Study On Sequence Stratigraphy And Sedimentary Paleogeography Of Upper Triassic Yanchang Formation 8-6 In Southern Ordos Basin,China

The Triassic Yanchang Formation of Ordos Basin is the representative example of Petroleum sedimentary stratum of China,in which Ansai,Zhijing and Jingan oilfield have been found these years,and is full of develop space of exploration.In Yanchang 8-6 period the southern part of Ordos Basin has expireinced rapid lake growth and decline process.There is research significance about the tight sand symbiosis with the oil shale which is reservioed lots of oil and gas resources.This paper is written relied on the project of China Petroleum & Chemical Corporation.Combined with sedimentary petrology,sequence stratigraphy and paleogeography research methods,the aothur do the research about stratigraphy and palaeogeography of the 8-6th oil group of Yanchang Formation in Upper Triassic.Using outcrop observation,logging combined with core data analysis tools,cycles isochronous sequence comparison and petrographic analysis techniques in the Zhengjing,Binchang,Xunyi-Yijun and Fuxian area,the author selected the main section to do stratigraphy comparation and established sequence framework and get some research achievements as follows:1.Sedimentary rock in this area are terrigenous rock,which are mostly feldspar sandstone and lithic feldspathic sandstone,in which lithic content in sandstones is generally low as less than 20%.Content of feldspar and quartz is high in sandstone.Sandstone cements are mostly calcareous cement,content of miscellaneous is not more than 15%.Feldspars in sandstone are mostly acidic plagioclase.Sphericity of quartz is different due to depositional environments.The maturity of rocks is relatively low in southwest region compared to the northeast region;2.The Ch7 deepwater mud shale has developed a wide range in southern part of the basin,which shows significantly higher natural gamma and low SP potential in logging curves and has great significance in stratigraphic correlation.In a comprehensive study of high resolution sequence stratigraphy principles and logs sequence boundary division,the author uses outcrops and logging of integrated core section for detailed cycle division.The Chang 8-6 section can be divided into four sedimentary cycles.Overall Ch8 and Ch6 are in the low stand system.The stratigraphic framework is established based on the research above;3.Most regions of the basin are lake sedimentary facies.By using of light and heavy mineral characteristics,the ratio of thickness of sand to tratum,it can be found that there are three resources area from the northeast,southwest and the southeast of the basin.The resources of southwest part are much closer compared to the northeast part.It is performanced of braided river delta in the southwest while meandering river delta in the northeast part.Due to the combined effects of tectonics,climate and provenance,there are not tipical lacustrine sedimentary belts;the sedimentarty structure is complex;4.There are gravity flow deposits in Ch8-Ch6 period,which are showed as typical turbidite sequence,slump structures and sandstone with no significant bedding structure.The third one is recogniazed as sandy debris flow,which mainly deposite in front of the delta-front.There is some argillaceous clumps float in sandstone.And the sandstone has a low mature structure.It is been thought that central part of the basin slope could be the most favorable exploration targets area.In this paper the Ch8-Ch6 period sequence stratigraphic framework in Yanchang Formation was established,sedimentary evolution of ancient geography was clarified,and distribution of deep-water gravity flow of sandy debris flow deposition system are analyzed,to improve the on the overall level of awareness sedimentary paleogeography in southern part of Ordos Basin.It is the potential of the preferred target area of recent mid-term exploration and development,and could be a guid to exploration and development of the basin deploy in deepwater face.