Natural fractures in the Longmaxi Formation,Weiyuan block:Characteristics,distribution patterns and forming episodes
To comprehend the major interesting reservoirs for both exploration and development of unconventional oil and gas in Sichuan Basin,natural fractures in the Longmaxi Formation,Weiyuan block,southern Sichuan Basin,were taken as objectives.Their development characteristics and forming timing were analyzed by means of core observation,thin-section identification and geochemical experiments.Additionally,both development types and forming episodes were defined.Re-sults show that(ⅰ)these fractures can be mainly classified into three types of structural fracture,hydrocarbon-generating overpressured fracture and bedding fracture;and(ⅱ)there exist multi-episode fractures in this formation.Three conclu-sions have been made.First,Weiyuan area often witnesses natural fractures with smaller width and high proportion of hori-zontal-bedding fractures while occasionally high-angle structural ones with low effectiveness.Moreover,the density of frac-ture extension increases gradually with the increasing burial depth of the Longmaxi Formation.Second,based on tempera-ture calculation and structural depth evolution,it is demonstrated that these fractures were dominantly formed in the Indo-sinian,Yanshanian and late Himalayanian epochs.And carbon/oxygen isotope and inclusion experiments all confirm their existence in these three epochs.Third,in the study area,it can be found a small number of NW-strike shear fractures and bedding fractures of sandstone interbedded with mudstone developed in the Indosinian epoch,NW-trending shear and hy-drocarbon-generating overpressured fractures near faults or at anticline high in the Yanshanian epoch,nearly NW-and NWW-strike shear fractures also close to faults or at anticline high in the late Himalayanian epoch,and nearly S-N-strike tensile fractures developed at anticlinal core in the late Himalayanian epoch,respectively.Finally,based on the geome-chanical setting of Weiyuan block,the characteristics,forming episodes and distribution patterns have been figured out for these natural fractures,which may provide technical support for the reservoir evaluation,favorable selection,sweetspot as-sessment together with exploitation of the Longmaxi Formation in this block.
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