Study On The Control Effect Of Tectonic Features On The Enrichment Of Shale Gas In The Longmaxi Formation In Southeastern Sichuan

As a frontier of shale gas exploration and development of China,the southeastern Sichuan region has become an important pillar of natural gas production in southwestern China in recent years.This paper focuses on the hydrocarbon-forming material base and structural characteristics of the Lower Silurian Longmaxi Formation in the southeastern Sichuan Basin.Based on this,the control effects of structural features on shale gas enrichment are analyzed and its enriched favorable area is predicted.The study shows that the thickness and depth of the Longmaxi Formation in the southeastern Sichuan Basin vary greatly,and the overall thickness is between 0 and 600 m.And the thickness of the shale rich in organic matter can be as thick as 100 m,and its distribution shows a good consistency with the distribution of sedimentary facies.Most of the buried depth of the Longmaxi Formation in the study area exceeded 6000 m,but under the influence of tectonic activities and peripheral basement faults,the buried depth of some areas is less than 3000 m.The hydrocarbon generation capacity of shale is also affected by organic matter type,organic matter abundance and organic matter maturity.According to the analysis of the collected field and core samples,it is found that the value of ?13C in the Longmaxi Formation of the study area is mainly distributed in the interval of-28‰?-32‰,which can be divided into saprolite(?)and humus saprolite(?1);the TOC values are distributed between 0.07%and 8.67%,with an average of 1.62%;the distribution of Ro ranges from 1.63%to 2.87%,with an average of 2.29%,reaching a high maturity stage.TOC and Ro control the gas content of shale formation to a certain extent,and their distribution has a high similarity.The overall gas content in the study area is more than 1.5 m3/t.It can be seen that the Longmaxi Formation shale in southeastern Sichuan has good hydrocarbon generation potential.The enrichment of shale gas is largely controlled by structural features.This study divides the research area into two region of eastern Sichuan and southern Sichuan according to their structural features.The boundary of the study area was controlled by the Huayingshan fault zone,the Qiyaoshan fault zone and the Tiexi-Wuxi fault zone.In the eastern part of Sichuan,the structure in the west of Qiyaoshan fault is characterized by an ejective fold of which the anticline is tightly closed and the syncline is widely sustained.According to the fold shape and direction,this aera is further divided into three sections:south,middle and north.The south section is affected by the Sichuan-Yunnan meridional tectonic belt,forming a north-southnorth-eastward gentle tectonic belt;the middle section is a typical high-steep fold belt in eastern Sichuan,which is characterized by the parallel north-south ejective folds;north section is in the north of Wanzhou,forming the south arc of the convergent double arc that converges eastward and spreads westward under the joint action of the Daba mountain arc and the Bamian mountain arc.The direction of this section is mainly from northeast to near east-west.On the basis of the extension of the Huayingshan anticline in the south of Sichuan,it can be divided into three groups according to the trend of the folds in this area:east-west,north-east and north-south.It also can be seen that the degree of fold deformation is significantly weaker than that in eastern Sichuan.The faults developed in the study area are mostly associated with folds,and exhibit strong squeezing and steep dip angles.The cracks are mainly small and medium-sized cracks,and the dip angle is relatively flat.On this basis,the influence of structural features on shale gas enrichment is analyzed.It is generally considered that the gentle anticline is more conducive to shale gas enrichment than the syncline.But the study area is a typical high-steep fold belt,the syncline area is better than the anticline.The existence of deep faults in the study area(such as the Qiyaoshan fault)has a destructive effect on the enrichment of the shale gas in the Longmaxi Formation,while the faluts associated with the folds has little effect.At the same time,the degree of crack development also provides sufficient space for gas enrichment.Finally,considering the hydrocarbon-generating material foundation,structural characteristics,regional roof and floor conditions of the study area,three favorable shale gas enrichment areas are selected,which are located in Changning,the south of Luzhou and Fengjie.