我国浅层生物气气藏的压力特征及成因探讨

生物气是有机物处于低温未成熟阶段，在厌氧细菌的生物化学作用下产生的天然气，它与成熟至过成熟阶段形成的热解气的化学成分有着明显差异。“八五”后期，继柴达木盆地、渤海湾盆地和杭州湾地区发现生物气气藏之后，我国又在云南第三系的杨林、陆良、保山等盆地钻获商业性生物气气流，并获得了一定的储量。对国内已发现的生物气气藏统计分析发现，此类气藏压力系数绝大多数为０．９～１．１，属于常压气藏。常压特征形成机理，在于此类气藏地层具有相对开启的水文地质条件（地层成岩作用弱，孔隙度、渗透率高，盖层的封盖能力相对较差，砂岩百分含量高）；经受的地层温度低（４０～６９℃）；构造作用弱（发育生物气气藏的新生代盆地被改造程度较低，基本保留了原始沉积盆地的状态）；气藏高度小（圈闭高度与圈闭层埋深的比值一般为５％～６％，最大不到７％）。鉴于我国浅层生物气气藏的压力特征，钻探中不宜使用密度过高的钻井液，否则会造成对气层的污染，甚至破坏储集层。结合钻井安全系数的规定，在此类气藏钻探中，钻井液密度不应超过１．２５。图１表１参１６（梁大新摘

Pressure characteristics of shallow biogenic gas reservoir and their genesis.

Biogenic gas is a type of natural gas formed by the biochemical action of anaerobic bacteria on organisms at low temperature and nonmatured stage, and differed clearly in their chemical compositions from those thermally created gas formed at matured or over matured stage. In the late period of the eighth FiveYear Plan, after the discovery of biogenic gas reservoirs in Qaidam, Bohai bay and Hangzhou bay basins, commercial biogenic gas reservoirs have been discovered is Tertiary basins, such as Yanglin, Luliang and Baoshan basins in Yunnan, and certain amount of gas reserves are obtained. A statistical analysis reveals that the formation pressure coefficient of these reservoirs lies between 0.91.1, and belongs to that of normal pressure reservoirs. The formation mechanism of such normal pressure gas reservoirs is that they lie in those basins with relatively opened hydrological condition (low diagenesis, high porosity and permeability, relatively weak capping action of cap rooks, high percentage of sandstone contents); relatively low temperature (4069); weak tectonic action (relatively weak reformation of Cenozoic basins as that the original shape of the sedimentary basins are retained basically); low structural closure (ratio of structural closure to their burial depth is 5%6% generally and are not greater than 7%). Due to this formation pressure characteristics, high density drilling fluids should not be used in drilling operations to avoid formation damage. With a safety factor specification in drilling operation the density of drilling fluid used in penetrating such gas reservoirs should not exceed 1.25.