Paleothermicity and hydrocarbons generation modelization in the eastern part of the Precaspian Basin and Pre-Uralian Basin (Kazakhstan)

Abstract

The Precaspian basin is the main oil province in Kazakhstan, where there are such giant fields as Tengiz, Karachaganak, Astrakhan and Kashagan. In previous works have been identified source rocks associated mainly with the sub-salt complex. However, the issue of such parameters as the depth of the oil window remains unresolved, because the geothermal gradient is low and the studied source rocks have a low maturity. By the study of the organic matter in these sediments and simulation of genesis of hydrocarbons by Petromod 1D software (IES Gmbh; now Slb), we wanted to determine the depth range of oil window for essential source rocks. This area is composed of three parts from Devonian to late Carboniferous: the Precaspian Basin subdivided into a basin (claystones, mudstones and calciturbidites) and a platform (limestone and storm sandstones), and the Pre-Uralian Basin (turbidite sandstones). For the studied sector the values of the Ro are low and varies from 0.4 to 0.7 and Tmax lies between 420 and 445°C. The simulation of the heat flow was calibrated by Vitrinite reflectance with different kinetics according to the type of organic matter – Vandenbroucke et al. (1999)’s model was chosen for the type III (Brent) and for the type II (KCF). The low value of the heat gradient can be compensated by the geological duration during which source rock undergoes the transformation. In the sector studied the phase of generation of oil can correspond to the level of evaluation, corresponding to value 0.5% of Ro and even 0.4% for the algal type or mixed OM II/III. For the studied formations, the major part of hydrocarbons was generated from the maximal phase of burial. We can distinguish four zones in which the depths of the beginning and end of the oil window are different: the western zone (basin) from 4-4.5 km to 7-9 km, the zone of slope and border from 3-3.7 km to 4.5-5 km, the zone of Preuralian basin from 2.8-3.3 to 4.5-5 km, and the eastern zone under the thrust of Ural (Izembet) from 2-2.8 to 3.5-4 km.