Depth of Diagenetic Processes and Lower Boundary of the Biosphere in the South Caspian Basin

Abstract

An assessment of the depth of the diagenetic zone and biosphere in the South Caspian Basin (SCB) is given based on a set of more than 400 measurements (lithological composition and reservoir properties of rocks, temperature, and pressure) on 41 onshore and offshore hydrocarbon fields; it is also based on about 200 isotope-geochemical analyses of gases and oils and hydrochemical analyses of formation waters. It is established that an extended diagenetic zone is formed in the SCB under conditions of an avalanche sedimentation rate in the Pliocene–Quaternary period and low temperature (temperature gradients were within 15–21°C/km). This is confirmed by the nature of the depth variations in the physical and mechanical parameters of the rock-fluid system in the SCB (water and gas saturation, porosity and permeability of sediments, temperature and pressure). From the point of view of oil and gas generation processes, the boundary between the end of the stage of late diagenesis and the beginning of the early catagenesis stage in the SCB is located at a depth of about 4.5 km, which corresponds of a vitrinite reflectance value of 0.5% and formation temperature of about 80°C. According to the isotope-geochemical parameters, microbiological activity, and biochemical processes, the lower boundary of the biosphere is outlined at a depth of about 2–2.5 km, which corresponds to a temperature of 45–50°C.