Petrophysical Modelling of Carbonate Reservoir from Bombay Offshore Basin

Abstract

The integration of well logs with laboratory measurements derived from core to analyse the reservoir characteristics of WELL P, of Bombay Offshore Basin, India. The reservoir properties such as permeability (k), Porosity (ϕ), shale volume (Vsh), lithology, water saturation (Sw), net pay thickness and other parameters were determined from well logs. The petrophysical model was derived from depth X150 m to X400 m. But the main focus of analysis were four pay zones named as Zone A (X195 m to X212 m), Zone B (X226 m to X282 m), Zone C (X299 m to X310 m) and Zone D (X338 m to X374 m). The logs used for analysis were CGR, NPHI, RHOB, LLD, DTCO and DTSM. Fluid types were identified by NPHI versus RHOB crossplot, VP, VS (vp/vs) versus DTCO crossplot and neutron/density log signatures which indicate the absence of gas and the presence of oil and water in the given well. The signatures of DTCO and DTSM followed each other indicating the absence of gas in the pay zones. Wet resistivity quick look technique was applied to locate the hydrocarbon-bearing zones of interest and any crossovers on density and neutron logs as indicators of the presence of oil zones. Saturation cross plots (Pickett plot) was used to determine the saturation exponent (n), cementation factor (m), tortuosity factor (a), and formation water resistivity (Rw), a prerequisite to their use in the determination of water saturation from Archie’s equation. The main lithology in the region of the study was limestone (calcite) and shale (illite). Shale although present but was in a very small amount. The final volume fractions obtained of each mineral mainly calcite and dolomite from the petrophysical model were compared with those obtained from X-ray Diffraction (XRD). The average porosities of four zones were found to vary from 14.1 to 16.9% which indicated good porosity for a carbonate reservoir. The average water saturations of zones varied from 0.631 to 0.667. The results of the study indicate that the zones where the porosity is good, the measured permeability turns out to be poor (less than 5 mD) which suggests that the pores perhaps are not interconnected. This could be true for carbonates. Thus, the measured values need to be compared with the porosity and permeability values obtained from other laboratory measurements like Routine Core Analysis (RCA) and MicroCT scan to study the connectedness and non-connectedness of the pore-system in the cores.