Characteristics of Pre-Cenozoic Bedrock Geothermal Reservoirs and Their Distribution In Leiqiong Area

The Leqiong area includes the Leizhou Peninsula in Guangdong Province and the northern part in Hainan Province. It belongs to the continental area of Beibuwan Basin and formed on the margin of continental margin since Cenozoic. Previous studies shows that four pre-Cenozoic structural layers of Beibuwan Basin have been divided: Precambrian crystalline basement, Caledonian fold basement, Hercynian - Indosinian fold basement and Yanshan fold basement. Due to the surface of the continental area of Beibuwan Basin (Leiqiong area) is covered by a large number of Cenozoic volcanic rocks, the basement of Leiqiong area is not clear, which restricts the exploration and evaluation of deep geothermal resources in this area. Through field geological exploration, data collection and arrangement of drilling data, laboratory experimental data, combined with the surrounding geological conditions of Leqiong Depression, on the basis of previous research results, the pre-Cenozoic geothermal reservoir distribution map of Leqiong Depression was compiled, and the distribution of concealed granite geothermal reservoir was analyzed. 3 types of geothermal pre-Cenozoic geothermal reservoirs in Leiqiong area are summarized: Mesozoic sandstone reservoirs, Upper Paleozoic carbonate reservoirs and granite reservoirs. The Mesozoic sandstone reservoirs are mainly cretaceous, which are fluvial and lacustrine deposits, composed of purplish red coarse clastic rocks, fine clastic rocks, pyroclastic rocks, volcanic lava and a small amount of tuff. They are mainly formed in the fault basin of active continental margin, distributed in the south of Leizhou Peninsula and North Hainan area, and their thickness can reach more than 2000 m. At present, a geothermal well was drilled in YongFa area, southern Leiqiong area, encountering Cretaceous sandstone at 232m, with water flow of 696.38 m3/d, wellhead water temperature of 40℃ and well depth of 1000.61m. The upper Paleozoic carbonate geothermal reservoirs are mainly distributed in Weixinan sag, Northwest Beibuwan Basin. In Leizhou Peninsula, only carbonate interbeds of upper Paleozoic are found in Well Xu 2 of Maichen Sag. The granite geothermal reservoirs are mainly distributed in the Zhanjiang depression, Yunlong uplift and discovered from wells in sags (such as WAN10, Weizhou 12-3-1, Wushi 1, N16, N6, QCK16, QZK02, QZK10, etc). Combined with previous MT gravity, magnetic data and wide-area electromagnetic data, it is considered that concealed granites are widely distributed under the Cretaceous in each sag of Leiqiong area, which is the main type of deep geothermal resources exploration in this area. The prediction of concealed granite can provide a basis for the future deep geothermal exploration and the optimization of favorable areas.