Sub-Basalt Imaging Reveals Deeper Plays Offshore India

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30279, “Revealing Deeper Plays, Offshore Kutch, India: A Success Story of Sub-Basalt Imaging,” by S.K. Biswal, N.N.B. Naidu, and S. Basu, ONGC, et al., prepared for the 2020 Offshore Technology Conference Asia, originally scheduled to be held in Kuala Lumpur, 2-6 November. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission. The Deccan Traps volcanic province of India is considered one of the largest basalt-covered regions in the world but is essentially unexplored because of the limitations of conventional marine streamer P-wave seismic acquisition in imaging structures both intrabasalt and sub-basalt. In the complete paper, the authors demonstrate that, even with legacy marine streamer surveys, an appropriate work flow of combining suitable advanced technologies can help to overcome the long-standing challenges of sub-basalt imaging. The reprocessed data show clear uplift in sub-basalt imaging, and inversion results validate the quality of the new data in relation to the well logs. Introduction The Kutch offshore basin is characterized by the presence of the Deccan Traps, a large igneous province of up to 2000-m-thick basalt lava flows. These lava flows have hindered successful imaging of sub-basalt Mesozoic sediments for hydrocarbon exploration. To date, no single technique has been found to produce considerable improvements in deeper image quality. The solution lies in an appropriate combination of advanced technologies. The project consists of three legacy data sets acquired in 2004, 2010, and 2014 in the shallow-water area (water depth ranges from 25 to 50 m). Two of the surveys were shot in the north/south direction with six streamers having 100-m separation, 25-m shot spacing, 12.5-m receiver spacing, and 6-second record length. The third survey was acquired oblique to these with a similar acquisition geometry; however, it featured sparser 25-m receiver spacing and 8- second record length. These surveys were matched and merged before migration to ensure a seamless image across the surveys in the post-migration domain. A tailored processing work flow improved existing data quality significantly and provided new insights into the sub-basalt geology, thereby opening a new play to exploration and production. Challenges and Work Flow Sub-basalt imaging challenges include transmission losses, scattering, complex wave kinematics, prevalent multiples, interference effects, and variable illumination caused by high and variable acoustic impedance of thick heterogeneous basalt layers. The tertiary sedimentary sequences overlying the Deccan Trap consist predominantly of carbonates, shale, and fine-grained clastic sediments, accompanied by channels and nearly vertical faulting. The geological complexities from the water bottom to the base of the basalt present a substantial geophysical challenge to successful deeper imaging and require an appropriate work flow to mitigate them. Broadband processing, including de-ghosting, can increase the signal-to-noise ratio across the broad range of frequencies in the seismic bandwidth and can enhance the lower frequencies required to achieve enhanced imaging at sub-basalt targets. Demultiple methods can reduce the presence of surface-related and interbed multiples that prohibit reliable interpretation of Mesozoic sediment; imaging methods can focus the recorded data when used in conjunction with an accurate Earth model that captures the velocity complexities of carbonates, shale, basalt, channel, and faults.