The Study Of Depositonal Conditions And Its Significance In Petroleum Geology Of Shengli River Oil Shale In The Qiangtang Basin, Northern Tibet, China

Qiangtang basin is a residual basin, which developed on the Precambrian crystalline basement and Palaeozoic fold basement. It's mainly filled by marine strata. In the basin, there are three secondary structure units which are named the Northern Qiangtang depression, the central uplift and the Southern Qiangtang depression. The Shengli River zone is located in the Longweihu sag, which is a tertiary structure unit in Northern Qiangtang depression. The intensive tectonic movements, huge Neozoic-magmatic activities and big fault belt are absent in the sag. In recent year, the largest marine oil shale resource in China has been discovered in the Shengli River zone. The aims of this study are to enrich the theory study of oil shale in China and promote the development of oil and gas exploration in Northern Qiangtang depression, by studying the depositonal conditions and the significance in petroleum geology of Shengli River oil shale.Base on the characteristics of geophysics, regional tectonic setting and sedimentary setting, the depocenter pattern of Northern Qiangtang depression in Late Jurassic can be simulated by using the formation thickness from 1:250000 geologic map as control points and interpolating data by the updated Xie Be D method. On the base of depocenter pattern of Northern Qiangtang depression, we had studied the characteristics of lithofacies paleographic map during the Shengli River oil shale depostional period, by synthesizing the data of regional geologic setting, the section and the trenches of oil shale in Shengli River zone.Base on the study of the lithofacies paleographic map and the enrichment mechanism of organic in sedimentary matter, we conjecture the enrichment mechanism of organic in Shengli River oil shale coincidence with the layering-water-accumulation model, by integrating the characteristics of lithology, sedimentary water body an so on. The control action of depositional conditions are expressed by three steps, which are bay-lagoon, semi-closed lagoon and closed lagoon.The experimental analysis of GC-MS and Rock-Eval indicate that the thermal evolution of Shengli River oil shale had got the low maturity phase. The biodegradation degree of Shengli River oil shale is range rank 3 to rank 4. The steranes and terpanes in the Shengli River oil shale can reflect the geologic information effectively. The organic matters contented in Shengli River oil shale are mainly from phytoplankton, partly from bacterium, a little from bottom fauna and terrestrial plant. The sedimentary water body is salinity stratification. The oil shale sedimentary environments are anoxic and relatively hypersaline. The characteristics of C&O isotope in oil shale section indicate the climate in the Qiangtang basin region during the deposition of oil shales was warm and humid or sub-humid. However, it's hot and arid during the plaster formation sedimentary period.According to the regional tectonic setting, the distribution characteristics of stratum in Longweihu sag, the characteristics of fluid inclusion under NO.1 prospect hole in Qiangtang basin, the characteristics of Ro in organic matters in stratum an so on, we had got the geothermal gradient value is 2.559℃/100m when the oil shale burial to the largest depth in history, by the regression analysis method base on the Ro—Tmax model. The largest burial depth of Shengli River oil in history was 3194.4 m. The threshold depth in that time is 940.8±m upon the oil shale formation.Base on the semi-quantity study of denudation thickness by Ro—Tmax model, the formation thickness and the basin evaluation characteristics, we had got the characteristics of 1-D burial and thermal history of Longweihu sag, by using the Basinmod software which was developmented by Platte River Associates, USA. According to the burial history, the hydrocarbons expulsion simulating experiment of Shengli River oil shale, we are supposed that the Shengli River oil shale had got the expulsion threshold. Finally, we recognized that the Shengli River oil shale and the formation sediment in the same time may be the source rock in J3s—N21s(·) petroleum system and as cap formation in J2b—J2x(·) petroleum system in Longweihu sag.