Influence of diffusion from oil reservoirs to clay minerals in Bashkirian oil-saturated carbonate rocks

Abstract

© 2019, International Multidisciplinary Scientific Geoconference. All rights received. Oil deposits in Bashkirian strata regionally oil-saturated, within the Volga-Ural oil and gas bearing province. They contain up to 30% of hydrocarbon reserves. Bashkirian strata are represented by carbonate complexes of various lithological type. Only packstone and grainstone have properties of oil-saturated reservoirs, mudstone and wackestone are dense rocks. A characteristic feature of the Bashkirian carbonates is the wide presence of stylolites filled by greenish-gray clay minerals. It is known, that hydrocarbons can change parameters of the geological environments, and cause the transformation of clastic minerals. It was analyzed the composition of clay material from stylolites located at different distances from oil reservoirs. The main research method was the X-ray analysis. Terrigenous material entered the Bashkirian basin from one area of denudation. Therefore, the mineral composition of the clastic fragments did not change. Terrigenous fragments are represented by quartz, albite, orthoclase, muscovite and fine-grained Mg-Fe chlorite and illite. Аny significant deviations from this mineral association can be considered as a result of the transformation of terrigenous material under the influence of the emanations of oil reservoirs. Detailed studies of the mineral composition of material from stylolites showed that the intensity of secondary changes of the clastic and clay components increases as they approach oil reservoirs. The most sensitive are clay minerals to the emanation of hydrocarbons. The degree of crystal disorder of chlorite and illite increases, with the approach to oil reservoirs. This is evident from the increase in asymmetry and broadening of reflections of minerals on X-ray diffraction patterns. Changes are also observed in the composition of clastic minerals, mainly feldspars. The X-ray spectra show how the relative content of albite and orthoclase gradually decreases. Near the oil reservoirs, where the stylolites are saturated with hydrocarbons, a wide peak of the illite-divermiculite mixed-layer phase appears at the site of illite reflection. Diagnostic reflections of chlorite completely disappear. Tossudit appears instead, this is mixed-layer chlorite-montmorillonite clay mineral. Reflections of feldspar are fixed as a background line. At the same time, in stylolites appear diagenetic dolomites and pyrite. Such transformations of clay and clastic material in stylolites are caused by the migration of light hydrocarbons through them Hydrocarbons stimulate the activity of anaerobic microorganisms and change conditions of mineral forming.