Biomarker Characteristics And Oil-source Correlation Research Of The Reservoirs In Southern Qiangtang Depression

Qiangtang basin is located at the central area of the Tibet-Qinghai Plateau, and it is the largest marine basin with the most probable oil-gas prospective. The oil-gas exploration of Qiangtang basin could be used to evaluate the next big oil-gas zone and the next energy base. The oil-gas exploration in Qiangtang basin started in 1970 s and lots of researches on the petroleum geological settings have been carried out. Based on these previous researches, this paper strive to find out the basin effective hydrocarbon source rocks and their forming mechanism, clerifying reservoir space-time evolution characteristics, the reconstruction of reservoir accumulation mode according to the ground reservoirs of Buqu formation and petroleum geology and oil-gas geochemistry methods. Presently, the following acquisitions have been stated:Combined systematic study of the 32 ground reservoirs on Qiangtang basin, precise section controlling surface oil-rich reservoir with the representative drilling data, this paper acquired that the major reservoir belt of southern Qiangtang basin is Buqu Formation and the dolomite is the main reservoir. The top and bottom boundaries of Buqu formation are both easily recognized through the lithology of Xiali Formation and Shaqiaomu Formation. With typical sections controlling the thickness of oily belt and the thickness between the top and bottom surfaces, the oily zone in Buqu Formation are found to be composed of three layers, which distributed seperately on the lower, middle and upper part of Buqu Formation. Organic Geochemistry research supports this view. Besides, oil-oil comparison shows that different interval of oil and gas filling way existence difference: the top and bottom of the main filling the third class of crude oil, respectively, in turn, filling the second central crude oil and the first kind of crude oil.After field research and thin section of the oily dolomites, the authers deduced that the oily dolomites were the product of the grain bank limestone through burial dolomitization, and the spatial distribution of dolomites is controlled by the platform edge grain bank. Based on the location of the grain bank beds in Buqu Formation, the section sequences and the spatial distribution, a sedimentary pattern of the dolomite reservoirs in Buqu Formation is put forward: the platform edge grain bank is developed in Sairen-Angbacundong area during the initial stage of the Buqu period. As sea level rising, platform edge line moved towards north, and might form platform grain bank and stromatolite reef between Xiaogaxiaona and Rigaerbao. The platform edge might have reached Zharen in north in the max flooding period. The platform edge reef and bank in Long’eni area indicates a depression process and southern movement of platform. The platform edge grain bank belt in Geluguanna area had migrated at least 10 km towards south, and the belt had been staggered from the Bagedijiari platform edge shoal belt. Two distinct reservoir belts separately in south and north ware formed.Based on the comparison of the biomarkers among the carbonate source rock, clastic source rock, oil-shale source rock and coal measure source rock, and combined with oil source comparison within different types of crude oil, the crude oil in Buqu Formation reservoir are found to be inherited with oil-shale source rock. It is possible that the oil-shale series or black rock series are the effective hydrocarbon generating layer. The evaluation are carried out that the oil-shale series have a considerable hydrocarbon generating potential.The result of oil-source rock correlation indicates that various crude oil all come from Shale rock source rock, and the difference dues to hydrocarbon charging histories in the reservoir. Combined with dolomite reservoir development model, the upper and bottom oil-bearing strata of dolomite reservoir are thought to lie in Southern Qiangtang depression south area near hydrocarbon- producing sag, however, the middle oil-bearing strata lie in the north far from hydrocarbon- producing sag. Vertical migration is faster than long-distance lateral migration due to that Carbonatite reservoir is filled with vertical fissures, which is the main form of the Hydrocarbon expulsion in Effective source rocks. Hydrocarbon first migrates vertically into the upper and bottom oil-bearing strata, then the late high-mature crude oil will push the early and relatively low mature ones to migrate in planar form, causing the third kind of crude oil, the second kind ones and the first kind ones charging into the reservoir successively.The above findings indicate that there are two possible sets of advantageous source-reservoir-cap assemblage in Southern Qiangtang Depression. But up to now, massive hydrocarbon generation-migration-accumulation processes just has been found in the former one: Quse Formation oil shale Source Rock—Buqu Formation dolomite reservoir—Xiali Formation mudstone cap rock assemblage. Based on the understanding on the petroleum geologic condition, and combined with breakthrough basin exploration experience, this paper raise two accumulation models for Southern Qiangtang depression: denudation-exposure model and thrust-nappe model.