Tectonic And Exploration Prospect In Carboniferous Of Qaidam Basin

Qaidam Basin is surrounded by the Qilian Mountains, Altun Mountains and Kunlun Mountains. In this area, few geological research works and oil and gas exploration of low level in the upper Paleozoic have been done. In this paper, guided by structural geology and sedimentology, we conducted in-depth research on the structural characteristics of the Qaidam Basin, distribution characteristics of sedimentary strata in the Upper Paleozoic, and the development characteristics of hydrocarbon source rock, using integrating outcrop, seismic and drilling data. Moreover, by analyzing a large number of test data, we also evaluated the Carboniferous exploration prospect in the Qaidam Basin, which will have great significance for guiding the Upper Paleozoic oil and gas exploration work in Qaidam Basin.Through the 18 field outcrops in the field observation and comparison, combined with interpreting and tracking seismic profiles, we found that the Upper Paleozoic strata is not only exposed in the periphery of basin, but a large number of Upper Paleozoic strata also exist within the basin. And thick sedimentary strata exist near the Delingha area and the southern Amunike Mountains.By studying on the geological section of the whole region, we found that Qaidam basin performance tectonic pattern as a basin within which there is a mountain, and in turn among of the mountains there is a basin. And there formed a multi-uplift, multi-depression, and uplift and depression alternate distribution of tectonic pattern. The basin contact surrounding mountains by the fault, and the mountain thrust to the basin, whose direction point to the basins, while the fault surface tend to the side of the mountain, and form the tectonic style about North thrust -South tend and south thrust - north tend. Basin contacts the Altyn Tagh fault in deep level and in shallow level the fault overlap the basins. From studying on the fault, we divided the whole basin into three geological systems: the anti-S-fault system of northern basin, pressure torsional fracture system of north Kunlun Mountain and the weak deformation fault system in the central of basin. Based on the study of the principle of division about tectonic features, transportation of the depocenter, distribution of the oil and gas, lithology character and fluctuate of the bedrock and the character of the abnormality in gravitation and magnetic field, we divided the Qaidam Basin into east and west units by the line of the Ganshen-Xiaochaidan where the characters of sedimentation and tectonic deformation start to defer. The tectonic deformation is more intense in the west than in the east of the Qaidam basin. Then we divided the Qaidam Basin into six first-order units: the obduction zone in south of Qiliang mountains, the obduction zone in north of Kunlun mountains, Oulongbuluke uplift zone, the Yiliping, Sanhu and Delingha Depression.From the study of paleocurrent data that is collected in the field and geochemical behavior of Neopaleozoic volcanic rocks, it is concluded that the land sources of the eastern part of the basin generally come from the north, which reflects that the direction of transgression is from the south to the north and the depocenter lies in the southern part of the basin. The tectonic settings were intraplate tension in later stage of collision in Late Devonian, intracontinental tension in Early Carboniferous and ocean-continent underthrust in Late Carboniferous. The Chaidamu Block Mass developed a series of slim graben-like basin or ocean trough and formed the prototype of Chaidamu central uplift (Niubiziliang ancient land) because of tension from Late Devonian. The Carboniferous depositional basin, with a depression and uplift palaeogeographic framework, is a differential subsidence basin, which formed by Hercynian movement on the basis of Late Devonian rift trough. It was a rift basin deposited with neritic facies carbonate rocks and was extensively transgressed. Lagoon, fan dalta (or braided river dalta) of transitional facies and coal-bearing deposition of paludal facies were formed around the ancient land. Permian period had the same sea area as that of Carboniferous Period, which still located in epicontinental faulted depression and belonged to clasolite-carbonate construction with stable shallow marine deposition.Through the analysis of field samples for testing, the results show that by boundary with the Shenglikou-Xitieshan line, the Upper Paleozoic strata widespread corruption in the northwest of Qaidam Basin, while the eastern region is relatively weak metamorphism. Moreover, a strong regional metamorphism does not have hydrocarbon potential. Carboniferous source rocks which are mainly dark mudstone, carbonaceous mudstone and coal may have the ability of hydrocarbon. Carboniferous dark-colored mudstone, carbonate rocks, coals and carbonaceous mudstone are well developed in the eastern Qaidam basin. Regional geological surveying and organic geochemical analysis indicate that Carboniferous hydrocarbon source rocks consist of the main part of dark-colored mudstones and carbonate rocks among which coals and carbonaceous mudstones probably have hydrocarbon-generating capacity. The dark-colored mudstones have a poor to moderate grade content of organic carbon in lower Carboniferous, but in upper Carboniferous there are good or better grade content of organic carbon and the organic matter is mainly ofâ…¡2 andâ…¢types, whereas the limestones have a low content of organic carbon, and the organic matter is also of â…¡2 andâ…¢types. Therefore, they belong to poor to moderate hydrocarbon source rocks. The dark-colored mudstones evaluated that are better in organic carbon develop in tidal flat, lagoon and marsh-phase sedimentary fans. The horizon that is good in evaluation is the middle of Huitoutala group, Kuluke group and below of the Zhabushagaxiu group. All the source rocks are in a peak period of oil generation and expulsion with a moderate maturity of organic matter and have good petroleum prospects, except those in the Dulan and geermu area that contain post-mature organic matter and are in a post-mature, dry gas-generating stage.