Abstract

Realistic imaging and conceptualization of reservoir models relies on linking petrophysical properties to geologic processes. A deliberate strategy adopted in development wells of North Jotana revealed the occurrence of oil in MU-III sequence, hitherto unknown in the area. This involved meticulous examination of drill cuttings and sidewall coring of low resistivity sands. Interesting zones, thus identified were corroborated by test results. Lamina scale heterogeneity comprehended by pore-size distribution and irreducible water saturation revealed the influence of depositional processes in masking the conventional oil response on electrologs. Sandstones deposited in the lower toe part of deltafront deposits of a tide-dominated delta system in North Jotana have indicated lowdown of resistivity owing to predominance of dispersed clay resulting in higher microporosity. SEM studies revealed grain coating and pore filling with clays occluding the porosity. Increased surface area associated with disseminated clay coating of the grains and filling the pores holds more irreducible water, thus increasing water saturation, which in turn reduces resistivity values. Resistivity logs respond to total water present in the pores and the high water saturation is due to bound water. Evaluation of such low resistive pays requires the interpreters to discard the myth that water saturations above 50% are not hydrocarbon productive. In new wells, close monitoring of well cuttings and sidewall cores backed up with qualitative resistivity- porosity overlay and real time petrophysical examination are imperative for deciphering zones that merit production tests.