Overpressure zones in relation to in situ stress for the krishna-godavari basin, eastern continental margin of India: Implications for hydrocarbon prospectivity

Abstract

An analysis for over pressure zone (OPZ) prevailing in parts of the Krishna-Godavari Basin (KG-B) at the Eastern Continental Margin of India (ECMI) is found promising from the viewpoint of its hydrocarbon potentials. Pressure coefficients estimated from pore pressure studies reveal that there is a rather extensive (lateral) OPZ in the study area than hitherto expected with maximum pressure coefficient of 1.31 or more. The stress magnitudes like vertical stress (Sv), minimum horizontal stress (Sh) and pore pressure gradient (PPG) and fracture pressure gradient (FPG) are predicted from well log data for 15 available wells distributed over an area of 6022 km2 in KG-B. The wells are drilled to depths of 3660 m on-land (#Wells 1–9) and up to 4000 m in offshore (#Wells 10–15). The PPG ranges from 11.85 to 13.10 MPa/km, whereas, the FPG varies from 17.40 to 19.78 MPa/km in sediments penetrated by the wells displaying normal pressured sediment to a significantly higher value of 19.78 MPa/km for the over-pressured sediments. The values of vertical stress gradient (VSG) varies from 14.67 to 23.10 MPa/km, whereas, the values of Sh magnitude varies from 64 to 77 % of the Sv in normally-pressured to over-pressured sediments. VSG, PPG and FPG tend to decrease with corresponding increase in water column for the studied offshore wells. These results are utilized for constructing contour maps for observing the variations in the VSG and in the OPZ-top, also for constructing PPG contour map in 3D along the vertical section connecting all 15 wells extending from onshore to offshore regions. Any significant increase in pore pressure means the decrease of effective horizontal stress in respect of depth. As a result, the safety windows or safe mud-weight windows (the difference between PPG and FPG corresponding to particular depth interval in a well) will also decrease with the increase of PPG and FPG. Analytical approach adopted above is then critically examined to recommend how a priori steps based on petrophysical characters of a formation are closely monitored in time and optimum mud weight maintained during drilling.