Reconciliation of Core and Log Data Analysis in Very Thin Reservoirs of Krishna Godavari Basin, India

Abstract

Deep water Tertiary reservoirs of Krishna Godavari basin are generally very complex and heterogeneous, ranging from massive thick sands to highly laminated very thin sand/shale sequences beyond the vertical resolution of imaging tools. Verification of log-derived quantities in very thin reservoirs is frequently difficult because of bias and scatter introduced by non-uniform or incomplete plugging of the available over 750 meters of conventional cores. As laminar shale does not occupy or alter the intergranular sand porosity, the conventional way of validation of log derived porosity and water saturation with core data showing excellent correlation does not ensure the correctness of petrophysical evaluations. In this paper, it has been demonstrated that there are potential problems associated with the integration of petrophysical evaluation and core data. Sources of errors are mainly due to the use of different parametric systems viz. using of the terms shale volume and clay volume interchangeably and validating log derived shaliness with clay abundance from XRD core data. Shaliness derived from logs cannot be directly correlated with such core data because neither de-averaging of lithology on log analysis nor averaging of core data is possible for the effective core-log integration. Average hydrocarbon pore volume in laminated reservoirs is linearly related to the average cumulative net sand fraction or shaliness of the formation. Laminar sand porosity and water saturations are independent of shaliness or net sand. Excellent correlation of log derived deconvolved data sets like porosity and water saturation with core measurements does not validate the correctness of hydrocarbon pore volume estimation, which is linearly related to net sand. Accurate estimation of hydrocarbon pore volume requires an accurate calibration of laminar sand/shale volumes, which can be precisely validated only with lamcount data.