Geological and Geophysical Integrated Study of Western and Central Tripura for Identificatrion of Hydrocarbon Plays and Prospects

Abstract

Though the geological maps of Tripura region is prepared by using geological survey and seismic survey including satellite image and interpretation of aerial photographs by geoscientists of Oil and Natural Gas Corporation (ONGC) during the period 1968 to 2010, the challenges of understanding the hydrocarbon prospectivity in that region is still remaining. In this research work an attempt has been made to understand the petroleum prospects and drilling challenges of Central and Western Tripura using integrated techniques from regional information, seismic data, geochemical data outcrop data and pressure data. Tripura is a more gas prone area however possibility of presence of oil over middle Bhuban needs serious examination. Examining the seismic sections, field mapping, photo geology, drill logs, cuttings and cores and integrating all the data have lead to development of seismogeologic model of central and western Tripura. Aeromagnetic survey conducted by ONGC in 1988 has helped to bring out the basement configuration. The Bouguer gravity anomaly map shows that Tripura as a whole and the study area lies on regional gravity high trending NNE-SSW. Landsat data and balanced cross section are analyzed to understand the seismic signatures and sub-surface geological structures. To understand the various geological formation and sand shale percentage distribution in the study area, a systematic study of isopach map of middle Bhuban formation, lower Bhuban formation, upper Bhuban formation, Bokabil formation and Surma sequence have been prepared. The prediction of pore pressure is a major goal of wildcat well planning. In the study seismic velocities are used to predict pore pressures by using modified Eaton’s methods over the synclinal and flank part of Atharamura to understand the pressure succession towards the anticline. The data from two seismic sections, synclinal and flank parts of the Atharamura anticline are collected. Both seismic sections are oriented in the directions of West – East and input data contains, Two – Way Travel time and average (normal) velocity values for various Common Depth Points (CDPs). In this study we observed that, these predicted pore pressures from velocities on the flank lead to a reasonable match when plotted with offset well measured pore pressures. It is also observed that the most of the wells which has shown the pressure match have been located in the western side of the Atharamura. Through this study we concluded that Tripura region is characterized by single pressure source and distributed evenly in all the anticlines in this region. We suggested that, to plan a safe well over Atharamura, prediction of pressures on the top of Anticline is essential. To understand seismic anisotropic parameters of the sedimentary rocks in the study area, four core samples have been analyzed. The anisotropic analysis suggested that the sedimentary rocks in the study area are highly anisotropic. The Thomsen’s parameters are also estimated from these four core samples. The study also suggested that Poisson’s ratio and Young’s Modulli are strongly affected by the Thomsen’s parameters. The linear relationships among the Thomsen’s parameters are also established for each core samples. To substantiate the anisotropic analysis investigation was corrected on to develop structural disposition of stratigraphic horizons and faults. Detachment folding followed by backthrust with detachment at the base of middle Bhuban is the plausible model that worked best as an answer to anisotropy. The subsurface geometry when integrated with surface geochemical study, geophysical study, pore pressure understanding and anisotropic understanding of rocks lead to a fairway model. The model will help exploarationist to convert leads into prospects and prospects to drillable prospects.