GEOHISTORY, THERMAL HISTORY AND HYDROCARBON GENERATION HISTORY OF THENORTH-WEST SOUTH CASPIAN BASIN

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 f low/compaction model. The model consists of three parts: a geohistory model; a thermal history model; and a hydrocarbon generation model. In put data for the model are commonly used depth values of geological pa rameters: lithofacies, formation thicknesses, stratal ages, porosity, permeability, pore pressure, temperature, total organic carbon content , type of organic matter, vitrinite reflectance and other maturity ind icators. In order to bracket oil generation in terms of timing and dep th location, two extreme thermal histories were modeled: paleoheat flo w values at one half and twice the present-day values with a linear va riation through time to present day measured values. The most intensiv e 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 th e west (onshore part of the study area), and 8-11 km in the central an d southwestern parts of the area. Below the 'oil window' cracking occu rs of oil into gas. Rapid sedimentation, which took place in the middl e Pliocene through Quaternary, results in overpressure build-up across all of the study a rea. Maxi mu-m values of excess fluid pressure of 300-400 atm at 6-10 km a re reached in the central and southeastern pa rts of the area, where the sedimentation rate is the highest. Isobaric contours of excess pressure a re then at shallow depths compared to t he previous history of the area. Because there is a laterally decreasi ng trend of excess pressure, oriented from the central and western par ts 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 flow s towards the northeast with hydrocarbon accumulations of gas and ligh t oil likely to be more prevalent in the northeastern sands. (C) 1997 Elsevier Science Ltd.