Terrestrial LIDAR Based Folding And Fracturing Mechanism Analyzing And Fracture Network Modeling

With the development in oil and gas industry,especially the shale gas revolution has dramatically changed the energy landscape in the world.The exploration and development of fractured oil and gas reservoirs has put forward new problems in traditional areas of geology,such as the development order and deformation mechanisms of fractures during rock deformation,which has long been a focus of research for structural geologists.However,due to the limitations of detection methods,people cannot accurately characterize and clearly reveal the crosscutting relationships and true distribution of fractures within the rock,which cannot meet the requirements of quantitative description of fracture distribution and connectivity in exploration of fractured oil and gas reservoirs.Despite the use of LIDAR systems for surveying fractures in rocks has become increasingly popular,quantitatively based and agreed upon procedures for assessing the accuracy of fracture orientation and number data extracted from LIDAR scans are not well established.In this study,we propose a fracture fabrics-based methodology to assess the data accuracy from LIDAR surveys.Three parameters(mean direction,dispersion,and number of each fracture set)were used to quantitatively determine the fabric characteristics of fractures in a contoured polar stereonet.We also discuss factors affecting LIDAR fracture measurements(including bed thickness,lithology,weathering,fracture surface roughness,size and shape).A general scheme for the quality control of fracture data in LIDAR fracture surveys is finally proposed.In this study,we have identified the types and crosscutting relationships of fractures in complicated fracture networks in the western Sichuan basin in China,determined their development order and formations.A concept model of folding and fracturing evolution is proposed.During pre-folding layer-parallel shortening(bedding dip angle < 20°),bedding perpendicular conjugate shear fractures are first developed,and transverse tension fractures occurred shortly after them.With the continuous increase of layer-parallel shortening,a switch between the minimum and the intermediate principal stresses occurred,resulting in formation of small-scale thrust faults.At the early stage of folding(bedding dip angle < 30°),the deformation mechanism of rock layers is marked by tangential longitudinal strain,under which widespread longitudinal tension fractures occurred.As the folding continues and the dip angle of rock layers increase from 30° to 40°,flexural flow gradually dominates in limbs,conjugate Riedel shear fractures began to appear due to top-to-the-hinge layerparallel shear.As the folds get tighter(bedding dip angle > 40°),new shear fractures occur as a consequence of shear systems derived from the reactivations of early smallscale thrust faults.The summarized intersecting relationships of fractures caused by different mechanisms during folding indicate that the fractures tend to be developed independently in each deformation system and intersect with each other in a certain law.Intersecting relationships of shear fractures resulted from the same deformation system are abutment according to their development order,while from different deformation systems or with later tension fractures,intersecting relationships are mainly crosscutting.To evaluate the influences of fracture fabrics and abutting relationships on rock permeability,outcrop-scale discrete fracture network models(DFN)were constructed based on results from both traditional and LIDAR fracture surveys.It was found that fracture density is the most important affecting factors on rock permeability.Fracture orientation dispersion and abutting relationships influence the rock permeability in a similar way: permeability increases as orientation dispersion and termination percentage increase,and reaches its maximum when orientation dispersion(95% variability limit angle)is about 15° and termination percentage is 10%.Then with the increase in orientation dispersion and termination percentage,the rock permeability decreases.