Geochemical,Petrophysical Properties And Its Effecting Factors Of The Wufeng-longmaxi Shale In Southeastern Margins Of The Sichuan Basin

In southeastern margin of the Sichuan Basin,the Wufeng-Longmaxi shales was significantly different due to the difference in sedimentary environments,and it had undergone differential geological evolutions with different tectonic movements in different positions.Therefore,it is suitable for the study on the factors that may influence shale petrophysical property,such as sedimentary environments and various late-stage reformation(tectonic reformation et al.).In this paper,in order to obtain the Wufeng-Longmaxi shale core samples from different sedimentary environments and different tectonic types,three shallow wells were drilled in Sanquan town and Anwen town of Chongqing City,Shuanghe town of Sichuan Province.Fresh shale core was obtained for nearly 400 meters,and the samples were densely sampled.In order to study the variations in the characteristics of organic matter,mineral composition,pore size distribution and pore morphology of shale,shale samples were analyzed by LECO CS-320 Carbon and Sulfur Analyzer,Rock-Eval 6,X-ray fluorescence spectrometry,3Y microphotometric system,X-ray diffractometer,high pressure Hg porosimetry,scanning electron microscopy,surface area and pore size analyzer,respectively.The pore volume and specific surface area were analyzed by non-local-density functional theory(NLDFT)method.Based on all these results,we try to reveal the geochemical characteristics,petrophysical properties and controlling factors of the Wufeng-Longmaxi shale formed in different sedimentary environments in southeastern margin of the Sichuan Basin.At the same time,the effect of late-stage reformation,such as calcite filling and tectonic reformation,on shale petrophysical properties and their mechanisms are also discussed.In this paper,the main conclusions are obtained as follows:1)High-quality shale interval(TOC>2.0%,clay<40%)with high silicon content,low clay content and rich organic matter mainly occur in the Wufeng Formation and the bottom of the Longmaxi Formation(Rhuddanian)which are favorable for exploration.2)The organic-rich shales usually have higher porosity,pore volume and specific surface area.However,when TOC>5.5%,the pore volume of nanopores decreases.Additionally,the pore volume of nanopores is negatively related to the CaO content(R~2=78%)because the carbonate cements in high calcareous shale could fill into microfractures,mesopores and macropores.The total pore volume(0.35~100nm)decreased by about 1/4 when the CaO content increased by 10%.3)The late-stage tectonic reformation would have a great influence on the petrophysical properties of shale and the preservation conditions of shale gas.The pore volume of the larger pore size in shale samples was easy to be compacted in compressional tectonic settings,while the shale becomes denser and porosity decreases.Under the long time tectonic extrusion environment in Qijiang,the total pore volume of inorganic pore(TOC=0)decreased by 17%,the total pore volume decreased by 40%at TOC=5.5%.Therefore,the higher the organic matter content,the greater the compaction rate.The detachment layers were generally developed in the Wufeng Formation shale.There are abundant micro-fractures in the detachment layers which can provide efficient lateral migration pathway for shale gas and the pore volume increased by about 1/3.The extensive occurrence of detachment layers in the Wufeng Formation provides a reasonable explanation of why shale gas usually accumulated in positive traps,such as anticlines,but escaped more easily in negative structures.Therefore,positive structures with moderate depth are favorable structural types for shale gas exploration,and their structural high parts are usually favorable locations.