Bazhenov suite as an "intermediate" hydrocarbon reservoir in fault zones
Bituminous deposits of Bazhenov horizon contain a significant hydrocarbon potential. According to the VNIGNI data, in 2016 the resources estimation of the territory of Bazhenov suite distribution made 9,6 billion tons, half of the volume of which was attributed to hard-to-recover oil. The World Energy Agency estimated oil potential geological resources of Bazhenov suite at 140 billion tons (recoverable oil resources - 20 billion tons). The main reason for the low efficiency of the resources’ development is the sharp variability of the rocks lithologic-mineralogical and physicochemical properties along the lateral and the vertical due to the various facies conditions of their sedimentation, the topography of the marine basin bottom in the late Jurassic - Early Cretaceous age, the influence of fluid migration processes and other reasons. The present paper is devoted to the identification of the rocks-reservoirs role of the matter composition and microstructure of the of the Bazhenov Formation in the zones affected by fluid dynamic processes associated with tectonic activity. The authors of the paper carried out a complex lithological-mineralogical analysis of Bazhenov horizon deposits in different regions of the Western Siberia. In the sections studied, the following layers and packs of permeable rocks with a biomorphic structure were identified: secondarily altered radiolarites and pelecypodal coquina, which, according to well logging data (WL) have the characteristics peculiar to fine-grained sandstones and siltstones. These layers are well distinguished on logging curves by abnormal readings of neutron gamma log (NGL), acoustic log (AL), less frequently - self-potential logging (SP logging) and infrared logging (IL). The epigenetically altered radiolarites with an open work, relict porous structure, pelecypodal coquina, coccolithophorite marls, secondary limestones and dolomites (developing in radiolarites) with openwork loose structure, that form the suite, possess the improved reservoir properties. Radiolarites were formed in the muddy depressions enriched with organic matter, that the sea bottom relief inherited from negative structures of the Pre-Jurassic basement, partly located within the over-rift channels, pelecypodal coquina - mainly on the underwater uplift slopes. It is assumed, that rocks epigenetic changes causing an increase of a reservoir permeability and porosity characteristics are associated with an increase in the intensity of fluid-dynamic processes during activation of faults affecting the sedimentary cover rocks up to Bazhenov horizon. Subvertical migration of the solutions, containing deep hydrocarbons (HC), occurred in the faults zone. These hydrocarbons penetrated the transformed interlayers of Bazhenov suite biogenic rocks through the systems of different-scale fracturing and filled the void space. Thus, these interlayers could serve as reservoirs for allochthonous hydrocarbons even before the generation of oil by the Bazhenov suite itself. The process of forced saturation of some interlayers in the layered strata occurred in combination with the restraining capacity of others. In the main zone of the oil formation, free pores, caverns and cracks were filled with products generated directly by Bazhenov suite. Directional pressure growth (from the bottom to the top), intensive transformation of clay minerals and filling of the suite biomorphic collectors, resulted from the repeated and, possibly, prolonged inflow of gas-thermal and hydrocarbon-containing mixtures along the transporting faults in tectonically active zones led to fluid fracture of rocks. Under the influence of the above-mentioned processes, the fluid-restraining properties of clay packs both inside Bazhenov suite and in the overlying sandstones of Nizhne-Kulomzinsk sub-suite were disturbed, and hydrocarbons of different genesis (allochthonous and autochthonous) accumulated in the suite’s rock migrated to the overlying reservoirs with improved reservoir’s characteristics. From this point of view, Bazhenov suite can be regardsed as an "intermediate" hydrocarbon reservoir for the Neocomian deposits that lie above.