Abstract
The Sichuan Basin is one of the richest oil and gas basins in China. The Middle Permian units (the Qixia and Maokou Formations) in the northwest Sichuan Basin have great potential for gas exploration. A new thermal history was reconstructed using the integrated thermal indicators of apatite and zircon (uranium–thorium)/helium ages, zircon fission tracks, and vitrinite reflectance data. The modeled results indicated that the northwest Sichuan Basin experienced gradual cooling, during which the heat flow at Middle Permian time (70–90 mW/m2) decreased to its current level of approximately 50 mW/m2. This study used basin modeling to reconstruct the paleo-pressure, which showed that the Middle Permian in the northwest Sichuan Basin generally developed overpressure. The pressure evolution of the Middle Permian can be divided into three stages: (1) a slight overpressure stage (T2–T3), (2) an intensive overpressure stage (J1–K2), and (3) an overpressure reduction stage (K2–present). Oil cracking and rapid tectonic subsidence are key factors that affect overpressure. The evolution of temperature–pressure has great significance with respect to hydrocarbon accumulation.
Highlights
The Middle Permian is the main stratum of oil and gas exploration in the Sichuan Basin, and exploration began in 1950s (Zhou et al, 2016)
In 2014, high-yield industrial gas was produced from the Middle Permian dolomite reservoirs in Well ST1 in the northwest Sichuan Basin (NWSB), which was a great breakthrough in natural gas exploration in the Middle Permian system (Shen et al, 2015)
The Sichuan Basin was a stable cratonic basin during the Sinian–Paleozoic (Guo et al, 1996), during which the thermal regime was stable and the tectonic and magmatic activities were weak. This prompts the question: what caused the high thermal state in the Middle–Late Permian? There are two possible reasons: (1) The Emeishan Large Igneous Province (ELIP) was erupted during the Middle–Late Permian, which has been attributed to the effects of a mantle plume that produced complex related magmatic activity (He et al, 2006; Xu et al, 2004)
Summary
The Middle Permian is the main stratum of oil and gas exploration in the Sichuan Basin, and exploration began in 1950s (Zhou et al, 2016). The ZHe ages of sample SS-1, SS-3, and GX-8 are younger than the stratigraphic age, which indicates that the maximum paleo-temperature was within the PRZ of zircon and effectively records a Late Triassic cooling event. The thermal modeling results of samples SS-3 and GX-8 show that the Longmen Mountain area was characterized by long-term deposition from the Permian to the Triassic, during which the burial temperature of these two samples gradually increased (Figure 5). This area experienced cooling beginning at 210 Ma (Late Triassic). The pressure evolution of the Middle Permian can be divided into three stages (Figure 10): (1) a short overpressure stage from the Middle Triassic to the end of the Triassic, during which excess pressure was approximately 10 MPa; (2) an overpressure stage from the Early Jurassic to the Late Cretaceous, with maximum excess pressure of approximately 50–70 MPa in the fold zone and approximately 10–20 MPa in the fault zone; and (3) an overpressure reduction stage from the Late Cretaceous onward, during which overpressure disappeared in the fault zone but remained in the fold zone
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