The mineralogy of the clayey-silty siliceous rocks in the Bazhenov Shale Formation (Upper Jurassic) in the west Siberian Basin, Russia: The role of diagenesis and possible implications for their exploitation as an unconventional hydrocarbon reservoir

The Bazhenov Shale Formation (BSF) of the west Siberian Basin is considered to be a promising unconventional hydrocarbon reservoir, acting as a source, store and seal of the oil and gas contained therein. It has a high content of organic matter, ranging from 2 to 20% (average 8–12%), and was deposited in a marine environment at an average water depth of 2220 to 2500 m where depositional conditions were of an intensely reducing nature. Deposition took place over a period of 8 to 10 million years mainly during Upper Jurassic times. This study reports the mineralogy of 83 core samples representative of the clayey-silty siliceous rocks of the Bazhenov Shale Formation (BSF) taken from six wells in the south-east of the west Siberian Basin. The mineralogy of most samples was found by X-ray diffraction (XRD) to be strikingly similar with most being dominated by quartz, and with albite, mica-like minerals (illite plus mixed-layer illite/smectite (I/S)) and pyrite commonly occurring in subsidiary amounts. Kaolinite and chlorite occur intermittently, usually in no great quantity. Throughout all six wells, plagioclase feldspar (albite) predominates over K-feldspar which is only of sporadic occurrence. The clay (< 2 μm) fraction of available core samples largely consists of illitic material comprising a combination of true illitic mica and mixed-layer I/S with R 3 stacking. Both optical microscopy and scanning electron microscopy (SEM) show the BSF to be composed of a mixture of detrital and authigenic minerals. Thus, quartz occurs as a detrital mineral and as the crystallized product of radiolarian tests subsequent to deposition. The micaceous (muscovitic) material seen in thin section is clearly of terrigenous origin but the fine-grained illitic material is considered to be at least partly diagenetic . With regard to feldspars, although thin sections show some to be detrital, it is considered that albite is also likely to be a diagenetic mineral, as it is in many sandstone reservoirs. In many of the core samples albite is the only feldspar detectable and it is improbable that provenance could yield feldspar detritus consisting only of pure albite over such an extended period of time. Other minerals in the BSF must also be regarded as diagenetic rather than detrital. This is certainly true of the carbonate minerals and pyrite, and the book-like aggregates and euhedral morphology of kaolinite observed by SEM and TEM are also indicative of a diagenetic origin. Finally, the illitic clay mineralogy of the BSF is considered in a general way in the context of their permeability, particularly as it relates to their anisotropic structure and pore water chemistry. It is also suggested that the siliceous nature of the Bazhenov shale makes it a suitable candidate for the creation of enhanced permeability through hydraulic fracturing technology, although matrix permeability may be adversely affected by reactions involving the external basal surfaces of the illitic clays exposed in slit-like pores.