Abstract
Fluid charging and formation pressure evolution are of great significance for shale gas exploration and development. Taking the Longmaxi formation shale in southeast Chongqing as the research object, the fluid activity and pressure evolution process were studied by means of thin section observation, fluid inclusion analysis and testing, and basin numerical simulation. The results show that the area of the southeast of Chongqing, the Longmaxi formation of dike filling minerals, is mainly quartz vein and calcite part of the veins and arteries, and vein body captures the rich organic inclusions in minerals, sees fgas-liquid two-phase brine inclusions and methane inclusions, combining with the temperature and salinity of brine inclusions, and shows that the area has experienced two phases of oil and gas filling, the first period of gas-liquid fluid. The second stage is gas fluid. Combined with burial history and thermal maturation history, it is found that the filling time of oil and gas is late Cretaceous and Paleocene, respectively. In the Cretaceous, the pressure coefficient of Longmaxi formation reached the maximum value of 2.2 and remained above 1.5 overpressure after pressure unloading in the later period. In terms of the frequency of inclusions, the charging amount of the first stage is relatively large, and the charging time corresponds to the peak period of the palaeo-formation pressure. At the same time, fluid charging can timely supplement the pressure after unloading, which is conducive to high and stable production of shale gas.
Highlights
Since 2008, China has explored multiple large marine facies shale gas fields in the southeast Chongqing area of Sichuan Basin (Figure 1), with shale gas resources exceeding 2:3 × 1012 m3 (Sinopec, 2020); successful shale gas exploration has been achieved in the middle-shallow strata of the Lower Silurian Longmaxi formation
Breakthroughs have been made in the exploration of deep shale gas (3500 m ~4500 m) in Yibin and Luzhou areas of Sichuan Basin [1, 2], making Sichuan Basin the most representative and potential area for marine shale gas exploration in south China [3,4,5]
To clarify the mechanism of shale gas fluid charging and preservation in the Longmaxi formation during the multistage tectonic evolution in southeast Chongqing, the controlling effect of formation pressure evolution on shale gas reservoirs can provide an important reference for shale gas exploration in other areas of Sichuan Basin [15]
Summary
Since 2008, China has explored multiple large marine facies shale gas fields in the southeast Chongqing area of Sichuan Basin (Figure 1), with shale gas resources exceeding 2:3 × 1012 m3 (Sinopec, 2020); successful shale gas exploration has been achieved in the middle-shallow strata of the Lower Silurian Longmaxi formation (burial depth 2000 m ~3500 m). Breakthroughs have been made in the exploration of deep shale gas (3500 m ~4500 m) in Yibin and Luzhou areas of Sichuan Basin [1, 2], making Sichuan Basin the most representative and potential area for marine shale gas exploration in south China [3,4,5]
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