Overpressure development and oil charging in the central Junggar Basin,Northwest China: Implication for petroleum exploration |
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Authors: | XiYuan Cai |
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Institution: | 1.China Petroleum & Chemical Corporation,Beijing,China |
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Abstract: | The Junggar Basin is one of the largest and most petroliferous superimposed petroleum basins in China. The central depression
area has become the frontier field for petroleum exploration. The characteristics of potential source rocks and reservoir
sandstones, and the pressure regime in the central Junggar Basin were studied. Permian shales are dominated by hydrogen-rich,
oil-prone algal organic matter, and Jurassic mudstones are dominated by hydrogen-poor, higher-plant derived organic matter.
These source rocks are widespread and have been mature for hydrocarbon generation, suggesting good to excellent exploration
potential, both for crude oils and for natural gases. The deeply buried Jurassic sandstones usually have low porosity and
permeability. However, sandstones beneath the Jurassic/Cretaceous unconformity display relatively high porosity and permeability,
suggesting that meteoric water leaching had improved the quality of the sandstones. Overpressure developed over much of the
central Junggar Basin. The overpressured rocks are characterized by slightly increased interval transit time, low sandstone
permeability, increased organic matter maturity, and high relative hydrocarbon-gas contents. Mudstones in the overpressured
system have quite the same clay mineral compositions as mudstones in the lower part of the normally pressured system. Overpressure
generation in the central Junggar Basin is best to be explained as the result of hydrocarbon generation and fluid retention
in low-permeability rocks. Petroleum generated from Permian and Jurassic source rocks could migrate laterally through preferential
petroleum migration pathways and accumulated in structural traps or lithological traps in the overpressured system, or migrate
vertically through faults/hydraulic fractures into the overlying, normally pressured system and accumulate in structural or
lithological traps. Therefore, commercial petroleum reservoirs could be potentially found in both the overpressured system,
and in the normally pressured system. |
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