Features of bitumen-containing sandstones of the volga-ural oil and gas province according to electron paramagnetic resonance and gamma spectrometry data

Abstract

© SGEM2017. All Rights Reserved. On the territory of the Volga-Ural oil and gas province in the Permian sediments are concentrated significant resources of bitumen-containing rocks, which are of interest as a source of unconventional oil. In this connection, it is very important to clarify the conditions for the formation of bituminous strata and their productive areas. High efficiency in the study of oil-bearing rocks has radio-spectroscopic methods for determining the structure of the oil deposit and the nature of its oil and bitumen saturation, not only in carbonate rocks, but also in sandy-clayey varieties. In this paper are considered the results of analysis of reservoir rocks by electron-paramagnetic resonance (EPR) in combination with gamma spectrometry. The EPR method is used to study diamagnetic crystals where the paramagnetic centers are the impurity ions of Fe3+ and Mn2+ in calcite, dolomite, electron-hole centers in them, radiative E'-center in quartz, free radicals of organic matter of coal and oil series. These paramagnetic centers reflect the conditions and environment for the formation of minerals, the degree of degradation of the syngenetic material of rocks, reflecting different stages of post-sedimentation changes of rocks. Bitumen deposits belong to the Ufimian (Kungurian-ICS) stage (sheshminskiy horizon) of Permian period, which are represented by cross-bedded fine-to-medium-grained, polymictic sands and sandstones. According to the mineral composition, they belong to the Greywack group. In the clastic material of sandstones, there are grains of quartz, feldspar, mica, as well as particles of volcanic rocks, which was introduced from the Ural Mountains that were collapsing in the Permian time. Cement in sandstones is clayey-carbonate by composition, sometimes carbonate; by type - porous, pelitomorphic, basal. The mutual arrangement of grains of detrital minerals determines the poorly expressed banding of rocks. The intergranular space of the detrital component is largely filled with cement matter. The EPR method makes it possible to determine the paramagnetic centers reflecting the stages of diagenetic transformation of rocks and the degradation of organic matter. According to the EPR data, it was found that in the studied samples there are 2 types of organic matter, of oil and coal series. The oil substance has a migratory nature and, most likely, is associated with the oil rising from deeper horizons (Carboniferous deposits). The organic matter of the coal series has a syngenetic origin and is represented by the remains of vegetation (algae) and is deposited along with the primary sediments. One of the most important parameters is the radiation E'-center, which is observed in trigonal quartz. The E'-center creates if the trigonal quartz is irradiated by radioactive elements. The content of natural radionuclides of the 238 U ( 226 Ra) series, as well as 232 Th and 40 K was estimated in the samples under study by gamma spectrometry. The EPR spectra of the ion-radicals E' of quartz in the profiles of the studied wells are highly correlated with 238 U ( 226 Ra) and 40 K. Herewith, their content prevails in the upper and lower parts of productive horizons. This suggests that the source of irradiation is in the rock itself. Such a source can be potassium feldspar, as well as insignificant concentrations of uranium. Uranium can be supplied as part of groundwater of the red-colored Ufimian deposits, which have the properties of alkaline solutions, and precipitate on contact with bitumen-bearing rocks.