Depositional System Of The Triassic Yanchang Formation In Southwestern Ordos Basin

New breakthrough has been made in oil exploration of Yanchang Formation in the southwestern Ordos Basin,then the concern of the Mesozoic oil and gas exploration has aroused again.Precise and systematic study on provenance of the sediment,sedimentary microfacies,distribution of sand and hydrocarbon source rock has become important basic subject.Base on the theories of sedimentology,geochemistry and petroleum geology,the data of the outcrops,core and well,and the advanced experimental methods.The provenance and sedimentary characteristics have beed analyzed.The distribution of sand,reservoir and hydrocarbon source rock has been study in order to reveal the regularity of hydrocarbon enrichment.The provenance of the Chang 10-Chang Formation in South,western Erdos Basin is mainly from the North Qilian and North Qinling orogenic belt.The main mother rock of the provenance is metamorphic rocks of the Longshan Formation and Qinling Formation,the Paleozoic and early Mesozoic magmatic rocks.It indicates that the source contribution of the Neoproterozoic strata is weak by detrital zircons LA-ICP-MS U-Pb dating,and the source supply of the Neoproterozoic strata disappears gradually after Chang 10 period.The source contribution of the Middle Proterozoic strata decreases gradually in Chang 10?Chang7 period,but the source contribution of the Paleo Proterozoic and Archean increases gradually.During the Chang 10?Chang7 period,it develops braided fluvial deltas-lake depositional system in Southwestern Erdos Basin,then divides into 4 types of sedimentary subfacies and 7 types of sedimentary microfacies.The major sedimentary facies is braided fluvial delta plain during the Chang 10 period.In Chang 9?Chang8 period,the lake change to deep,the braided fluvial delta front facies appear.The major sedimentary facies is deep lake during the Chang 7 period,the braided fluvial delta front facies appear only in the Western and Southern area.The distribution of good sand body is obviously controlled by sedimentary microfacies,which presents the trend of NE-SW direction.Base on the analysis of lithology and zircon U-Pb isotopic dating,the Longmen magmatic rock has been study.It shows that the lithology type of the Longmen magmatic rock is syenite and monzonite.The age of magmatic intrusion is 242.3Ma.Comprehensive research show thatthe southwest Erdos basin is not a typical foreland basin during the Triassic Yanchang period,and which exists extensional environment in some area,especially in Chang7 period.The main types of the Chang 10?Chang7 reservoir are lithic feldspar sandstone.The intergranular pores and dissolution pores are main pore space of the Chang 10?Chang7 reservoir.The reservoir is low permeability and ultra-low permeability reservoir,which is mainly controlled by provenance and sedimentary microfacies.It is shown that the shale of Chang7 is main hydrocarbon source rock in the area by hydrocarbon source rock research.lt is good condition for the oil reservoir,and the hydrocarbon accumulate in trape of Chang 7and the lower stratum.Lithologic reservoir and structural-lithologic reservoir are main types in the area.The distribution of reservoir was mainly controlled by sedimentary microfacies and sand,hydrocarbon source rock,fault and fracture.The development of the hydrocarbon source rock is the key factor to control the distribution of oil reservoir,the sedimentary microfacies and the distribution of sand body are the basic conditions for oil and gas accumulation.Fault and fracture can effectively improve the migration condition,and it is very important for hydrocarbon accumulation in lower reservoir.