The Tectonic Characteristics Of Changning Area, South Of Sichuan And Crack Characteristics Of Longmaxi Formation Of Silurian System

In recent years, shale from the lower Cambrian Jiulaodong Formation and thelower Silurian Longmaxi Formation in Sichuan basin has aroused people’s greatattention and has been the focus of various researches with Longmaxi Formation thathas been widely distributed in the south of the basin turning to be the right zone for thestudy of the numerous problems found in the exploration and exploitation of the shalegas. The in-depth study of the tectonic characteristics in this area and the features anddevelopment rules of the shale fracture in Longmaxi Formation will not only contributeto the task deployment and the exploitation work in terms of the exploration of shalegas in the research area, but also play a practical and significant role in the study andexploration of the shale gas in the whole Sichuan basin. However at present, noin-depth analysis has ever been made on the tectonic background of the shale fracturedevelopment in Silurian Longmaxi Formation in Changning Region of SouthernSichuan. People haven’t established a perfect and effective method to recognize thedifferent shale fractures, which has led to the incomprehensive analysis on the mainfactors that control the development of the shale fracture and the regularities ofdistribution with few researches having been made on the different formation stages ofthe shale fracture and the various fracture development patterns. Therefore based on theprevious researches, we shall analyze briefly in this article the deformation behavior inChangning region of Southern Sichuan and the stress field that dominates suchdeformation through the research on the paleostructure evolution in Changning regionon the basis of the existing tectonic characteristics. Then through such a research method, based on which basic geological theory, drilling data, field data andgeophysical data will be utilized comprehensively to have an integrated study of thefracture characteristics of Silurian Longmaxi Formation in Changning region, we shalldiscuss the development rules of such fractures with the application of variousdisciplines and theories including structural geology, tectonophysics, well logging andphysical geography to make an in-depth analysis of the research area by means ofpoint-line-area combination. Actually in this article, through the research and analysismethods describes as above, we shall come to understand the tectonic characteristics ofChangning region in Southern Sichuan, the fracture features of Silurian LongmaxiFormation and its development rules:(1) According to the tectonic evolution sectionspresented through the2D seismic sections of Changning region, we could find that themain deformation period of Changning structure is in the late Yanshan-Himalayanorogeny, when the research area is affected by the Himalayan orogeny after theformation of the Jurassic stratum to create a northwest-southeast anticline with the corein the territory of Xingwen County and a series of reverse faults beginning to arise inthis area. In fact, the vertical movement in the early stage has laid a foundation for theformation of the northwest-southeast structure.(2) Through the planar paleostructureevolution characteristics of Changning region, we shall realize that: before theformation of the upper Permian series, Changning region has been featured with arather stable tectonic framework with the whole region experiencing a slow uplift.However Leidaping-Guanyintang and Anshunchang-Shiziyan in the central-northarea have been uplifted slightly higher than the other areas in the region, demonstratinga trend of increasingly expanding and merging. Moreover judging from thepaleotectonic map for the medial Ordovician top boundary before the formation of thePermian system, we could find that these two relatively independent tectonic highs, sayLeidaping-Guanyintang and Anshunchang-Shiziyan in the central-north area havebeen connected with each other in the peripheral area to create a northwest-southeastanticline structure in Changning region. Meanwhile according to the paleotectonic mapfor the medial Ordovician and Silurian top boundaries before the formation of the upperPermian system, we could find that during this period, the anticline in north-centralChangning has been expanded further in the area, which has accounted for around1/4of the total region area, making it become the main tectonic highs in this region. In fact,such an opinion has been proved by the analysis result from the sedimentary-tectonicevolution sections of Changning region.(3) After have a careful observation andmeasurement on the various field observation points in Changning region, we could find that: the fractures developed in the Silurian Longmaxi Formation mainly consist ofthe interlayer lamellation fracture, the shear fracture that is vertical to the layer and thefracture developed partly to form a skew angle with the layer.(4) The research on theimaging logging of A Well turns out that: high-conductivity fracture has beendeveloped mainly in Longmaxi Formation with the statistics showing that most of thehigh-conductivity fractures are medium or high-angle fractures since the dip angles ofthese fractures are mainly between65°~82°.(5) With the combination of the analysisresults about field outcrop, rock core and imaging logging, it turns out that: thedevelopment and distribution rules of the shale fracture in Longmaxi Formation ofChangning region vary much from different tectonic positions with the characteristicsof fracture development diversified as well in different layers of the same position. Infact, the main factors that control the fracture development in Changning region includethe multistage composite or combined tectonic stress field and the tectonic positionwith the content of the brittle minerals in the rock having been one of the influentialfactors as well.(6) The numerical simulation shows that: the maximum longitudinalprimary stress will increase with the increase in the depth and strain will bedemonstrated mainly through volume expansion. Since the presence of the geologicalfault has played a significant role in the stress-strain distribution rule, therefore wemight assume that fracture development in the research area has been affected by thefault development.(7) Make a prediction on the planar fractures in the3D blockthrough the curvature method, the result turns out that: there’s a big difference in thedevelopment of the upper and lower fractures in the Longmaxi Formation with theupper sub-member of the fracture having been well developed, while the fracture in thelower sub-member has hardly been developed with only some cracks that are paralleledto the fault having been found nearby the major fracture.