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The widespread Carboniferous KT-Ⅰ dolomite in the east margin of the Pre-Caspian Basin is an important hydrocarbon reservoir.The dolomite lithology is dominated by crystalline dolomite.The δ18O values range from -6.7 1‰ 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.