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Seismic Expression of the Canyon Fill Facies and Its Geological Significance---A Case Study from Ariyalur-Pondicherry Subbasin, Cauvery Basin, India*
By
S.K. Roy Moulik1 and G.K. Prasad1
Search and Discovery Article #10125 (2007)
Posted May 7, 2007
*Adapted from extended abstract prepared for presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007
1Oil and Natural Gas Corporation Limited, Dehradun,India ([email protected] )
Two-way time contour mapping at the top of Cretaceous-Tertiary boundary (K/T boundary) and the isochronopach of Sequence K3C reveal a canyon network at the K/T surface of Ariyalur-Pondicherry sub-basin, Cauvery basin, located on the southeast coast of India. At the end of Cretaceous Period, there was tectonic reorganization resulting in uplift causing marine regression throughout the basin. This caused incision of exposed shelf and the formation of submarine canyon at the shelf edge. Subsequently the canyon was filled during late Paleocene times. Five different types of canyon fill facies have been identified based on the seismic expressions of the canyon fill. Integrating the available well data with seismic and lithological interpretation of different types of canyon fill facies has been made and presented diagrammatically in this paper. Somewhere it is sand rich, at some places shale rich, and sometimes it consists of alternating sand and shale facies. The porosities and permeabilities of the sands are quite good as evidenced by the test data. A conceptual depositional model of these sands has been postulated. This model shows that sands were deposited in submarine canyon formed under the influence of gravity-driven mass transport processes like slumps/slides and debris flow.
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Studies on ancient submarine canyon system has gained importance since early 1970’s due to its prolific hydrocarbon occurrences as reported from different parts of the world. This paper presents an ancient example of a unique Paleocene canyon system with its various fill facies from Ariyalur-Pondicherry sub-basin, the northernmost graben of Cauvery basin located on the east coast of India (Figure 1). Based on seismic expression coupled with well calibration, five different types of canyon fill facies have been discussed and a sand filled area within the canyon has been delineated. Sands within the canyon have good petrophysical characters as indicated by well data. This is exceedingly critical in petroleum exploration in understanding the variety of canyon fill facies in order to direct exploration toward optimum stratigraphic traps and potential reservoirs.This study will certainly minimize the exploration risk to target the potential reservoirs within the canyon in this part of the basin. It should be mentioned that Tranquebar sub-basin, which is just due southeast of the present study area (Figure 1), is one of the prolific producer from a setting similar to that is mentioned above.
Cauvery basin, a poly-history divergent passive margin basin, is located in the southeastern part of the Indian Peninsula (Figure 1). It encompasses an area of about 25,000 sq.km. onland and 22,500 sq.km. offshore, extending up to 200m. isobaths towards the east. In the onland it extends from Pondicherry in the north to Tuticorin in the south. It is a symmetrical failed arm rift sandwiched between Peninsular India in the northwest and the Sri Lankan massif to the southeast, with its structural trend parallel to the adjoining Precambrian Eastern Ghat structural grain (Chari Narasimha et al., 1995). The basin is bounded by a steep basin-bounding fault on the western side, trending northeast-southwest, parallel to the axis of the basin (Figure 1). The Cauvery basin evolved in Late Jurassic-Early Cretaceous time as a result of rift-drift phenomenon of the then Indian plate from Gondwanaland. Evolution of this basin is genetically linked to the other simultaneously evolving extensional basins of East Coast of India; viz., Palar, Krishna-Godavari, Mahanadi, and Bengal basins (Rangaraju et al., 1991). Based on gravity data, the Cauvery basin has been divided by the previous workers into three depressions, each separated by intervening ridges; viz., Ariyalur-Pondicherry depression,Tanjore-Tranquebar depression, and Ramnad-Palk Bay depression (Figure 1).
Ariyalur-Pondicherry sub-basin hosts as much 6000 m of Early Cretaceous to Recent sediments as shown in the tectonostratigraphic chart of Figure 2. This graben has several major structural elements (Roy Moulik et al., 2006) (Figure 3), which are as below: · a) Northeast-southwest-trending narrow troughs, Vridhachalam and Chidambaram lows, mapped as basement level along western and eastern flank, respectively, · b) Andimadam horst and Neyveli high separated by a cross saddle run parallel to Vridhachalam low along western margin, · c) Bhuvanagiri nose in the central part of the main graben, · d) NE-SW trending main extensional fault, · e) Cross-faults trending NW-SE.
The focused study led to identification of 9 sequences (Roy Moulik et al., 2006); viz., K0, K1, K2A, K2B, K3A, K3B, K3C, T1, and Younger corresponding to Pre-Albian, Albian, Cenomanian,Turonian,Coniacian-Santonian, Campanian, Maastrichtian, Paleocene, and Post-Paleocene, respectively, for the entire sedimentary column of this sub-basin (Figure 4). Of the various sequence boundaries recognized in the basin, surfaces at the top of Turonian and the Cretaceous-Tertiary boundary are the most prominent and regionally correlatable surfaces, which helped in understanding the basin evolution. The depositional geometries of identified sequences mainly suggest three episodes of basin evolution as below: · 1) Extension Stage/Rift Stage (Late Jurassic-End of Turonian) Þ development of syn-rift sequences over the Precambrian basement during East Gondwana rifting (Sequences KO, K1, and K2 ). · 2) Early Thermal Subsidence Stage/Early Post-Rift Stage (Late Cretaceous and Paleocene) Þ minor NE tilt of the basin and development of the supersequence K3 and T1. · 3) Late Thermal Subsidence stage/Late Post-Rift stage ÞDominated by pronounced easterly tilt of the basin and development of Eocene and Younger sequences.
Over 3800 line-kilometers of two-dimensional (2D) seismic data, geo-scientific data of 30 wells, including sedimentological, petrological, and petrophysical input, and the available biostratigraphic data have been integrated in this study.
During the Late Maastrichtian-Early Paleocene period there was an extensive uplift of the Cauvery Basin. As a result, most of the shelf part (except the northern part; i.e., the study area) was brought under subaerial exposure (Kalyansunder et al., 1991), causing incision of the exposed shelf and the formation of a submarine canyon at the shelf edge. This has resulted in the development of the widespread erosional surface which is the prominent unconformity defining the boundary between the Cretaceous and Tertiary. Two-way time contour mapping on 2-D data on the Cretaceous-Tertiary boundary (K/T boundary) and the isochronopach of Sequence K3C (Figures 5 and 6) document a canyon network at the K/T surface. Similar submarine canyon accommodation occurs across many slopes and represents the space created by submarine erosion (Prather, 2003). Isochronopach map (Figure 5) shows that the thickness variation of sediments within the Sequence K3C (Figure 4) is almost uniform, on the order of 200-400 ms (represented by yellow color).Two isolated areas of greater thicknesses, on the order of 525-725 ms and 400-525 ms preserved in the northwestern part, are indicative of the main depocenters during deposition of sediments of K3C Sequence. Areas identified as the canyon are shown in blue color where the range of sediment thickness is on the order of 0-200 ms. However, the thickness of the adjacent areas varies between 200-400 ms. Therefore, it is evident that on an average about 200 ms thicknesses of older sediments has been eroded by the canyon. The time structure map prepared close to the top of sequence K3C (Figure 6) shows the fault geometry of the area. Two distinct major fault systems are clearly visible from the map. One set trending NE-SW is the main older extensional faults. There is another orthogonal fault system trending NW-SE that intersects the older NE-SW-trending set of faults. These faults are reactivated faults and are much younger than the syn-rift faults. Canyon geometry shows two sets of canyon systems defined by an unconformity (top Cretaceous unconformity) or master surface. The older canyon trending NE-SW, parallel to the trend of the older fault system, extends all along the basin from SW-NE. Two subsidiary canyons, trending NW-SE (which is also the trend of the younger cross faults), join the main canyon from the western boundary. Eustatic rise and fall of sea level coupled with local tectonic influences are the causes of the formation of the submarine canyons and subsequent deposition within the canyon. In general, the erosional activities were found to be more pronounced during the formation of the younger canyon system, as evidenced by the width of the canyons. Younger canyons are comparatively much wider than the older one. This canyon has been found to be filled by Paleocene sediments. One seismogeological section has been prepared along the seismic line S1-S2 (line location shown in Figure 1) to demonstrate the vertical and spatial distribution of canyon fill facies within sequence T1 (Figure 7). The internal seismic expression of the canyon fill deposits reflects acoustic impedance contrasts indicative of lithology differences within the canyon. Based on the seismic characters calibrated with the available well data, five different types of canyon fill facies have been identified (Figure 8). Interpretation of the various identified fill facies is summarized as below:
Thus it is seen that the high-amplitude and chaotic seismic events broadly corresponds with sandy lithologies. The sand/shale ratio map of the sequence T1 shows the sand-rich areas within the canyon (Figure 9). This map matches well with the lithology prediction from the expression of seismic facies. It is observed that submarine fan facies (Fill-III) which are very good reservoirs can be found at the distal part of the submarine canyon system, and sand deposition commonly coincides with periods of canyon erosion. Paleobathymetric curve (Figure 10) indicates that the sediments were deposited in deep water environment. Based on available cores, cuttings, and subsequent integration with electric logs and seismic data, depositional model for the canyon fill sediments has been proposed (Figure 11). Mass transport, gravity-driven processes, dominated by channelized debris flow with slumps and slides, have been identified as principal depositional processes in a bathymetry ranging from 200 to 250 m .This channelized mass transport model is a highly predictive one for locating sand-prone areas within the canyon, and it thus has an important implication for reservoir geometry and correlation. Therefore, this study would certainly minimize the exploration risk to target the potential reservoirs within the canyon in this part of the basin.
The authors are highly indebted to ONGC Limited for giving an opportunity to work on this project. Authors express their gratitude to D.K. Pande, Director (Exploration) for permission to publish this paper. This work could not have been successfully completed but for the valuable support and guidance provided by Jokhan Ram, Executive Director and Chief KDMIPE. Authors also acknowledge the encouragement given by Manoj Asthana, GM. Support/inferences taken from the reports of various authors is also gratefully acknowledged. Views expressed in this paper are that of the author(s) only and may not necessarily be of ONGC.
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