| Qiangtang Basin is located in the Tibetan Plateau and covers an erea about 22×104 km2.It is the lowest exploration basin of oil and gas in China land.Qiangtang Basin located in the eastern section of the Tethys structure,and has similarities with the Middle East region in the tectonic,geological evolution,sedimentary filling and oil and gas geological condition.There is no doubt that the greatly strategic significance of exploration and development of oil and gas resources in Qiangtang Basin to our country,especially the development of the national economy in Tibet.The black rocks,which are rich organic carbon deposits,include dark gray-black mudstone rocks,siliceous rocks and carbonate rocks and their corresponding metamorphic rocks.The black mudstone as the core problem of the formation mechanism is the main research object of the black rocks,and the main control factors of accumulation of organic matter are still very controversial.In recent years,the Upper Triassic organic-rich sediments have been considered as the most significant source rocks in the Qiangtang Basin due to their wide distributions and relatively high total organic carbon(TOC)contents.Our field survey reveals that the Upper Triassic Bagong Formation mudstones are considered as the most potential source rocks in the east part of the North Qiangtang Basin.Bagong Formation mudstones have relatively high TOC and can provide a valuable insight into source rock prediction in the basin.Meanwhile,the organic-rich sediments deposition in delta can provide a good example for understanding the mechanism of organic accumulation.This study investigates the Bagong Formation hydrocarbon source rocks based on the organic geochemistry,element geochemistry,mineralogy,sedimentology,palaeontology and other multi-disciplinary research approach.We evaluated the Bagong Formation hydrocarbon source rocks according to the abundance,type,maturity of organic matter;Analysis the source,maturity and formation environment of hydrocarbon source rock according to the characteristics of biomarker;Analyze the primary productivity,redox conditions,sedimentation rate,paleoclimate and paleoenvironment conditions of Bagong Formation mudstones according to the total organic carbon(TOC)content,mineral compositions,biomarker,palynology results as well as major,trace and rare earth element concentrations,and estimate the main controlling factors on organic matter accumulation of Bagong Formation mudstones.(1)The Upper Triassic Bagong Formaiton is dominated by dary gray to ash black argillaceous hydrocarbon source rock,which deposits are thick and widely distributed.Bagong Formaiton mudstones were deposited in a deltaic environment.The abundance of organic matter is medium,and the type of organic matter is mainly type ?2,and the degree of mature is mature to high mature.Through a compherensive evaluation of Bagong Formation mustones,the results indicate that the hydrocarbon source rock have good hydrocarbon generating potential.(2)The characteristics of the biomarkers of hydrocarbon source rocks in the Bagong Formation mudstones reflect that the source of organic matter is a mixed source,which includes both aquatic low biological sources and the input of terrestrial higher plants.The thermal evolution degree of hydrocarbon source rocks is high,and most of the samples have reached the late stage of the oil stage.The hydrocarbon source rocks are mainly deposited in the marine-continental transitional facies.(3)In the present study,primary productivity indices(Porg and Babio)reveal that the Upper Triassic Bagong mudstones were formed in the water columns of medium to high nutrients,which are generally favorable for organic matter production.On the contrary,redox indicators suggest that Bagong Formation mudstones were formed in the oxic-suboxic environments,which generally against the organic matter conservation.Organic matter enrichment was controlled mainly by primary productivity,which increased as a result of abundant nutrient flux of surface water in the deltaic environment.However,the oxic-suboxic environments and relatively fast sedimentation rate are adverse to the preservation of organic matter.In addition,the clay minerals in Bagong Formation mudstone are relatively high(35-70%,avg.49%),which has a certain adsorption effect on the organic matter.In the Bagong Formation mudstone samples exhibit high CaO(1.7-18.0%,avg.10.6%)concentrations,which could lead to the dilution of organic matter.In summary,organic matter enrichments of Bagong Formation mudstone was controlled mainly by primary productivity,and the high content of clay mineral has a promoting effect on the organic matter.The oxic-suboxic environment and high sedimentation rate are bad for the preservation of organic matter,and caused the organic carbon of Bagong Formation mudstone not high. |
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