| The Qaidam Basin, surrounded by the Qilian Mountain, Altyn Mountain and Kunlun Mountain, located in the NE of Tibetan Plateau. It has an average elevation of2700m. In this area, few geological research and petroleum exploration of Carboniferous have been done at present as a result of the complexity of evolutionary and low exploration degree of the basin. There were only a few wells drilled up to the Carboniferous. The seismic exploration degree was rarely low and the target bed recognition was also difficult. In recent years, as a new exploration layer, the Carboniferous attracted people’s attention with the further petroleum exploration. Through incessant effort and researches, the Carboniferous will be the important superseding layer of the petroleum exploration in Qaidam Basin. The previous researches were mostly on the stratigraphic characteristics and sedimentary structure of the outcrops area on the boundary of the basin. The further study of the distribution and geochemical characteristics of the Carboniferous source rocks was little in previous researches, which influenced the exploration process of Carboniferous in Qaidam Basin.In this paper, on the basis of the field geological investigation and the research data, guided by petroleum geological, geochemistry and geomechanics, followed the theory of controlling of the tectonic systems on the source rocks, we conducted further researches on the source rocks about the distribution and geochemical characteristics of it, using the intergrating outcrop, seismic and drilling data. The characteristics of structural systems and its controlling function on the source rocks was discussed in the paper.The outcrops geological observation and comparison indicated that the Carboniferous source rocks widely developed in most area of Qaidam basin. The max thickness of900m was located in the Oulongbuluke Mountain in the north margin of basin, the thickness of source rock in the Gerermu-Wutumeren-Gansen area of the southern margin was about400-500m as zonal shape distribution along the basin margin, The thickness in the northwest and the southeast became thin.The dark mudstone, carbonate rocks and coals were well developed in Qaidam basin. From the study of geochemical analysis assay data of the samples, the dark mudstone had an organic carbon content range of0.09%～7.0%with the average of1.28%, among which there was13.9%samples’organic carbon content over0.75%. The Wanggaxiu area had a high organic carbon content with moderate to good source rocks of73.3%, followed by the66.7%in the Zhabusagaxiu area. The organic matter was mainly of Ⅲ and Ⅱ2types and the thermal maturity reached up to mature～high mature stages. Thus, the dark mudstone belongs to moderate to good source rocks. The carbonate rocks had a low organic carbon content, with average of0.37%, and the organic matter was Ⅱ1～Ⅱ2type, the hydrocarbon generation potential was poor. The coal has a low organic carbon content with a relative high content of desmocollinites. So it has hydrocarbon generation capacity to some degree. Considering the samples were from the surface occurrence, generally speaking, the Carboniferous dark mudstone and carbonate rocks were belong to moderate to good source rocks and the coal might had hydrocarbon generation capacity.Through the outcrops geological observation, it was the first occurrence of the liquid oil seepage in Carboniferous of Qaidam Basin which found in the calcite veins of Carboniferous limestone in Wanggaxiu coal mine in Derlingha. Through the analysis of the mass chromatogram of oil seepage, the tricyclic terpane has a normal distribution with C23as the main peak, its Pr/Ph value was0.64and had a high abundance of gamacerane. In distribution patterns of C27, C28and C29regular sterane, it presents a "V"distribution, the level of C27was the higher than the C29. The isomerization parameter of C29sterane was obtained as0.39and Ts/Tm value as1.12, which imports that the oil seepage was mature. The thermal maturity of the oil seepage reached up to mature stage as the isomerization parameter of C29sterane was obtained as0.39and Ts/Tm value as1.12. It showed the source rocks was formed in the brackish environment, which mainly came from lower hydrobiont such as bacterial and algae, parts from the terrestrial plants. The oil-oil and oil-source rock correlation of oil seepage and Carboniferous source rock underground and surface indicated that the oil seepage was similar to crude oil in Mabei-2well (1463m). It is inferred that the oil seepage was authigenic crude in Carboniferous. This study showed that it ever had hydrocarbon generation in the Carboniferous in Qiandam basin.Through the analysis of the structural systems in Qaidam Basin, it developed several structural systems such as the Western Regions, latitudinal and the Altyn structural belt. The forming and developing of Qaidam basin and the distribution of the hydrocarbon was influenced by the evolution of the structural systems. The distribution of Carboniferous source rocks was mainly compound controlled by the Western Regions, latitudinal and the Altyn structural belt.The max thickness was located in the Oulongbuluke Mountain in the north margin of basin which influenced by the Western Regions structural systems. The thickness of source rocks in the southern margin as zonal shape distribution along the basin margin was influenced by the compound effect of the Western Regions and latitudinal structural systems. |
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