Study of petrophysical properties of deep-seated Meso-Cenozoic sediments within the North Apsheron system of uplifts

This article presents our research on the depth dependence of the reservoir properties oil and gas bearing deposits of the Meso- Cenozoic age within the Apsheron uplift. The petrophysical features and patterns of change at depth of the Meso-Cenozoic sediments of the deposits of the Apsheron Bank and Western Apsheron were studied. Despite the fact that the history of the discovery and processing of the abovementioned deposits dates back to the 1950s, some geophysical features associated with these deposits still remain relevant. In particular, from literary sources it is known that the clay content of oily reservoirs of the Apsheron Archipelago tends to grow in a southeast direction. Reservoirs consist mainly of rounded, semi-rounded and angular rounded grains of isometric shape. The average carbonate content in the Apsheron Bank is up to 16.2%, and the average carbonate content in Western Apsheron is up to 13.8%. The average porosity here is 22%, with slight differences in the lower and upper sections. Due to the presence of carbonate material as cement, carbonization in sand reservoirs varies between 2.9-31.8%, and the porosity coefficient is 6.8-25.6%. In general, in deposits of the Archipelago, sand reservoirs have high permeability, on average characterized by permeability coefficients of 139*10-15 m2. From the results of our study based on the core samples from wells drilled in the territory, the general porosity of rocks involved in the depth-section is characterized by unstable values. Nevertheless, in general, there is a tendency to decrease in porosity in depth. So, in particular, in the context of the Apsheron Bank with an average value of porosity of 8%, the pattern of its decrease is described by the linear equation Y = – 0.16x + 12.59 and is on average 0.6% for every 100 m. For Western Apsheron with a sharper the change in porosity in depth, the average value is 14.2%, and the tendency to decrease with increasing depth is weaker and is 0.25% per 100 m. To determine the causes of the observed variability of porosity at different depths, we studied the effect of the degree of carbonate content of various rock samples on porosity. As a result, in both deposits (Absheron Bank and Western Apsheron) the inverse proportion of the porosity of the rocks and the values of carbonate was confirmed. Although the separate carbonate contents negatively affect the porosity of reservoir rocks, the combined effect of carbonate and clay contents differs for the studied deposits.