An Estimation Method of Geological Reserve for Thin Sand-Shale Bed Reservoirs

Abstract

Hydrocarbon volume calculation of thin sand-shale bed reservoirs is often challenged by the two factors. One is unreliable net pay thickness estimation at well locations mainly because of low resolution of conventional logging. The other is loose prediction on net sand distribution contrary to depositional laws. This paper presents a more accurate method of geological reserve estimation in view of the above problems in thin bed reservoirs. When log interpretation is carried out, the preliminary results could not clearly differentiate thin muddy or silty interbeds from net oil sand. On the other hand, some oil-bearing intervals with weak log response are missed. To resolve above problems, the interpretation criteria is improved to broaden the range of electrical parameters indicating oil intervals. And for more precise calculation these oil zones are further divided into three types combined with the core description. Based on oil-bearing grade classification of core, NTG is obtained respectively in terms of oil zone types. By means of the log interpretation chart and the new established index, different types of oil zones are identified. In order to precisely characterize the oil-bearing sand distribution, individual sand layers are separated from each other by detailed well correlation. Isochore maps of oil sand are delineated constrained by facies analysis results. 3-D lithology models, integrated with isochore maps, are constructed to replace the conventional 2-D maps to more accurately estimated the geological reserve. Each geocell in 3-D model is given the corresponding NTG based on the classification of oil zones. Porosity and saturation models, which are used in reserve estimation, are also constructed. The calculation result of geological reserve shows a meaningful difference from that by the previous method.