Analysis and study of the Sheshminskian horizon dated to the Ufimian age is very important considering that commercial accumulations of heavy oil on the territory of the Republic of Tatarstan are confined to the Permian formations, in that number, to the Sheshminskian sand sequences. The Sheshminskian horizon consists of the upper sand sequence P1ss2 and the lower sand/clay sequence P1ss1. Heavy oil accumulations are confined to the increased thickness intervals, which form subparallel, mostly, northwest trending bar-like structures. Frequent and inhomogeneous interbedding of unconsolidated sandstones and tight rocks complicates correlation of pay intervals, tight sands, and shale breaks adding to exploration and production challenges.
The authors concentrated on study of the geologic cross-section of heavy oil accumulations and classification of geologic cross-sections types of the Sheshminskian sand sequences. Different approaches to classification of geologic cross-section types were considered, including those based on grain-size distribution, number of productive interlayers in the cross-section, combined analysis of SP curves and detailed core description, lithological characteristics and PVT analysis.
Finally, we developed a method providing for a combined analysis of logging data, calculated reservoir properties, lithological core description, and laboratory core analysis. This method can be used for complex-geology sediments represented by inhomogeneous interbedding of permeable interlayers differing in thickness with tight lithofacies.
The sand sequence was divided into two parts, the first part represented by sandstone with similar lithological and reservoir properties through the thickness, and the second part represented by tight sand, or intercalation of tight and unconsolidated sand. In terms of proportion of homogeneous and inhomogeneous parts, four types of cross-sections were recognized: I, II, III, and IV, which in case of occurrence of shaled out sandstones and thin shale interlayers were subdivided, in turn, into four subtypes – Ia, IIa, IIIa, and IVa.
Well logging data were used for analysis. On the territory under study, the sand sequence within the heavy oil pool boundary can be represented by all types of geological cross-sections, or one or two types can prevail. The prevailing type of cross-section is controlled by a number of factors: structural framework, tectonics, cycles of oil generation, oil migration behavior.
The results obtained can be used to estimate oil reserves, to plan reservoir engineering programs, and to assess oil and gas potential of the territory.
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