Carboniferous Shale Distribution And Fracture Characteristics In Eastern Qaidam Basin

With the extent of shale gas exploration deepening, Eastern Qaidam BasinCarboniferous as a new layer system of exploration has caused enough attention. Theshale fractures not only directly affect the shale gas production, but also as aimportant fractor of determing the quality of shale gas reservoirs. Therefore, thispaper studies the distribution, sedimontation and tectonic development feature of theCarboniferous shale in eastern Qaidam Basin. On this basis, it comprehensivelyanalyses the fracture characteristics and the development stage of fractures.The shale well developed in Shihuigou, which has four sedimentary ficies(tidalflat, lagoon, swamp and restricted platform) and four lithofacies(siliceous shale,calcareous shale, carbonaceous shale and silty shale).This paper analyzes the development status of149fractures in ZK1-1core, highangle fractures accounted for85.91%of the total, which have49.22%are filled withcalcite; while low angle fracture is almost completely unfilled. The maximum fracturelength is40cm, the minimum is3cm, and at10cm has the greatest probability of50%.Frature aperture range is0.2mm-5mm, and the maximum is7mm.There are126tectonic fractures of two shale outcrops observed in Shihuigou,high angle tectonic fractures (70%), followed by vertical fractures (16%), low angletectonic fractures (14%) and horizontal fractures are rarely seen.78%of the126tectonic fractures are large cracks (>20cm),13%are macro cracks (10-20cm),9%arefractures (4-10cm), and few micro cracks are observed. The first outcrop has75fractures, and the second outcrop has51fractures. Each shale outcrop is divided intotwo groups, the fracture attitude of two groups in first outcrop are225.71°∠49.11°and103.31°∠68.49°, mean fracture length are0.388m and0.55m, standarddeviation of fracture length are0.4902m and0.334m. There are11.492fractures permeter (one-dimensional density of fracture) and10fractures per meter of each groups.Besides, the fracture attitude of two groups in second outcrop are226.82°∠7.46°and98.78°∠90.00°, mean fracture length are0.827m and0.291m, standarddeviation of fracture length are0.4827m and0.2644m. There are14.667fractures permeter and2.333fractures per meter of each groups. Based on measurements offractures obtained in the first shale outcrop, the discrete fracture network model is established by an inverse method for identifying and analyzing fractures developmentstatus in study area.There are three categories of fractures according to geological surveys and coreobservation, namely, tectonic fracture, diagenetic fracture and atectonic fracture. Fromthe sagmentation, limitation and combination relationship, it indicates that thefractures formation have two stages: late Hercynian and early Yanshan. Controlling bythe maximum principal stress of NE-SW, late Hercynian has formed fractures ofNE-SW in trend, and early Yanshan has formed fractures of EW under control bymaximum principal stress of EW.