滇黔桂地区新生代走滑拉分裂陷盆地生物气成藏规律

BIOGAS RESERVOIR FORMATION LAW IN CENOZOIC STRIKE-SLIP TENSIONAL RIFT-SAG BASINS IN YUNNAN—GUIZHOU—GUANGXI REGION

我国西南边陲滇黔桂地区的一批小型走滑拉分裂陷盆地普遍发育生物气藏。在各盆地的深洼及其周围地区 ,烃源岩富含草本有机质 ,厚度大 ,埋深 4 0 0～ 12 0 0m ,地温 30～ 6 0℃ ,孔喉 10～ 14 0 μm ,水介质pH值6 .8～ 8,SO42 -及NO3 -离子浓度很低 ,十分有利于甲烷菌的大规模繁殖 ,加之该区带具多套生储盖组合 ,发育大量构造、构造—岩性、岩性圈闭 ,断层密度低 ,具有良好的生物气成藏条件。而在盆缘地区 ,烃源岩及盖层不发育 ,砂岩过于发育 ,断层密度高 ,富含SO42 -、NO3 -水体十分活跃 ,封盖条件差 ,不利于生物气藏的形成。因此 ,生物气藏主要分布于深洼及其紧邻的过渡区带 ,气藏埋深介于 4 0 0～ 12 0 0m。

Biogas reservoirs have been commonly found in a large number of small-sized strike-slip tensional rift-sag basins in Yunnan - Guizhou - Guangxi region. In the deep sags of the basins and their surrounding areas the thick hydrocarbon sourc rocks are rich in herbaceous organic matters, their burial depths being 400-1200 m, geotemperatures 30-60 °C and pore-throat sizes 10-140 μm; and their sedimentary water mediums were very beneficial to the large scale procreation of mathane bacteria, being of pH values of 6, 8-8 and low SO 42- and NO33- concentrations. In addition, there are several source-reservoir-seal assemblages and a great number of structural traps, structural-lithological traps, lithological traps and a few faults in the areas, so that the conditions of forming biogas reservoirs are very good. However, the biogas reservoir formation are unfavourable around basin-margin zones, because of undeveloped hydrocarbon source rocks and caprocks, overdeveloped sandstones, many faults, very active exchange of water being rich in SO42- and NO3-, and poor caprock quality. In a word, the biogas reservoirs are mainly distributed in the deep sags and their adjacent transition areas and the burial depths of the reservoirs range from 400 to 1200 m.