N. Ruhovets
Halliburton Logging Services, Houston, Texas
ABSTRACT
Existing high-resolution log interpretation techniques were primarily developed for evaluating thinly laminated shaly sand reservoirs, where effective porosity depends mainly on the volume and distribution mode of the clay. These techniques cannot be applied to complex carbonate and sandstone reservoirs where porosity also depends on lithology and degree of cementation, and so may change abruptly from one bed to another, even in clean formations.
A special high-resolution interpretation technique has been developed for complex reservoirs containing carbonates and highly cemented sandstones. Required input logs are induction (or other deep-investigating resistivity logs), density, neutron, gamma ray (or another standard shale indicator), an unfiltered Pe-index, and a high-resolution resistivity log such as a dipmeter or high-frequency dielectric.
Initially, shale volume and its mode of distribution (dispersed or laminated) are determined at the high-resolution level. Shale volume is then integrated to the medium-resolution level and used to conventionally compute effective and total porosities from density-neutron crossplots. The integrated shale volume is also used to calculate effective and total water saturations. Effective porosity at the high-resolution level is determined from a shallow investigating high-resolution resistivity log, using apparent mud filtrate resistivity computed continuously at the medium-resolution level. Water saturation in the thin beds is determined from linear relationships established at the medium- or low-resolution level between effective porosity and water-filled effective porosity. Lithological composition of laminated complex reservoirs is determined at the vertical resolution of the unfiltered Pe-index, assuming that in thin beds no more than two minerals are present.
The paper presents a discussion of the principles underlying this new interpretation technique. Applications to log interpretation in complex reservoirs are then illustrated using three field examples having different lithological compositions.