A Study On The Characteristics And The Causeof The Organic-rich Shale In The Extensional And Compressional Settings

The shale gas with organic-rich mudstone as a carrier is a new type of unconventional oil and gas resources,and its accumulation model has broken through the original oil and gas geological concept.The development of the shale gas industry has caused changes in the global energy landscape and is related to national energy security.The Sichuan Basin is a key area for the exploration and development of marine shale gas in China.In particular,the cumulative gas production in the Fuling shale gas field has exceeded 20 billion cubic meters,indicating the strong shale gas resource potential of the Early Paleozoic marine shale in the Sichuan Basin.Organic-rich shale is also a record of geological and historical events,especially the shale in the plate-bound and orogenic belts.Its formation,development and distribution are the comprehensive responses of plate movement,paleogeographic evolution and orogenic process.Therefore,the study of the characteristics and formation laws of organic-rich shale not only has important theoretical significance,but also has significant practical value.Dabashan Mountain is located at the northern edge of the Sichuan Basin.It is the junction of the Yangtze Plate and the Qinling Plate.The front edge of the Dabashan Mountain is the transition zone from the Dabashan Mountain thrust fold belt to the Sichuan Basin.Its early Paleozoic Shuijing Formation(Lujiaping Formation)and Wufeng-Longmaxi Formation organic-rich shale developed the basic geological conditions for shale gas formation.This paper takes the Lower Paleozoic marine organic-rich shale of the Dabashan Mountain front as the research object,and based on the shale gas geological route survey,profile measurement,seismic data acquisition and interpretation,combined with relevant test analysis,based on the characteristics of organic mud shale formation.The sedimentary characteristics,geochemical characteristics,reservoir and gas-bearing characteristics are studied in detail.From the response of regional palaeogeographic characteristics of the nature and tectonic movement of the surrounding basin,the different formation mechanisms of the Shuijing Formation(Lujiaping Formation)and the Wufeng-Longmaxi Formation shelf in the front of the Dabashan Mountain are summarized.By comparing the distribution law and material composition difference between different basins and basin margins,combined with the structural deformation characteristics of the Dabashan Mountain front,the distribution law of organic-rich mud shale is analyzed,and the genesis of the early Paleozoic rich organic mud shale in the front of the Dabashan Mountain is finally established.The Lower Paleozoic in the frontal area of the Dabashan Mountain develops two sets of organic-rich marine shale shale layers,the Lower Cambrian Shuijing Formation(Lujiaping Formation)and the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation.The organic-rich shale develops at the bottom and has a large thickness variation,which is gradually thinned from the front edge of the Dabashan Mountain to the basin.Take the Chengkou fault,Pingba fault,Jimingsi fault and Tiexi-Wushan hidden fault as the boundary,the frontal area of the Dabashan Mountain is divided into three structural deformation units: the Dianhua thrust belt,the fault fold belt and the slip-recessed belt.The distribution of organic-rich shale in various structural deformation units is obvious,the abundance of organic matter is generally high,the type of kerogen is mainly type I,the maturity of organic matter reaches high-over-mature stage,and it has good hydrocarbon-generating conditions.The Paleozoic marine organic-rich shale in the forefront of the Dabashan Mountain is a continental shelf sedimentary deposit,but the material composition is quite different from that in the basin,with obvious “high siliceous” characteristics,and the Lower Cambrian Shuijing Formation(The Lujiaping Formation)is higher in siliceous content than the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation,which often appears as a thin layer of siliceous form at its bottom.According to the combination characteristics of lithology,it can be divided into: sandy shale,carbonaceous shale,siliceous sand and siliceous carbon.The porosity and permeability of the two sets of shelf shale are low,and the organic pores and micro-cracks develop,which has good reservoir conditions.The shale mineral component has a high content of brittle minerals and the shale is highly compressible.According to the characteristics of complex orogenic belts in the front of the Dabashan Mountain,using sedimentary basin analysis,plate motion and tectonic dynamics analysis,combined with the Rodinia continental pyrolysis-Xingkai ground fission movement and the Huaxia paleo-expansion-Guangxi movement process,the analysis of the Lower Paleozoic The formation mechanism and material composition difference of two sets of organic-rich mud shale sheds in the boundary;according to the structural deformation characteristics of Dabashan Mountain,combined with the latest exploration results,discuss the shale gas enrichment rules in different tectonic deformation units,and summarize the distribution of organic-rich shale Regularity,the formation pattern of marine organic rich shale under the Paleozoic in the forefront of the Dabashan Mountain:(1)The different basin properties of the South Qinling Basin during the formation of two sets of shale,(Rift Basin-Shuijing,Foreland Basin-Longmaxi),are the basis for the formation of two sets of organic-rich mud shale.(2)The Lower Cambrian Shuijing Formation(Lujiaping Formation)is distributed from the edge of the plate along the extension of the trough and the fault to the interior of the Craton Basin.It has a “platform interfacial” paleogeographic pattern and is the paleo-geographical response of the Rodinia continental dissolution and the Xingkai rifting movement.The sedimentary sediments of the Upper Silurian Longmaxi Formation are controlled by the central and southern Paleocontinent in the middle of the Sichuan Basin,and have obvious paleo-geographical patterns of the“longitudinal and interlaced”,which is the ancient Chinese ancient land expansion-Guangxi movement.Geographic response.(3)The difference in material composition of the continental shelf is the material manifestation of differences in basin properties and tectonic dynamics.The siliceous content of the two sets of organic mudstones is relatively high.The Shujing Formation(Lujiaping Formation)is affected by the submarine volcanic activity of the rift basin;the Wufeng-Longmaxi Formation shelf is affected by the orogenic island arc.(4)The structural deformation of the front edge of the Dabashan Mountain is the key to the enrichment of shale gas in the organic matter.Due to the fracture failure,the shingling belt and fault fold belt have poor structural preservation conditions,and there is a small amount of shale gas enrichment in the disc area between the fragments and the broken anticline.The slippery fold belt develops concealment and intra-slip slippage,increases the connectivity between organic-rich mud shale,and does not connect the surface to destroy the gas reservoir.It has obvious characteristics of“pressure without breaking”,and has better performance on the broken anticline.Constructing storage conditions.Combined with the characteristics of the organic-rich shale in the Dabashan Mountain and the shale gas enrichment law in the tectonic deformation zone,the favorable area of the Zhangheping-Shaitianba and the favorable area of the Tianba-Wenfeng-Zhuyuan are delineated.The Wufeng-Longmaxi section of the Wuxi2 well in the Tianba-Wenfeng-Zhuyuan favorable area has achieved good gas test results,confirming the potential of shale gas resources in the area of the sloping anticline in the detached anticline is the key area for further exploration.