Geomechanics Helps in Identifying Hydrocarbon Potential and Compaction Disequilibrium of Eocene Reservoirs – Case Study of a Geologically Complex Plateau in South Asia

Abstract

Abstract Reservoir geomechanics is essential for cost-effective drilling in complex areas, reducing risks related to wellbore stability problems, and other issues related to abnormal pore pressure, efficient reservoir modeling and enhanced oil recovery (EOR) plans. The information of major sources of overpressure generation like tectonics, under compaction, hydrocarbon accumulation and presence of an effective seal is necessary to apply reservoir geomechanics. The present study comprises of structural interpretation and comparative petrophysical evaluation of the Chorgali Formation with special emphasis on reservoir geomechanics for this Eocene reservoir in an oil field. This field is in the Potwar Plateau, included in the list of areas of the world containing over-pressured sedimentary rock. The main objective of this research work is to determine the pore pressure using well log data for two wells. In this model sonic transit (DT) time is correlated to recognize the compaction history for more accurate pore pressure prediction and to analyze the dependence of compressional wave velocity on vertical effective stress, stiffness of material and pore pressure. Pore pressure prediction by using well log data is much more affordable and gives continuous and extensive range of data than direct measurement by modular dynamic tester, drill stem test and repeat formation tester, etc. One dimensional (1D) well concentric mechanical earth model (MEM) is developed for reservoir geomechanics of Eocene carbonates. Seismic interpretation results reveal thrusting and popup structures in the area due to compressional tectonic regime. Petrophysical analysis discloses that the Chorgali Formation has better reservoir characteristics as compared to the Sakesar Formation. Compaction disequilibrium in argillaceous layers and hydrocarbon accumulation in reservoir zones are primary sources of overpressure. The well log-based pore pressure estimation shows good agreement with the in-situ measured formation pressure obtained from modular dynamic tester (MDT). The variation in rock physics properties such as bulk modulus, Young's modulus, shear modulus and Poisson's ratio are also modeled to delineate the high pore pressure zones. Reservoir geomechanics results illustrate that the predicted abnormal pore pressure is mainly due to significant hydrocarbon potential in the Chorgali Formation. On the other hand, shales deposited in the Sakesar Formation during Early Eocene are over-pressured due to compaction disequilibrium.