Tomography and geochemistry of low-permeable high-carbon Domanik-type strata in the south of the Timan–Pechora sedimentary basin

Widely spread within the limits of the Timan–Pechora sedimentary basin, low-permeable high-carbon strata of the late Devonian–early Carbonic time (middle-to-upper Frasnian, Famenni an and Tournaisian levels) formed during late transgression–early regression of the sea under conditions of intense progradation of the carbonate platform west–eastwards from the Timan Ridge towards the Ural Ridge. The stra ta in the relatively deep part of the marine sedimentary basin, within a wide stratigraphic range and with high content of organic matter attract interest of researchers as oil and gas source rocks and as oil and gas reservoirs (with hydrocarbons enclosed in nonconventional low-porous and low-permeable “shale” reservoir rocks). The geochemistry and micro tomography reveal some features of the structure and composition of the Domanik horizon rock mass: the organic carbon content Сorg ranges from 1 to 13.5 % but there are some interlayers with Сorg to 25–27 %; all test samples belong to type I or II of organic matter at a maturity from MK2 to M3-4; the comparison of the rock porosity before and after the chloroform–bitumen extract analysis reveals a regular increase in the values of the general and open porosity, which is the most typical of samples from exposures, as well as to the Domanik samples from shallow depths and low catagenesis zones PK3–MK1-2; the highest porosity in all zones of depths and catagenesis is intrinsic to the Domanik samples with a carbonate mineral matrix. The analysis of the permeability and porosity of the Domanik sediments is required for the detail description of the organic carbon distribution over the section, and for the assessment of its catagenetic maturation by a set of geochemical indicators.