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The Complicated Petroleum System in Sinian-Lower Paleozoic, Central Sichuan Basin, China

Haifeng Yuan, Yong Liu, and Guosheng Xu
Chengdu University of Technology, Chengdu, Sichuan, China

Since the Sinian Dengying Formation gas reservoir was discovered in Weiyuan (Sichuan basin, China) in 1964, there has been no major breakthrough in oil and gas exploration in Sinian-Lower Paleozoic in Sichuan Basin. Over the past two years, the commercial oil and gas flow was found in the Sinian Dengying Formation and Cambrian Longwangmiao Formation in the Gaoshiti-Moxi structural belt in central Sichuan basin, indicating the great exploration potential in Sinian-Lower Paleozoic in Sichuan Basin.

There are mainly two types of hydrocarbon source rock developed in central Sichuan basin, including carbonaceous shale in the Lower Cambrian Qiongzhusi Formation and algae dolomites in the Sinian Dengying Formation. The carbonaceous shale in the Lower Cambrian Qiongzhusi Formation are the dominant source rocks, which contain 1.01%-6.57% organic carbon (average 1.78%) and are 90-120 m in thickness. These carbonaceous shale is characterized as containing typekerogen with determined vitrinite reflectance values in the range of 2.5%-3.7%, thus are in an overmature stage. The algae dolomites in Dengying Formation have also been proven as minor source rocks. These algae dolomites contain abundant blue-green algae with 0.10%-3.67% organic carbon (average 0.93%), thus are in an over-mature stage. The matrix porosity of dolomite reservoir in the Dengying Formation is relatively low, mainly between 1% and 3%. The reservoir pore space including fracture and solution opening are fully or partially filled with asphalt. The asphalt biomarkers indicate that the asphalt mainly comes from the carbonaceous shale in the Lower Cambrian Qiongzhusi Formation. The Lower Cambrian Longwangmiao Formation is dominated by dolomites, mainly consisting of porous reservoir and fractured reservoir. The carbonaceous shale in the Lower Cambrian Qiongzhusi Formation feature a large capillary pressure measured and are the direct caprocks of gas pools in Sinian Dengying Formation. The gypsum bed in Middle-Lower Cambrian is the direct caprocks of gas pools in Longwangmiao Formation. The overburden includes the Cambrian, Ordovician, Permian, Triassic, and Jurassic strata. The traps are litho-structural-traps developed in the background of giant uplift of nosing structure and formed in the late Caledonian movement, with continuous transformation and reconstruction by later tectonic movements.

The complicated petroleum systems have been identified in the Sinian-Lower Paleozoic, central Sichuan basin: the known Qiongzhusi-Dengying systems and the known Qiongzhusi-Longwangmiao systems. They are most important for natural gas exploration in the basin.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013