Low-resistive sandstone reservoirs in the Attaka field

Abstract

Abstract The Attaka Field is a giant oil field in offshore East Kalimantan. Reservoirs in this field are multi-layered sandstones eposited in deltaic to shallow marine environments during Middle to Late Miocene time. Within this field, high-resistive as well as low-resistive hydrocarbon bearing sandstone layers are present. Low resistivities occur in both thick-and thin-bedded sandstones, implying that shoulder-and dipping-bed effects are not the only reasons for the low resistivity readings. High irreducible water saturations in micro pore spaces and bound water associated with high clay content also lower the resistivity of pay sands. In the thin bedded shaly reservoirs, shoulder-bed effects have complicate low-resistivity problems. The conventional logs, such as the dual and phasor induction, cannot resolve true resistivities in sand layers less than 5 feet thick. To derive resistivities that are closer to the true formation resistivities, Unocal employs an in-house deconvolution program for dual induction and other older logs, and an enhancement program for phasor induction logs. Unocal has tested a magnetic pulsed induction log which shows higher resolution than the older induction logs. In addition, utilization of high sampling rate lithodensity and neutron logs results in more accurate porosity values in thin bedded reservoirs than can be achieved with older porosity logs. Determination of fluid contents of low-resistive sandstone layers is based on qualitative log evaluations, correlations, Repeat Formation Tester (RFT) samples, and Drill Stem Tests (DST). Formation evaluations also incorporate core analysis data. Using new logging technology and integrated formation evaluations, Unocal has mapped a significant increase in reserves from the low-resistive sandstone layers. These reservoirs are among the primary targets for the current development plan.