Sedimentation conditions of the lower cretaceous deposits of the Northern Caspian.

Abstract

This research identifies sedimentation settings of Lower Cretaceous (Aptian-Albian) terrigenous sediments of the North Caspian, which are oil and gas reservoirs. These rocks are characterized by irregular distri- bution of pelitic and silty fractions, which leads to ambiguous results of petrophysical analyses. In order to correctly interpret the geophysical data, a detailed lithologic analysis of the rocks was carried out. In the studied area in the Early Cretaceous time marine conditions prevailed: coastal, shallow-marine, relatively shallow shelf lithofacies with signs of active hydrodynamics, as well as relatively deep-water shelf lithofacies of low hydrodynamics. The study of sections reveals a change from shallow-water sediments to deeper-water sediments and vice versa, which allows us to conclude that the sea level fluctuates periodically. Lithologic analysis also showed the predominance of shallow-water and coastal sediments in the Aptian time and deeper-water, shelf sediments in the Albian time. Consequently, we can assume the presence of local sea transgression in this study area during the Lower Cretaceous.