Research On The Fracture Characteristics Of The Lower Silurian Longmaxi Formation Shale In Southeastern Chongqing

According to the exploitation of Shale gas in Sichuan basin,we have found out that the development of cracks is important to the preservation of shale gas.This thesis concentrates on the crack features of the shale of Longmaxi formation of lower Silurian series in the southeast area of Chongqing,supplying the datas for advanced exploitation of shale gas.From northwest to southeast,the folds in southeast Chongqing are ejective folds,transition type folds and trough-like folds,consecutively.The research area has been affected by multiple phases,including Tongwan movement,Caledonian movement,Variscan movement,Indo-china movement,Yanshan movement,Himalayan movement,and etc.In Yanshan movement,the research area was squeezed by the intensest stress,and formed a large of tectonic features with trend of northeast.Initially,we observed six sections in the field and cores of 7 wells,then classify cracks into tectonic cracks and non-tectonic cracks with types and causes.The small size and medium size in length own the large part of cracks.The small size in width own the largest part of cracks.Further more,high angle cracks and oblique cracks own the large part of cracks of Longmaxi formation in the southeast of Chongqing.After restoring its original horizontality,the trends of cracks include the north north west,north north east,and nearly from east to west,and the north north west owns the largerst of them.The unfilled cracks own the largest part of the cracks of outcrop shale in southeast of Chongqing,then is the fully filled.Relatively,partly filled cracks are less than them.The forms of filler includes calcite,pyrite,quartz,etc.The calcite own the largest part of the fillers,then is quartz.However,fully filled cracks is the major cracks of cores of shale,unfilled cracks and partly filled is less,relatively.The forms of filler includes calcite,pyrite,quartz,etc.The calcite own the largest part of them.The fact that the ?13C of the cement in crack is deferent with the ?13C of the surrounding rock indicates that the carbonate in cracks was dissolved and precipitated in a unclosed system,and the ?18O of the cement in crack lower than the ?18O of the surrounding rock,which means the cement had formed in a higher temperature than the surrounding rock.The governing factors of the form and distribution of cracks in shale can be classified into internal cause and external cause.The internal cause includes lithology,mineral component,mechanical properities,thickness of stratum,organic carbon content,etc.The external cause includes regional tectonic stress,tectonic position,the sedimentary diagenesis and abnormal formation pressure caused by the hydrocarbon generation process,etc.Analyze the result of cracks matching with various stage,acoustic emission experiment and inclusion temperature experiment,and combine with the tectonic evolution of the southeast of Chongqing,we have found out that the southeast of Chongqing has went through three phases of effects of structural stress field include the indo-china period,Yanshanian and Himalayan,which has formed three phases of cracks.Combining the feature of the tectonic evolution of the southeast of Chongqing with the genetic mechanism of cracks and intersected relations of cracks in field,we can summary the effect of the stress field to the cracks.It shows that the intensest Yanshanian tectonic movement creates the most developed cracks which trends are north-northwest,north-westwest and northeast.The research shows that the cracks have developed best in the middle and distal of the Longmaxi formation,the reasons are the different lithology,the content of brittle mineral and organic carbon in the lower Longmaxi formation is higher than upper Longmaxi formation,and the strata thickness is thinner in the lower Longmaxi formation.The cracks developed better from the northwest to southeast in the southeast of Chongqing,the reasons are the content of brittle mineral is gradually goes up and the strength of tectonic feature is gradually goes higher..The research shows out:the research area has six areas where the cracks are developed well,these areas are usually in the anticlinal axis and near the large-sized faults,the observation of cores and outcrop shows that the cracks always developed well in these areas.Cracks have developed well in more areas,which always in synclinal shaft and near secondary fault.The cracks developed less in the areas of anticlines and synclines which has weaker tectonic deformation.