This paper presents an integrated petrophysical characterization of a representative set of complex carbonate reservoir rock samples with a porosity of less than 3% and permeability of less than 1 mD. Laboratory methods used in this study included both bulk measurements and multiscale void space characterization. Bulk techniques included gas volumetric nuclear magnetic resonance (NMR), liquid saturation (LS), porosity, pressure-pulse decay (PDP), and pseudo-steady-state permeability (PSS). Imaging consisted of thin-section petrography, computed X-ray macro- and microtomography, and scanning electron microscopy (SEM). Mercury injection capillary pressure (MICP) porosimetry was a proxy technique between bulk measurements and imaging. The target set of rock samples included whole cores, core plugs, mini cores, rock chips, and crushed rock. The research yielded several findings for the target rock samples. NMR was the most appropriate technique for total porosity determination. MICP porosity matched both NMR and imaging results and highlighted the different effects of solvent extraction on throat size distribution. PDP core-plug gas permeability measurements were consistent but overestimated in comparison to PSS results, with the difference reaching two orders of magnitude. SEM proved to be the only feasible method for void-scale imaging with a spatial resolution up to 5 nm. The results confirmed the presence of natural voids of two major types. The first type was organic matter (OM)-hosted pores, with dimensions of less than 500 nm. The second type was sporadic voids in the mineral matrix (biogenic clasts), rarely larger than 250 nm. Comparisons between whole-core and core-plug reservoir properties showed substantial differences in both porosity (by a factor of 2) and permeability (up to 4 orders of magnitude) caused by spatial heterogeneity and scaling.
The paper discusses the issues of interaction of the organic matter and the siliceous-carbonate mineral matrix in unconventional reservoirs of the Upper Devonian Domanik Formation of the Upper Kama Depression of the Volga-Ural Basin. The Domanik Formation is composed of organic-rich low-permeability rocks. Lithological and geochemical peculiarities of rocks were studied using light microscopy, X-ray diffraction analysis (XRD), scanning electronic microscopy (SEM), and evaporation method. Organic matter was examined by the Rock-Eval pyrolysis with quantitative and qualitative evaluation of generation potential and maturity degree. Integrated analysis of results of lithological and geochemical studies allowed identifying intervals in the studied section where organic matter can form a complex association with the siliceous-carbonate matrix. It was fixed experimentally that in some cases the mineral carbonate matrix and the organic matter form a one-whole high-molecular compound. The authors supposed that in the course of sedimentation, organic matter is immobilized into the structure of the mineral carbonate matrix. At the deposition and diagenesis stage, the carbonate matter interacts with acids of the organic matter and forms natural organo-mineral polymers. Special physicochemical properties of such organo-mineral associations shed new light onto the problems of producing from hard-to-develop nonconventional carbonate reservoirs and evaluating the associated risks.
The present work is devoted to geochemical studies of the Bazhenov Formation in the north of the West Siberian Petroleum Basin. The object is the Upper Jurassic–Lower Cretaceous section, characterized by significant variations in total organic carbon content and petroleum generation potential of organic matter at the beginning of the oil window. The manuscript presents the integration of isotopic and geochemical analyses aimed at the evaluation of the genesis of the rocks in the peripheral part of the Bazhenov Sea and reconstruction of paleoenvironments that controlled the accumulation of organic matter in sediments, its composition and diagenetic alterations. According to the obtained data, the sediments were accumulated under marine conditions with a generally moderate and periodically increasing terrigenous influx. The variations in organic matter composition are determined by redox conditions and terrigenous input which correlate with the eustatic sea level changes during transgressive/regressive cycles and activation of currents. Transgression is associated with an intensive accumulation of organic matter under anoxic to euxinic conditions and insignificant influence of terrigenous sources, resulting in the formation of rocks with oil-generating properties. During the regression periods, the terrigenous sedimentation increased along with the dissolved oxygen concentration, and deposits with low organic matter content and gas-generating properties were formed.
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