Outcrop Fracture Description Of North-dipping Part Of Xinjin Xiongpo Structure

In Western Sichuan Depression clastic gas-bearing formations, natural gasresources amounted to1.8～2.5×1012m3, and the formations are the main natural gasproduction base in Sichuan Basin. Xu2now has been the industrial capacity in manyformations, and it belongs to cracks-porosity reservoir. The permeability has beengreatly improved by existing fracture, so the fracture is the key to high production. Asimilar tectonic and similar lithological outcrop has been studied due to seriousdenudation of Xu2in outcrops. This dissertation starts studying from outcrops fracturecharacteristics in Chengdu Xinjin, and completes description of the distributionrelationship between the fractures and faults and description of the fracture pattern indifferent structural parts.All fracture data in63outcrop locations have been collected and described afteroutcrop observations. Besides, the crack occurrence, shape, plane characteristics,filling characteristics, opening width, length and other parameters were counted andanalyzed. The match and combination relations between the different scales crack andthe faults, the different nature and different sizes of the cracks derived from fault havebeen analyzed, the fracture development in different structural positions has beencompared. Meanwhile, image analysis method to study the micro-fracturecharacteristics has been adopted. By analyzing the fracture development characteristicsand studying the geomechanics environment for generating the fractures, the basis forthe underground fracture evaluation has been found and provided. The factors thatcontrol fracture development of the study area is the combined effects of structuralcurvature, structural positions, fault and lithology.According to this study, the following understanding has been achieved: crackgroups included four groups mainly, which are North East, North West, North andSouth, East and West. Among which, the cracks is mainly North East direction in the southeast of the tectonic region. Northwest-Southeast direction domains in Northweststeep wing and southwest place close to the core of the construct location. Strike ofmost cracks has consistency with the strike of the fault when they are near the fault.Causes of fractures in the study area are mainly related to tectonic deformation and faultdevelopment. In west wing of tectonic zone, the fractures are developed much moreintensively due to complex and steep strata; while in the flat east wing, the structure isrelatively simple and stable, consequently, the development of fractures is weaker thanit in the West Wing. The vertical extension of the fissure length ranging from0.1m to5m with the maximum of10m, and it is common to see cracks penetrate through thestrata.In study area, the effectiveness of permeability in cracks reduced due to fillingphenomenon, mostly are filling with calcite, other filling with quartz and muddy. Thedensity of fracture development in the study area are mainly related to lithology, in allthe profiles of cracks existing in both sandstones and mudstones are more frequent inthe sandstone. According to the statistics, the development of fractures in the sandstonehas a negative correlation with bed thickness in some degree, while has no significantrelationship in mud. Besides, the development of cracks is also constrained by theprincipal stress field. The maximum principal compressive stress orientation in thestudy area in the Triassic period is NW-SE; in the Jurassic period is NE-SW; in the latePaleogene, the main stress is horizontal compress stress due to the impact of theLongmen Shan fault slip activities in western Sichuan region. Because the outcropshave similar geological conditions and fracture development characteristics with Xu2,the conclusion could be used as the basis and foundation of fracture development inXu2reservoir.