Discussion On The Original Sedimentary Range In The Southeastern Part Of The Hydrocarbon-rich Depression In The Ordos Basin

The hydrocarbon rich depression in the Yanchang period of the Middle-Late Triassic in the southern Ordos Basin has a high abundance of oil and gas resources,control the generation and distribution of oil and gas in the south.It is the main unit of oil and gas occurrence and exploration in the basin.However,there are still Late Triassic strata with a wide distribution range,different thicknesses and organic-rich rocks in the southwestern outside the distribution range of the remaining strata in the present basin,Henan area.The relationship between the two areas is still inconclusive.The Upper Triassic in western Henan is used as research object.The distribution range,thickness distribution,sedimentary facies characteristics,paleogeographic environment and oil-generating rock characteristics,etc.,are analyzed by field geological survey,sedimentology,lithofacies paleogeography,rock mineralogy,geochemistry and other methods,to explore its relationship with the hydrocarbon rich depression of the Ordos Basin during the Yanchang period and the original sedimentary range of the Late Triassic.Geological outcrop,drilling,seismic and other data shows that the distribution of the Upper Triassic in western Henan is relatively scattered,the thickness of the stratum changes greatly,and there are multiple thicker sedimentary areas,which are generally parallel to the Qinling orogenic belt and are northwest-southeas exhibition.Geochemical characteristics indicate that the Late Triassic in the study area was formed in a warm,humid,fresh-brackish water environment.The Nanzhao,Wulichuan,and Yichuan areas developed semi-deep lake-deep lake facies deposits,similar to the Tongchuan area in the southern Ordos Basin.In Jiyuan and Yimacareas,only shallow lacustrine sediments were developed.Earthquakes,detrital zircons,major elements and other data show that the western Henan area had a similar structural setting to the Upper Triassic in the hydrocarbon rich depression in the southern Ordos Basin.The provenance in the Nanzhao,Wulichuan,Yichuan,and Jiyuan areas all came from the southern Qinling The orogenic belt and the North China Craton,and the provenance of Yima area mainly came from northern sources.At the same time,tuffs,soft sediment structures,calcareous concretion and other sedimentary phenomena similar to those in Tongchuan area developed in the dark mudstone strata in western Henan,indicating that they were in active geological background with the hydrocarbon rich depression in the southern Ordos Basin.Organic geochemical analysis of the Upper Triassic shows that the organic matter types of dark mudstones in the western Henan area are mainly type ? and ?.The abundance of organic matter is lower than that in Yichuan area.However,the thermal evolution degree is quite different.Among which Nanzhao and Wu The Lichuan area shows higher because of magmatic thermal activity.The comprehensive comparison shows that the the source rocks in Nanzhao and Wulichuan areas are similar in quality to Tongchuan area,and the source rocks in Yichuan area are similar to Hancheng area.In summary,it is believed that the Western Henan area and the Ordos Basin belonged to the same large sedimentary basin during the Middle-Late Triassic and the main body belongs to the same hydrocarbon-rich depression.The southeast boundary of the source rock development area exceeded the Yichuan and Nanzhao areas.of which the Nanzhao,Wulichuan and Tongchuan areas were in the deeper center partof the hydrocarbon-rich depression in the Yanchang period,and the Yichuan,Jiyuan and Yima areas wrer in the edge of the hydrocarbon-rich depression.The later regional uneven uplift and the south-to-north thrust from the Qinling orogenic belt led to a wide range of stratain the study area undergoing intense and uneven erosion and transformation,and gradually formed the present scattered distribution pattern.The research results of this paper will provide an important scientific basis for further evaluation and exploration of hydrocarbon-rich sags in the Ordos Basin.