Geohistory, thermal history and hydrocarbon generation history of the north-west South Caspian basin

Abstract

Both the hydrocarbon potential of, and excess fluid pressure build-up in, the thick (25–30 km) sedimentary pile of the northwestern part of the South Caspian Basin were evaluated using a one-dimensional fluid flow/compaction model. The model consists of three parts: a geohistory model; a thermal history model; and a hydrocarbon generation model. Input data for the model are commonly used depth values of geological parameters: lithofacies, formation thicknesses, stratal ages, porosity, permeability, pore pressure, temperature, total organic carbon content, type of organic matter, vitrinite reflectance and other maturity indicators. In order to bracket oil generation in terms of timing and depth location, two extreme thermal histories were modeled: paleoheat flow values at one half and twice the present-day values with a linear variation through time to present day measured values. The most intensive oil generation occurs during the last 5.2 My, when the ‘oil window’ is confined to depths between 5–7.5 km in the northwest, 7–10 km in the west (onshore part of the study area), and 8–11 km in the central and southwestern parts of the area. Below the ‘oil window’ cracking occurs of oil into gas. Rapid sedimentation, which took place in the middle Pliocene through Quaternary, results in overpressure build-up across all of the study area. Maximum values of excess fluid pressure of 300–400 atm at 6–10 km are reached in the central and southeastern parts of the area, where the sedimentation rate is the highest. Isobaric contours of excess pressure are then at shallow depths compared to the previous history of the area. Because there is a laterally decreasing trend of excess pressure, oriented from the central and western parts of the area to the northeast and, in addition, the sand/shale ratio is increasing in the same direction, inferences that can be drawn on the most probable hydrocarbon migration directions suggest recent flows towards the northeast with hydrocarbon accumulations of gas and light oil likely to be more prevalent in the northeastern sands.