Abstract：

The estimation of paleotemperatures and paleopressures from methane inclusions in shale reservoirs can provide important clues for the study of shale gas overpressure and enrichment mechanisms. This study investigates the fracture cements from the Wufeng-Longmaxi Formation shales in the Luzhou shale gas field, Sichuan Basin. Petrographic observations show that the fractures are mainly filled with quartz and calcite. If both minerals exist, quartz predates calcite. A large number of high density methane inclusions were found in the quartz cements, suggesting that the formation of fractures and the cementation of quartz occurred during the gas generation stage. The opening of the fractures may be related to the increase of pore fluid pressure caused by gas generation. Using the methane Raman scatter peak v1(2909.97~ 2914.23 cm-1) of methane inclusions,the density of methane inclusions was calculated to be 0.277~0.305 g/cm-3. The homogenization temperatures of the coexisting two-phase gas-liquid inclusions are 242 ℃~258 ℃ (well L201) and 175 ℃~206 ℃ (well H202). Using the equation of state of the CH4 system, the trapping pressure for the high density pure methane inclusions was calculated to be 128~169 MPa,and the corresponding pressure coefficients reached 1.86~2.65, showing high overpressure conditions. The discovery of high density methane inclusions in the Luzhou shale gas field, and their high pressure and temperature conditions, indicate that the Wufeng-Longmaxi Formation shales had already been in an abnormally high pressure state before reaching the maximum depth(before or in the early stage of tectonic uplift). This finding provides important insights into the further development of fracture formation mechanisms and their effects on the preservation and enrichment of shale gas.

Key words：methane inclusions; fracture; Wufeng-Longmaxi Formation; shale; Luzhou shale gas field

Received： 2019-07-20

Corresponding Authors： liudd@cup.edu.cn

Cite this article：PAN Zhankun, LIU Dongdong, HUANG Zhixin, JIANG Zhenxue, SONG Yan, GUO Jing, LI Canxing. Paleotemperature and paleopressure of methane inclusions in fracture cements from the Wufeng-Longmaxi shales in the Luzhou area, southern Sichuan Basin. Petroleum Science Bulletin, 2019, 03: 242-253.

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