| The Qaidam Basin, 12 km2 in area and bounded by Altyn Mt., Qilian Mt. and Kunlun Mt., is located in the NE of Tibetan Plateau. The basin is one of the big petroliferous basins in the western China and is the highest petroliferous basin above sea level in the world. For its special geographic location, complicated tectonic setting and the ample resources, the Qaidam Basin has been one of the hot research areas to the earth scientists and petroleum geologists.After more than half century exploration 25 oil fields have been found and 6 hundred millions reserve have been proved. The studies show that Qaidam Basin has its special tectonics and complex revolution history which are different from the basins in the middle and eastern China, and even quite different from the other basins in western China and the other similar type basins in the world. The studies of petroliferous basin tectonics is very important for oil exploration. The tectonic subsidence controls the distributions of sags and the oil-producing center. Deformations form different oil and gas traps. Therefore, the research of geological setting, evolution, basin tectonics, faults and their controls on the oil and gas distribution have important scientific values, enrich the theory of petroleum geology, and have guiding importance for oil and gas exploration in Qaidam Basin.Based on the studies on the basement and cover of Qaidam and Tarim basins, and the tectonic reconstructions on the both sides of Altyn fault, the Qaidam Basin has a uniformed Precambrian basement. The concept of"Western Craton"has been put forward, which describes that there existed a combined craton consisted of Qaidam, Tarim and Alex Blocks during Paleozoic. The Qaidam and Tarim basins had been connected each other before Cenozoic.There generally occurs a thrust fault between Qaidam Basin and surrounding mountains. To the front of Altyn Mt. the Cenozoic sediments overlapped on the southern slop of the Altyn Mt. on the surface, but the basement rocks from Altyn Mt. overthrusted on the basin sequences in the deep. However, the Kunlun Mt. and Qilian Mt. overthrusted on the basin sequences in the mountain fronts. There occurred main four major tectonic events during Meso-Cenozoic, they are Late Indosinian, Yanshanian, Himalayan and Neotectonic events of which the Yanshanian, Himaliyan and late Early Pleistocene events are more important to the formation of oil and gas in Qaidam Basin.Qaidam Basin is a typical inner continental lacustrine sedimentary basin. The basin has been filled by more than 17000m thick Meso-Cenozoic sediments from Jurassic to now with 213 Ma long history. There are three petroliferous systems in Qaidam Basin. In the northern area of the basin the petroliferous system consists of Middle-Lower Jurassic source rocks and Mesozoic and Tertiary reservoir and cover rocks. The petroliferous systems with only Tertiary reservoir and cover rocks were mainly developed in the Mangai depression, and with the Quarternary reservoir and cover rocks only in Sanhu depression.The surface structures consist of a series of en echelon"reverse S"fold belts with NWW or NW trending. The anticline is narrow, but synform wide. These en echelon"reverse S"fold belts mainly develop in the northern areas. The trending of the belts from west to east changes from NWW to NW, then to NWW. The anticline locations move toward SE from north to south. Every"reverse S"fold belt consists of several anticlines, and every anticline consists of several small heights. The western head anticlines of the"reverse S"fold belts are tight with SW dipping axial plane. The eastern tail anticlines of the"reverse S"fold belts are wide with NW dipping axial plane. The formation of the"reverse S"fold belt developed gradually from the head to tail.The Meso-Cenozoic anticlines in Qaidam Basin are divided into two types. One type is basement swell-controlled anticlines which inherit the basement swells. There often develop a wide depression between anticlines and forms pectination anticline system. The sediments on the axial areas are think and coarse-grained. These kind of anticlines formed early and are located close to the source rock sags with ample hydrocarbon, therefore, they have generally high hydrocarbon concentration. The other type anticline is shallow cover fold which formed about early Pleistocene in the deep depression of the central basin without control by basement swell. The anticlines in Sanhu sag, such as the Tainan anticline, are late shallow fold, but with syn-sedimentary characters. The anticlines are wide and are Quaternary gas-producing structures in the basin.From Miocene the basement in the NW part of the basin started gradually uplifting. The depocenter moved toward SE. The basin receded from the west and overlaid to the east. The anticlines of the cover were formed from deep to surface and from west to east with the time. The height of the anticline in the deep immigrates toward NW relative to the height on the surface.Folding events were getting stronger form Mesozoic to Cenozoic. The evolution of Qaidam Basin is divided into three stages. The first stage is Jurassic-Cretaceous, during this period the Qaidam Basin and the SW part of the Tarim Basin were connected as one extensional half-graben basin, the depocenter were located in the NW and small with relative thick Jurassic-Cretaceouse sediments. The second stage is Oligocene-Pliocene during which the Qaidam Basin reached the biggest deposition area. With the uplifting of the Altyn Mt. from late Miocene, the basin depocenter was moved toward east more than 240km. During the Quaternary (the third stage) the depocenter was in the Sanhu areas in the SE part in which more than 3500 km thick lacustrine sediments were deposited.Multiple tectonic events formed different genesis and style structure frameworks, the overprinting of these structures is very important for capturing the oil and gas to form reservoir. The typical examples of this kind of reservoir are Gasikule oil field, Huatugou oil field and Shaxi oil field and so on in the western part of the basin, which are all large oil fields and located in the overprinted areas of the NWW and NNW structures.The thrust faults in the basin were well developed, which are so-called spade thrust faults with high dip angle but small offset close to the surface and lower dip angle but relative large offset in the deep. They often have growing fault features. Some slide thrust faults have high dip angle but large offset close to the surface and lower dip angle but relative small offset in the deep, they are often disappear in the Oligocene mudstone layer and not influence the deep structures. The faults have preferred orientation horizontally. In the NW part of the basin the fault system distribution is banded from N to S, sectioned from W to E and layered from surface to deep.The reservoir formation in the Qaidam Basin has close relationship with the fault development. The large scale faults control the formation and deformation of the whole basin, and also control the distributions of oil-producing sags and sedimentary facies, they provide different geological settings to form different trap structures. The fault systems control the petroliferous system, their development and continuing activity control the location, shape and distribution of the traps.The fault system controls the distribution and development of the petroliferous zones. Fault-controlled extension structures, flower structures and nappe-thrust structures control the distribution and development of the most traps in the basin. The petroliferous zones occur often together with fault belts. Fault is the pass way to connect source rocks to traps, the traps close to the faults which connect to source rocks are generally good reservoirs.Late Himalayan movement generally breaks the original reservoirs and results in the re-transporting of oil and gas, which forms the secondary reservoirs in the shallow level or overflow to the surface to form oil seepage. Therefore, the faults during Himalayan period not only break the original reservoirs but also favor of forming secondary reservoirs.The oil fields in Qaidam Basin occur in oil-producing depressions or in the margin of the depressions. Oil fields are distributed in rings, e.g. the oil in inner ring and the gas in outer ring. Reservoirs are distributed along the fault zones and normally located on the height of fault-related anticline. The reservoirs occur vertically in different layers. In the SW part of the basin the reservoirs occur in E13 and E23-N11, in E32-N11 and N1-N2 in the middle part and in all Cenozoic layers in the northern margin area.The main exploration targets in the future should be focused on the Tertiary in the western part of Qaidam Basin, which has ample hydrocarbon resources and favorable conditions of forming reservoir. Many oil and gas fields have been found in this area, therefore, the area has a big oil exploration potential. More exploration works and the systematic research should be done in this area, including the analysis of the favorable petroliferous zones and the target selection of the exploration. While more detail explorations are carried out for complicated structures and lithological reservoirs in the SW part of the basin, the deep layer reservoir exploration and studies should be emphasized in the front thrust zone of Kunlun Mt., Shizigou– Youshashan - Dawusi structure belt, and Jiandingshan and Nanyishan in the NW part of the basin.The northern margin of Qaidam basin has potential to finding middle-scale oil fields, more seismic exploration and research works should be carried out in this area, especially in Lenghu and Maxian structure belts. The exploration for the Quaternary bio-gas should be focused on the small scale structures and lithological reservoirs.
|
PDF Full Text Request