Geochemical characteristics and genetic model of dolomite reservoirs in the eastern margin of the Pre-Caspian Basin

Abstract

The widespread Carboniferous KT-I dolomite in the eastern margin of the Pre-Caspian Basin is an important hydrocarbon reservoir. The dolomite lithology is dominated by crystalline dolomite. The δ 18O values range from -6. 71‰ to 2. 45‰, and average 0. 063‰, obviously larger than -2. 5‰, indicating low-temperature dolomite of evaporation origin. Stable strontium isotope ratios ( 87Sr/ 86Sr) range from 0. 70829 to 0. 70875 and average 0. 708365, very consistent with 87Sr/ 86Sr ratios in Carboniferous seawater. Chemical analysis of Ca and Mg elements shows that the dolomite has 9. 1 mole% excess Ca or even higher before stabilization. The degree of order of dolomite is medium-slightly poor, varying in a range of 0. 336-0. 504 and averaging 0. 417. It suggests that the dolomite formed under near-surface conditions. There are two models for the origin of the Carboniferous KT-I dolomite reservoir. These are 1) the evaporation concentration - weathering crust model and 2) the shoal facies - seepage reflux model. The former is mainly developed in restricted platforms - evaporate platforms of restricted marine deposition environments with a representation of dolomite associated with gypsum and mudstone. The latter mainly formed in platform edge shoals and intra-platform shoals and is controlled by dolomitization due to high salinity sea water influx from adjacent restricted sea or evaporate platform. © 2012 China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg.