216 / 2019-06-23 17:25:48

Characteristics of karst reservoirs in Leikoupo Formation of Xiangshui profile in Jiangyou, Hanzeng Town, Western

Western Sichuan, Leikoupo Formation, karst, fracture-cavity

Leikoupo Formation is completely outcropped in Xiangshui profile in Jiangyou, Hanzeng Town, Western Sichuan and it can be divided into 4 members. Lei 4 Member can be divided into 3 zones, among which, L43 zone is the main development area of karst reservoirs. Xiangshui profile in Jiangyou, Hanzeng Town is located northwestern Sichuan. Leikoupo Formation was deposited at the end of Middle Triassic time. At the end of the depositional period of Leikoupo Formation, early Indosinian movement occurred, when large-scale stratigraphic uplifting resulted in the seawater that was trapped at early time gradually withdrawing from the upper Yangtze platform. Thereby, the sediments of the Leikoupo Formation gradually exposed to the surface and weathering and denudation occurred. Tianjingshan Formation was composed of algal limestone. Leikoupo Formation was composed of gray, grayish white and grayish yellow thick dolomite at top. Tectonic karst breccias were observed in the field. Gypsum karst breccias were observed in L42 zone locally. At top, there were large karst caves and oblique fissures and interlayer fractures. Dissolution occurred along fractures, so beaded karst caves were observed. Leikoupo Formation was mainly composed of dolomite, limestone, limy dolomite and dolomitic limestone and the sedimentary facies was high-energy granular shoal facies. According to the results of microscopic observation, L43 zone was composed of micrite, crystal powder and fine-grained dolomite. Residual debris were observed, showing very obvious recrystallization. Two stages of tectonic micro-fractures were observed. The rocks were reformed into breccias of different sizes due to dissolution, then the micro-fractures of the first stage were filled by clean and bright dolosparite at late diagenetic stage. The structural fractures of the second stage cut the structural fractures of the first stage and original karst breccias. De-dolomitization occurred due to the leaching of fresh water. The dissolved caves and fractures were filled with clean and bright calcite at late stage. The intercrystalline pores and intergranular pores became dissolved fractures and caves due to dissolution, but most of which were completely filled or half filled by calcite, dolomite and argillaceous matters. Argillaceous sutures, algae and algae window pores were observed locally. L43 zone was about 120m from top to bottom. Obvious karstification occurred within 60m from the top. Fractures, caves and karst breccias were widspread, but part fractures and cave were filled. During the depositional period of Leikoupo Formation, the sedimentary facies was platform margin facies. The hydrodynamic conditions were better than that of deep water environment. The study area was in karst slope zone in response to uplifting and denudation. Reservoirs were well developed, with porosity of 2.6% and permeability of 0.024md.