| In the exploration and exploitation process of shale gas,we had found that tectonic evolution is very important to the enrichment of shale gas.Jiaosiba,which is the located in the middle of Chongqing,has a great produce of shale gas.However,the produce of shale gas in the southeast of Chongqing has no increase.It is possibly cause by a series of tectonic movements and the highly developed folds and fractures.By combining the field sections,cores and experiment data and regional geological data in the southeast of Chongqing,this paper focuses on the tectonic evolution and enrichment conditions of shale gas in the southeast of Chongqing,for forecasting the favorable area and providing geological basis for further exploration of shale gas in this area.The southeast part of Chongqing has undergone many tectonic movements,such as Caledonian,Hercynian,Indosinian,Yanshan and Himalayan.Most of the folds in the area are linear folds.From the northwest to the southeast,the folds in turn are the barrier-like folds,then the battlements folds,the last is the trough type folds.The direction of faults and the trend of folds are similar,both north northeast or north east,and mainly is northeast faults.The density of the fractures increase from north to south.According to the zonal characteristics of the internal fracture distribution and the distribution law of the first-order faults in the Southeast Yunnan structural belt,the entire structural belt is divided into three fracture systems:Qiyaoshan fault system,Pengshui-Qianjiang fault system,and Youyang-Xiushan fault system.Through acoustic emission experiment,inclusion temperature measurement experiment,c-o isotope test of filler in cracks and field crack stage and matching.We conclude that the development of cracks in shale,in the Southeast of Chongqing,is mainly influenced by Indosinian movement,Yanshan movement and Himalayan movement.In Indosinian period,the layers in the southeastern Chongqing had not been obviously folded and faulted.In Yanshan period,the tectonic pattern of the SouthEast in Chongqing had varied the most,and cracks with the alignment of the northwest west,north northwest and north northeast had developed.With the continuous actions of tectonic stress,the shear joint of section with the trend of north northeast had gradually formed the north north east inversion fault in this area.During the Himalayan period,the systems of fold and fracture formed in Yanshan period was superimposed and finally transformed into a combined superimposed tectonic system,with the trends of north north east and south north nearly.The enrich conditions of shale gas are mainly influenced by the maturity of organic matter of source rocks,the thickness of shale,regional cover conditions,roof and floor conditions,time and strength of structural transformation,etc.The Wufeng-Longmaxi Formation and Niutitang Formation in the southeastern Fujian region have high thermal maturity,moderate depth of burial(except slopes),extensive coverage of regional caps,large thickness,poor development of microfractures,low porosity and permeability,good roof and floor.The Baota Formation and Linxiang Formation under the Wufeng-Longmaxi Formation are a set of knob-like muddy limestone with a hard texture,which is a good floor.Although the floor of the Niutitang Formation is subjected to Tongwan movement and the underlying karst formation,which resulted a poor bottom plate condition,the enrichment conditions of the two target layers were generally better.According to the tectonic evolution characteristics and enrichment conditions evaluation index of study area.The optimum distributing areas of shale gas in the Upper Ordovician-Lower Silurian are the Puzi syncline in the southwest of Pengshui country,the Sangtuoping syncline in the east of Pengshui country,and the Quheba syncline in the southeast of Qianjiang country.The favorable area of the shale gas in Lower Cambrian is in the syncline or nearby,in the northwest of Youyang country. |
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