On the question of creation of secondary porousity in large-depth sediments of the south-east of the Dneprovsk-Donetsk depression

Abstract

Introduction. Currently, most researchers associate the main prospects of oil and gas potential with great depths. The most important component of the problem of oil and gas potential at great depths is the problem of reservoir availability. This problem still has no clear solution. From the point of view of the classical theory of stadial catagenesis, with depth, the primary porosity naturally and irreversibly decreases, and we can only talk about how quickly this happens. In this case, large depths appear to be a zone of fading oil and gas potential without any special prospects. The theory of superimposed catagenesis comes to the rescue, which shows that there are processes that naturally lead to the formation of secondary porosity, a secondary reservoir, and then large depths represent a special deep zone of gas accumulation.

According to the views of many researchers, including the authors of this work, quartz sandstones with regenerative quartz cement – quartzite sandstones - are the most promising in terms of reservoir properties at great depths.

The aim of the work is to establish the regularities of compaction of quartzite sandstones with depth, to identify anomalies in the compaction process and to try to interpret them using mineral indicators of hypogenic-allogeneic porosity formation – one of the varieties of superimposed catagenesis of great depths.

Previous study. The theory of metasomatic reservoir formation as a result of hypogenic-allogeneic porosity formation is one of the most developed. It is based on the penetration of a certain favorable composition of deep aggressive fluids into the reservoir during the period of tectonic-magmatic activation with leaching of some components and deposition of others. It is assumed that this process is accompanied by the crystallization of certain indicator minerals.

Result of work. The graphs constructed in the course of this work reflect both a natural change in the porosity of this type of Sandstone with depth, and show an anomaly associated with some superimposed process that allows for different explanations. It is natural to use indicator minerals to solve the problem, but the analysis shows that specific indicator minerals of hypogenic-allogeneic decompression have not yet been found, and all the minerals involved can be interpreted as minerals of one of the stages of stadial catagenesis. This does not disprove the possibility of the formation of a secondary metasomatic reservoir by the above mechanism, but it is not possible to prove its implementation by the indicator minerals noted in the studied samples.

Relevance. The question of the formation of secondary porosity is of great practical importance for assessing the prospects of gas content: either large depths are represented by the area of naturally decreasing gas accumulation, or the area of naturally occurring secondary reservoirs with good oil and gas prospects, which can be distinguished as a separate deep zone with its own specific power sources, reservoirs and cap rocks.