柴达木盆地第四系生物气形成机理、分布规律与勘探前景

从生物气成烃条件、运移方式和封盖机理等方面，研究柴达木盆地第四系生物气藏形成机理、分布规律，原始有机质物气生成模拟实验结果表明，原始有机质生物转化率 可以高达85%以上。虽然第四系烃源岩残余有机质丰度低，平均有机碳含量为0.3%，但低丰度烃源岩仍然可以形成大型生物气田。柴达木盆地中东部三湖地区生物气主要以水溶相运移为主，即在盆地南侧和深部生成的生物气溶解在地层水中，由南向北侧向运移，当到达北侧时，埋藏变浅，地层水矿化度变高，天然气从地层水中析出，以游离相形式垂向运移成藏。在柴达木盆地向北斜坡地区，同时存在物理封闭，饱和盐水封闭和动态封闭3种机理。柴达木盆地第四系生物气藏形成条件良好。三湖地区北斜坡仍然是今后生物气的勘探重点地区，东部察尔汗地区和北斜坡深层是生物气勘探的有前景的地区。图5表3参12

Formation mechanism, distribution feature and exploration prospect of the Quaternary biogenic gas in Qaidam Basin, NW China

The formation mechanism and distribution of the Quaternary biogenic gas fields in Qaidam Basin is studied based on the biogenic gas generation, migration mode and cap-rock sealing mechanism.The biogenic gas simulation experiments suggest that the conversion rate of some kinds of original organic matters may reach over 85%. Although the residual TOC (total organic carbon) values of the Quaternary source rocks in Qaidam Basin are only about 0.3%, but giant biogenic gas fields could still be formed. The biogenic gases in the Sanhu area, central to eastern Qaidam Basin, migrated laterally by water-dissolving phase. The biogenic gases generated in the southern side and deep section of the basin were dissolved in formation brine and migrated from south to north. Biogenic gases were separated from the formation brine and then trapped in free phase through the vertical migration because of the shallower buried depth and higher formation brine salinity. Three kinds of sealing mechanism coexist in the north slope of Qaidam Basin: physical property sealing, saturated brine sealing and dynamic sealing. The Quaternary biogenic gas reservoir in Qaidam Basin is provided with favorable conditions. The north slope of the Sanhu area is still a key play target for biogenic gases in future, and the eastern Chaerhan area and deep zone of the north slope in Sanhu area are potential play targets.