Organic Matter Pore Types And Characteristics Of Shale From Wufeng Formation To Longmaxi Formation In The Sichuan Basin

Sichuan Basin in the middle and upper Yangtze region has a huge amount of shale gas resources,and great breakthroughs have been made in exploration and development in recent years.Black shales are widely developed in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation,which have already became main shale gas exploration and development horizons in China.Shale organic matter is not only the parent material of gas generation,but also the main contributor of pores in the strata.Organic matter pores play an important role in controlling the enrichment and production of shale gas.However,there are still many controversies about the development carrier,influencing factors and contribution of organic matter pores to shale reservoirs.Therefore,it is the key to reasonably evaluate shale gas reservoir to clarify the types of organic matter pores and their formation control factors.Taking the Wufeng Longmaxi formation in Sichuan Basin as the research object,the rock types and hydrocarbon generating organisms of high-quality shale are studied by using scanning electron microscope and corresponding energy spectrum characteristics,the composition of organic matter in shale is refined,and the control effect of different hydrocarbon generating parent materials on the pore development of organic matter and their contribution to the hydrocarbon generation and reservoir of shale gas are discussed:The organic matter micro components are the fundamental factors controlling the pore development of organic matter.Based on the observation and analysis of organic maceral types in Wufeng-Longmaxi Formation,many organic maceral types(hydrocarbon generating parent materials)such as multicellular algae,unicellular algae,asphalt,graptolite and animal debris are identified.Micro nano pores are developed in hydrocarbon generating parent materials,but the pores developed by different hydrocarbon generating parent materials are quite different.Among them,1)the micro nano pores of multicellular algae are the most developed,which inherit the structure of primitive organisms and have obvious biological morphology characteristics.At the same time,they are also affected by the mineral intergranular pores.The pore size is 300 ? 800 nm,and the face rate is about 50% ? 80%;2)The pores of single cell algae are relatively poorly developed,and the pores of single algae(Archaea)are very small,only tens of nanometers,while the pore diameter of organic matter formed by the arrangement gap of monomers is 100 ? 200 nm,and the larger pore diameter is caused by the shedding of monomers,with a face rate of 5% ? 10%;3)Most of the asphalts are flocculent under electron microscope,which is filled in intergranular pores or cracks.The pores are poorly developed,and mainly develop spherical pores and pores.The pore size varies greatly from 500 nm to 3 ? It is not considered to be the main carrier of pore development of organic matter;4)Some macerals,such as graptolite and animal detritus,are well developed,and organic pores are very few,which can be ignored.On this basis,combined with the proportion of different types of organic maceral and total organic carbon content,the calculation model and formula of organic matter pore were established.Mineral composition,diagenetic stage and mineral grain support framework also have important influence on the formation and protection of organic matter pores.The degree of thermal evolution and formation pressure also play an important role in the development of organic matter pores.Combined with the identification and calculation of biogenic silicon,the evaluation method of shale enrichment and high yield favorable intervals is established.Finally,the favorable shale gas intervals in the study area are optimized by integrating the types of organic maceral,organic carbon content and biogenic silicon content.