Occurrence Of Overpressures And Its Implications For Hydrocarbon Exploration In

Abstract

Abstract Assam-Arakan basin having an area of approximately 100,000 sq km. is one of the main, proven and highly prospective petroleum province in the north eastern part of India. Upper Assam valley contains 7000 m of sediments under its alluvial cover. On the southern edge of platform, lies the mobile belt of Assam geosyncline with very thick sediments of the same age as in the shelf region. Over pressure horizons in northeast India occur mostly in the geosynclinal region in the western Tripura, Cachar as well as in schuppen belt and adjoining Arunachal Pradesh areas, mostly in the younger Tertiaries of Miocene-Pliocene age. In the Tripura area, abnormal high pressures to super high pressure horizons occur in Middle to Lower Bhubans of Surma group in almost all the structurs so far drilled. Deeper prospects are yet to be probed especially Barails which may have oil too, in case the popular belief of pressure regression brought out froni the PIVT studies, comes true. The studies, duly corroborated by weilsite evidences and seismics-VSP/PIVT for pressure prediction, have been quite successful for the pore pressure evaluation in this region. In the Schuppen belt (north-eastern part of Arunachal Pradesh) commercial production of oil is obtained from over pressure horizons of Mio-Pliocene age. Seismic and sonic log data, duly corroborated by the well site evidences, indicates very high pore pressure gradient up to 0.95 psi per ft in this area. Pressure reversal has also been observed in some of the structures isn the Tipams which may help in developing better porosities and likely to have good amount of exploitable reserves. High abnormal to super pressures are observed from Middle-Lower Bhubans, practically in all the structures of the Cachar area with pressure gradient reaching to almost geostatic or even exceeding. Compaction disequilibrium, aided partly by clay diagenesis has been responsible for generation of over pressures in Tripura area. However, in Cachar itnitial over pressuring took place due to compaction disequilibrium, subsequent major tectonic activity and uplifting of structures further intensified formation pressures. In the Schuppen belt of Assam and Arunachal Pradesh compaction disequilibrium as well as the tectonics has been responsible for generation of over pressures. The decreasing pore pressure gradients below the over pressures zones of Miocene-Pliocene age, as evidenced by the pressures regression plots of seismic/VSP and interpreted PIVT data, should encourage exploration for deeper prospect especially in the structures in Tripura and Cachar adjoining Bangladesh where oil is reported in some of the fields. P. 653