Scientists explain how shale oil-bearing rocks of the Volga-Ural region came to be

The researchers have studied the geochemical composition and conditions of formation of Domanik deposits (Semiluksk horizon) in the Volga-Ural oil and gas province, which made it possible to determine their oil-generating properties.

"Traditional oil reserves in our region are at a very late stage of development—they are close to exhaustion. In order to maintain the resource sustainability of the region, we need to look for new sources. High-carbon Domanik deposits are prospective of future oil production," says Anton Kolchugin, Chair of the Department of Mineralogy and Lithology.

In addition, according to the scientist, Domanik deposits, which are interbedded high-carbon carbonate and carbonate-siliceous rocks, are the oil-producing strata for most of the deposits in the Volga-Ural region, located in Udmurtia, Tatarstan, Bashkortostan, Orenburg Oblast, and Perm Krai.

"Domanik deposits are found throughout Tatarstan at depths of 1,200 to 1,800 meters. And the giant Romashkino field, one of the ten largest in the world, most likely owes its origin to them," Anton Kolchugin notes.

In order for oil production from Domanik rocks to be effective, it is necessary to study their mineralogical, lithological, and geochemical properties.

Core samples taken from wells located in different parts of Tatarstan were comprehensively studied at KFU using X-ray phase and X-ray fluorescence analysis, mass spectrometry, thermal analysis, and scanning electron microscopy. Special geochemical and pyrolytic studies, correlation analysis of data, and other studies were also carried out.

This allowed the researchers to glance into the distant past of the Earth and reconstruct the conditions that existed within the modern Volga-Ural oil and gas province 370 million years ago. It was during this period that sedimentation of high-carbon rocks in the Upper Devonian sea basin took place.

"We found out the main conditions and patterns of formation of these sediment complexes. At that time, on the eastern edge of the East European platform, there was a vast shallow basin, which in the Domanik period became somewhat deeper due to the general rise in the level of the world ocean. This contributed to the accumulation of specific carbonate-siliceous sediments, saturated with organic matter, which were subsequently transformed into high-carbon rocks, often called 'black shales'. Why did so much organic matter accumulate? The fact is that in the place of the modern Ural Mountains, there were volcanic ridges of the type of the Kamchatka and Kuril volcanic belts. They supplied a huge mass of ash material that fell into the reservoir. Large volumes of ash after volcanic eruptions were carried across the entire water area of the shallow basin. There was a rapid increase in biological productivity in this reservoir, because ash material is a very good nutrient medium for a huge number of microorganisms. In addition, the Devonian stage is marked by the active dispersal of vegetation on the continents. This stage is associated with the rapid spread of vascular plants in a continental environment that was still completely unoccupied by biodiversity. This contributed to the intensification of soil formation processes, the accumulation of organic mineral matter and its removal into the waters of the adjacent sea basin. It should be noted that the climate was hot and humid, and the basin was located in subequatorial latitudes. The removal of the organic mineral matter into the oceans could also provoke an explosion of bio-productivity in shallow shelf basins and, as a result, the accumulation of significant masses of organic matter on their bottom," elaborates Dr. Kolchugin. According to the geologist, various microscopic algae and other microorganisms (mainly plankton) living in the water column multiplied rapidly. After dying, they formed silts rich in concentrated organic matter, from which high-carbon rocks were later formed.

"Subsequently, under the influence of elevated temperatures and corresponding pressures, oil was generated and released, which formed oil deposits in porous and permeable rocks," concludes the scientist.

He sheds light on why it is extremely difficult to extract oil from Domanik deposits in Tatarstan, "On the territory of Tatarstan, these objects are in not very good temperature conditions. The reservoir temperature is approximately 30 degrees. In order for the extraction of shale oil to become possible, it is necessary to heat the reservoir to approximately 60 degrees and form artificial porosity and fractures in it using hydraulic fracturing, so that oil from high-carbon rocks penetrates into the cracks and micropores."

More information:
Geochemistry and formation conditions of the Domanik sediments (Semiluksk horizon) in the Volga-Ural petroleum province, Russia
www.sciencedirect.com/science/ … 199582262400342X?via%3Dihub

Provided by Kazan Federal University