Evaluating Commercial-Quality LRLC Reservoirs: A Case Study from Offshore Sarawak, Malaysia

Evaluating and commercializing low-resistivity, low-contrast (LRLC) reservoirs are increasingly at the forefront of the industry’s concern in diverse geological settings ranging from deep water laminated formations, high conductive formations and heterolithic and silty formations within coastal plain. Current scenario of plunging oil prices and cost optimization practice makes such reservoirs even more interesting as LRLC pay zones are examined via behind casing and no new wells needs to be drilled for production. Traditionally, low-resistivity formation is defined as having an apparent deep resistivity value of less than 5.0 ohm-m. Major challenges in these reservoirs include identification of the hydrocarbon interval, which is usually masked by the lack of resistivity contrast between the hydrocarbon and water zones and evaluation of their commercial-quality. To unlock the hidden potential of LRLC pay sands in the offshore Sarawak Malaysia, the effective integration of subsurface disciplines including petrophysics, geology and quantitative derivatives from seismic analysis is vital. This study covers geological perspective of low contrast reservoirs from an offshore oil field deposited in a lower coastal plain settings located within offshore Sarawak Malaysia. An improved understanding of the geological, petrophysical and geophysical parameters was achieved by adopting a holistic and multidisciplinary approach. This includes the integration of core, logs, rock physics modelled parameters, stratigraphic, depositional and lithofacies information along with stochastic inversion derivatives. The paper quantifies rock physics parameter uncertainties for LRLC pay zones and establishes a framework for LRLC reservoir characterization. Stochastic inversion derived P-Impedance and Vp/Vs ratio are used to predict fluid and facies probabilities for LRLC reservoirs, which then further integrated with stratigraphic information. The results offered an effective way of establishing analogues of producing and non-producing LRLC zones. Analysis of fluid and facies probabilities derivatives driven surface attributes is a way seismic can potentially contribute in indicating areas of relatively better or worse LRLC reservoir continuity. Identified LRLC reservoirs proved to be of commercial-quality and increased oil production to the extent of several hundred thousands of barrels over the years and currently producing.