Carbon and oxygen isotopic compositions, and palaeoenvironmental significance of saline lacustrine dolomite from the Qaidam Basin, Western China

摘要

Recently, significant progress has been made in tight reservoir exploration of petroleum in the western Eocene Qaidam Basin in Western China. Among these rocks, there are high-quality dolomite reservoirs, of which we studied the petrological, mineralogical, and geochemical characteristics. These analyzes revealed that the dolomite structures are mainly micritic, with a widely developed intercrystal pore. The pore radius measured less than 1 pm and the micritic dolomite delta C-13(PDB) values ranged from -4.5 parts per thousand to 0.2 parts per thousand, with an average value of -2.4 parts per thousand. These values are negatively delta C-13 biased when compared to other lacustrine carbonate rock in China, indicating that the braided delta supplies light carbon-rich surface water. Over time, the dolomite carbon isotopic composition changed due to dissolved carbon from organic matter, which produced CO2 via oxidation disintegration. In the case of light carbon, the rich organic matter resulted in a lighter isotopic composition of the dissolved carbon and modified the lacustrine deposit dolomite carbon isotopic composition indirectly. In addition, the micritic dolomite delta O-18(PDB) values ranged from -6.1 parts per thousand to 0.4 parts per thousand, with an average value of -2.6 parts per thousand. This value is positively delta O-18 biased compared to other lacustrine carbonate rock in China and was affected by the saline lacustrine environment. The oxygen and carbon isotope correlation, salinity, and temperature data reflect that the dolomite micritic formed in both a semi-open and semi-closed saline environment on a penecontemporaneous stage via calcite metasomatism in the sediment interface under a low-temperature. This type of penecontemporaneous metasomatism dolomite is widely distributed throughout the study area, and the intergranular pores possess a strong anti-compaction ability. This critical discovery of the plane distribution of the tight dolomite in the Qaidam Basin has provided multiple novel avenues of study within the field. (C) 2015 Elsevier B.V. All rights reserved.