Jurassic and Cretaceous clastic petroleum reservoirs of the West Siberian sedimentary basin: Mineralogy of clays and influence on poro-perm properties

Abstract

• Kaolinite is a predominant clay for Jurassic while chlorite for Cretaceous reservoirs. • Secondary chloritization might not have affected the coarse-grained lithic clasts . • A lowland relief predominated in Jurassic and volcanic upland relief in Cretaceous. The West Siberian Sedimentary Basin (WSSB) is the main commercial and the biggest sedimentary basin of Russia and plays a huge role in the Russian oil and gas industry. Three deep wells intersecting full sections of the basin main Jurassic and Cretaceous reservoirs have been studied. This basin consists of two major petroliferous sedimentary sequences: uppermost is of Lower Cretaceous (the Neocomian Sedimentary Sequence) and the oldest petroliferous sandstones of Jurassic age (the Urman-Togur-Tyumen sequence). All the main oilfields of the WSSB are found in sandstone reservoirs, and this paper describes their petrography and mineralogy, emphasizing on the nature and origin of the clay fraction (<2 µm). Each section was characterized by granulometry and mineral composition using optical microscopy, X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. The uppermost Lower Cretaceous sequence, that appears to be related to volcanogenic debris of high iron content, is characterized by abundant chlorite of similarly high iron content. The clay mineralogy of the petroliferous sandstones in the lowermost Jurassic sequence is not so chloritic but is more kaolinitic, a feature believed to be inherited from weathering of lateritic type soil profiles in the source area, as well as post-sedimentary authigenic conversion of feldspars.