Complex Lithophysical Typization of Bazhenov Formation Rocks from Core Analysis and Well Logging

Abstract

Abstract The paper is aimed at drawing a lithophysical typization of the Bazhenov formation rocks from core analysis and well logging suite and applying these types to assess the structure and oil bearing of geological section. As a result identified and substantiated are correlations of material composition, porosity, oil saturation, oil generation potential, and mechanical and physical characteristics of rocks estimated by core analysis and well logging. These correlations are related to different lithophysical rock types which are regularly alternating in a geological section and can be identified by applying the well logging suite. Identified are the following types: 1). fragile low-clay radiolarian silicites; 2). fragile low-clay carbonized silicites comprising authigenic calcite and/or dolomite; 3) plastic flagstone clay silicites and carbonate-clay silicites with bivalves; 4) plastic authigenic limestone and dolomite having a radiolarian structure. The overall thickness of types 1 and 2 within a geological section makes up 10 m, and that of type 3 – makes up 20 m and more. Content of organic matter (OM), abundance of uranium in OM, and generation potential of these rock types are growing from bottom to top of a geological section. These indications allow subdivide all geological sections into the upper and lower parts. All the above mentioned types of rocks are encountered both in the upper and lower parts of the geological section. Clay varieties of type 3 are predominating in the upper part of the geological section, they feature poor reservoir properties and high generation potential of up to 130 mg HC/g of rock. They are the major oil source rocks. Rock types 1 and 2 are predominating in the lower part of the geological section. They are characterized by a medium oil-generation potential of 10-30 mg HC/g of rock. These rock types mostly are reservoir rocks of the porosity of up to 16%. Rocks of the type 4 do not relate to reservoirs, they have a low oil-generation potential – 2-10 mg HC/g of rock. All the rock types can be identified and traced in the upper and lower parts of the geological section from well logging data. In the section of boreholes drilled in the areas of Surgut Swell and Salym megalithic bank both the number of interbeds and overall thickness of types 1, 2, and 4 are higher as compared to those in similar intervals of the geological section of boreholes drilled in the Krasnoleninsk Swell. Taking into account the regularities of depth and areal alternation of source rocks and potential reservoirs will enhance the reliability of geological exploration and production of HC accumulations of the Bazhenov formation.