三水盆地宝月地区干气的成因探讨

ORIGIN OF DRY GASES IN BAOYUE DISTRICT, SANSHUI BASIN

通过分析三水盆地宝月地区天然气的组分和甲烷的稳定同位素组成.探讨了该区干气的成因,并提出了一种生物成气模式.宝月地区一些天然气的干湿指数(C_1/(C_2+C+3)]在18至200间,甲烷的碳、氢同位素组成分别为-61.0‰——59.5‰和-235.9‰——174.1‰.气层埋藏浅(<900m),分布于宝月隆起构造的高部位.生油期后构造运动使宝月地区上升.原生油田水受到改造,再次形成了生物成气的地球化学环境.次生生物化学作用改造原油和湿气形成了干气.

Gas constituents and stable isotope compositions of carbon and hydrogen of natural gases from Baoyue area. Sanshui Basin, Guangdong Province, have been analyzed, and origin of the natural gases are discussed. The dry-wet coefficients (C1/ (C2+C3)) of dry gases in Baoyue area are more than 18, the largest up to 200. Stable isotope compositions of carbon and hydrogen of methane in dry gases are -61.0-59.5‰ (PDB) and -235.9-174. 1‰, respectively. The dry gases are found in the higher part of Baoyue upwarp. The reserviors of dry gas are shallowly buried (the depth is less than 900m) .Primary water of the oil field in Baoyue district have been reformed by precipitate water since the faults were formed by tectogenesis and volcanic activities during Eocene-Oligocene. The Na2SO4 type of oil field waters containing higher concentration of ions ( 50 to 100 g / 1) , which were found in neighbouring fault blocks, became NaHCO3 tye of oil field waters in which concentrations of ions are between 25 to 50 g / 1 in the fault blocks containing dry gases. The NaHCO3 type of water is of pH values about 7.5. Geochemical environment that is suitable for methanogenic microbe were reconstructed after the stage of oil generation as a result of reducing concentration of SO4- and changing pH values of the water. Stable isotope compositions of carbon and hydrogen of methane of the dry gases are uniform with those of biogenic methane. Methane in the dry gases is enriched in deuterium, which may be relative to depositional environment and process of biogenic methane. It is considered that an amount or hydrogen which reduces carbon dioxide to methane under the action of mcthan genie microbe, comes from ground water depleted in 12C. All of the geological and geochemical data indicates that the dry gases found in Baoyue area might be formed by microbe altering crude oil and wet gases, methane of the dry gases may be preponderantly generated through the passway of CO2 reduced by H2, which can be called as the modlc of formation of secondary- biogenic gas. Therefore, stable isotpe compositions of carbon and hydrogen of methane are not only affected by crude oil, wet gas and depositional environment, but alsoby isotope compo sitions of hydrogen of oil field waters, and controlled by biochemical fractionation; the value of dry-wct coefficient may reflect the grade altered crude oil and wet gas.