Mechanism of the Enrichment and Loss Progress of Deep Shale Gas: Evidence from Fracture Veins of the Wufeng–Longmaxi Formations in the Southern Sichuan Basin

Natural fractures caused by tectonic stress in shale can not only improve the seepage capacity of shale, but also become migration and loss channel free gas. Calcite, quartz other minerals fracture veins record fluid evolution information shale. Through analysis different types cements Silurian–Ordovician Wufeng–Longmaxi Formations southern Sichuan Basin, effect on gas construction or destruction was clarified. Geochemical investigations included diagenetic mineral sequences hole–cavity veins, paleo-pressure recovery Raman quantitative analysis, environments fluids traced rare earth elements (REE) signatures. The density, composition, pressure, temperature properties CH4-bearing inclusions were determined measurement thermodynamic simulations to establish trapping condition geo-fluids, so constrain periods accumulation. be summarized as follows: Cal-I→Qz-II→Cal-III. Cal-I bedding crystallized late Jurassic (~180 Ma), originated from hydrothermal origin fluid; Qz-II middle (~190 Ma); Cal-III high-angle precipitated during early Eocene (~12 derived atmospheric freshwater leaching. Pore pressure gradually increased. coefficient reservoir increased strong overpressure 160 Ma 86 Ma. Between 75 present day, pore reservoirs, having been affected activities strata uplift-erosion, have significantly reduced. Bedding slippage play a constructive role enrichment gas, slip permeability. High-angle shear usually cut through areas with activity, destroy sealing entrapment primary methane recorded process excess reduction, indicated partial