir heterogeneity. Sand body geometries and petrophysical properties in different sub-environments vary and so do vary the sand dispersal direction and sedimentation axis. This study shows that the sand facies varies from channel to tidal channel, then sand bars & heterolithics in between, blocky sands and deltaic sands. These vertically stacked facies could therefore be laterally associated with each other indicating a fluvial to shallow marine depositional environment. Broadly the palaeo-current direction is towards south-east; hence paleo-shoreline could be envisaged as E-W to NE-SW trending. The depositional settings indicate various reservoir types, each of which could have different reservoir characteristics. The different sand units thus interpreted for their depositional environment could be taken to static reservoir modeling where the petrophysical model needs to honor the geology of the sub surface in order to attain the neares approximation to the reservoir behavior. Reference: Bastia, R., An overview of Indian sedimentary basins with special focus on emerging east coast deepwater frontiers. The

Leading Edge (July 2006), pp. 818 – 829. Rao, G.N., Sedimentation, stratigraphy, and petroleum potential of Krishna-Godavari basin, East Coast of India. AAPG

Bulletin, v. 85, no. 9. (September 2001), pp. 1623–1643. Internal Core Reports on KG wells, GSPC Acknowledgements: Authors are thankful to the Managing Director, GSPC for their kind approval for this technical paper. They also acknowledge their colleagues at GSPC and Schlumberger for all their support in preparation of this manuscript.