The Effect of Bioturbation on the Petrophysical Characteristics of the Lower Cretaceous Rocks of Oil and Gas Reservoirs in the Northern Caspian Sea Region

Abstract

The Caspian Sea is one of the most perspective areas for oil and gas exploration. Significant hydrocarbon reserves are contained in polymictic sand reservoirs of the Lower Cretaceous (Aptian–Albian) deposits of the North Caspian fields, which are characterized by an irregular distribution of pelitic and silty fractions. This results in high residual water content and low permeability and poor consistency between filtration and capacity properties. This determines the high heterogeneity of reservoirs in hydrocarbon accumulations and unusually low specific electrical resistivities when producing water-free hydrocarbon inflows. As our studies have shown, the most important reason for the complex structure is the intense bioturbation of deposits virtually throughout the productive section, which actively affects the redistribution of clay material in the reservoir rocks and ultimately leads to a decrease in the resistivity of reservoirs to the point where they can be classified as low resistivity ones.