Accumulated conditions of shale gas in the Middle-Low Palaeozoic, Yangtze Platform, China

摘要

Shale gas is an important unconventional natural gas resource. There is probably abundant shale gas resource in the Longmaxi Formation of the lower Silurian, Sichuan Basin, West China. Longmaxi Formation is high quality source rock, its TOC(total organic carbon) up to the most vulue 6.5 percent; the Formation more than 1percent TOC is up to 105 meters. Because of high maturity, it cannot identify the kerogen type by element Carbon and Hydron ratio, rather than carbon isotopic value. The isotopic carbon values of Kerogens are more than 29.67 permillage, which showed One Type Kerogen. In the Longmaxi Formation, source rock is up to post maturity and lack of vitrinite, so vitrinite reflectance cannot measure the source maturity. It can measure bitumen reflectance, then calculate relevant vitrinite reflectance (Ro) by bitumen reflectance to identify the organic matter thermal evolution degree. The result showed the maturity degree of organic matter equal to more than 3 percent vitrinite reflectance (Ro), which showed the source rock yielded rich thermogenic gas during the geological thermal evolution. The source rock of Longmaxi Formation has some silt partly composed of quartz mineral, up to 40 percent, which is beneficial to gas reservoir. So, it has available shale gas developed elements because of rich organic matter, high thermal maturity degree, good pool. The geological analogy method is used to assess the resource potential. Sichuan Basin has drastically similar to Michigan Basin and San Juan Basin in basin type, seal conditions, reservoir states, source rock, matched condition and so on. So they are selected as standard basin. Sixteen parameters are used to appraise the shale gas pool geological conditions. Compared with Michigan Basin and San Juan Basin by analogy method, Longmaxi Formation shale gas potential is 0.06-0.36 billion cubic meter per square kilometer.