DISTRIBUTION OF CARBONATE CEMENTS IN THE SANDSTONES OF THE SOUTHEAST OF THE DNIEPER-DONETSK DEPRESSION IN THE ASPECT OF FORMING SECONDARY POROSITY IN THEM AT HIGH DEPTHS

Abstract

The main problem when searching for oil and gas at great depths is the lack of an economic reservoir. Since great depths correspond to high degrees of catagenetic transformations, the primary porosity of most rocks is reduced and all hopes are associated either with the relict primary porosity, the value of which progressively decreases with depth, or with secondary porosity. There are different views on the formation of secondary porosity in sandstones - the main reservoirs of the Dnieper-Donetsk Depression (DDD). Some of these views are briefly discussed in this paper. In all cases, carbonate cement plays an important role as its dissolution under the influence of various factors forms secondary porosity. The question arises as to how often sandstones with carbonate cement are found among the differences with a different cementing material, at what depths, in the sediments of what ages, are there many sandstones in which the formation of secondary porosity is possible by one or another mechanism. The aim of this work is to quantitatively assess the prevalence of carbonate sandstones among the sandy rocks of the southeastern part of the DDD, clarify the distribution of carbonate sandstones in depth and over the main stratigraphic horizons, and preliminary estimate the content of various carbonates in the cement of sandstones according to field and standard laboratory studies of core material. In the course of the work, data from more than 10 thousand samples were used, taken at various fields of the northern side and the northern near-zone zone of the southeastern part of the Far East region, approximately from the Belskoye field in the north-west to Kruzhilovskoye in the southeast inclusive. The sampling depth intervals for the samples used vary from 1 to 6 km, the age of the sediments they characterize is from Triassic to Devonian. The results obtained show that the content of carbonate in sandstone cement at great depths is high enough for its movement to cause the formation of secondary porosity on an industrially significant scale, which, in turn, provides good prospects for prospecting hydrocarbon deposits at great depths.