Abstract
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.
Highlights
In the context of persistently declining production of oil from conventional reservoirs, studies of oil shale are of high priority
To assess the effect of various mineral grains and organic matter on water saturation, the organo-mineral system was calculated into wt.% according to X-ray diffraction analysis (XRD) analysis and total organic carbon (TOC) was recalculated to OM due to kerogen type and maturity [19]
By results of XRD optical microscopy it was established that the prevalent component in the mineral composition is calcite—from 50 to 87%, amount of dolomite is up to 7%
Summary
In the context of persistently declining production of oil from conventional reservoirs, studies of oil shale are of high priority They are often confined to predominantly carbonate (siliceous-carbonate) deposits where hydrocarbons occur in situ, with no evidence of migration into overlying layers. Domanik Formation is represented by organic-rich carbonate shales These rocks are considered both as source rocks and unconventional reservoirs due to extremely low porosity and permeability with high amount of organic matter. In accordance with the concepts of the sediments-and-migration theory, these deposits are considered as “excellent” and “rich” petroleum-source rocks [3] Such hydrocarbon-saturated deposits are characterized of low porosity and permeability values [4], lack of structural and stratigraphic control of oil-bearing intervals, that makes it impossible to identify them by logging data. The main aim of the oil companies is to find affordable options for recovering hydrocarbons from such organic-rich shales, and to this it is essential to clearly understand the controlling factors behind the organic-mineral matrix
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