The Sedimentary Model And Hydrocarbon Generation Potential Of Yurtus Formation Source Rocks In Tarim Basin

The Tarim Basin is the largest inland petroliferous basin in China and one of the typical basins in developing marine hydrocarbon source rocks.The strata of Cambrian-Ordovician is the main source rocks in the Tarim Basin.The Lower Cambrian Yurtus Formation in Tarim Basin developed high-abundance and high-quality source rocks,and depositeded siliceous rocks with complex and diverse sedimentary facies at the bottom of Yurtus Formation,which has important research value of oil-generating potential and indicator of sedimentary environment.Selecting the Yurtus Formation source rocks as the research object,using petrology,rock geochemistry,organic geochemistry and sedimentology and other theoretical research results,based on field observations,geochemical analysis and other research methods,tectonic setting,sedimentary environment,sedimentary model of siliceous rocks and hydrocarbon generation ability of the Yurtus Formation source rocks have been systematically analyzed and studied.Sedimentary model of the Yurtus Formation siliceous rocks has been established,hydrocarbon generation ability of the Yurtus Formation source rocks has been evaluated.During the deposition of the Yurtus Formation,it was the extended passive continental margin environment,and the hydrothermal activity of the basin during this period was relatively active.Sedimentary parent rocks were mainly andesite,probably with the mixture of felsic volcanic rocks and granite.The parent rock was under the plagioclase weathering stage and underwented weaker weathering without significant sedimengtary-regenerated history.The Yurtus Formation was a set of sedimentary facies characteristics of the inner ramp-the middle ramp-the outer ramp-the inner ramp from bottom to top and constituted a complete transgressive-regressive cycle.Sedimentary paleo-seawater has the characteristics of low energy hydrostatic,high salinity,acidity and anoxia reduction.In some periods,paleo-seawater was characterized by sulfidation due to the hydrothermal activity that carried large amounts of acid gas?H₂S?.The Yurtus Formation siliceous rocks was less affected by source debris.High-salt,high-alkalinity and anaerobic seawater could dissolve large amounts of SiO₂,which were provided by silicon-rich hydrothermal fluids,rising silicon-rich ocean currents,and river inputs?dissolved silicon?.Acid gases and bioacids led to the precipitation of SiO₂,the deposition of SiO₂ resulted in the formation of silicalites dominated by hydrothermal fluids or ascending ocean currents at different layers.The hydrothermal and ocean current activities also provided favorable conditions for the enrichment and preservation of organic matter.The Yurtus Formation source rocks was a high abundance source rock with an average thickness of 20m.The upper source rocks was a potential source rock.However,due to generally high thermal evolution maturity,the hydrocarbon generation potential of the lower source rocks was poor.The main source of organic matter were algae,plankton and various types of bacteria,the organic matter type was mainly I-?.In addition,there was a high similarity in the biomarker parameters between the Yurtus Formation source rocks and the Bashituo oil,indicating that the hydrocarbon generation activities in the history had a significant oil supply value to the Tarim Basin.