鄂尔多斯盆地神木气田的发现与天然气成藏地质特征

The Shenmu Gas Field in the Ordos Basin:Its discovery and reservoir-forming geological characteristics

截至目前,鄂尔多斯盆地神木特大型气田已探明含气面积4 069 km^2、探明天然气地质储量3 334×10~8m^3,该区域有可能将形成万亿立方米级天然气储量规模,系统总结该气田的勘探历程、气藏地质特征及天然气成藏机理,可以指导该盆地以及其他盆地同类气藏的勘探。研究结果表明:①该气田产层主要为上古生界二叠系太原组,其次为山西组、石盒子组,为典型的致密砂岩气藏;②天然气类型主要为煤成气,其中甲烷平均含量为88%,不含H_2S;③气藏埋深介于1 700~2 800 m,具有多个压力系统,平均压力系数为0.87;④储层为大面积分布的河流—三角洲相砂岩,平均孔隙度为7.8%、渗透率为0.63 mD,喉道半径多小于1μm,具有强的应力敏感性和水锁效应;⑤不同阶段天然气成藏动力差异明显,早白垩世沉降阶段异常压力发育,成为天然气成藏主要动力,早白垩世之后抬升剥蚀阶段流体膨胀力成为天然气运移主要动力;⑥气藏具有超低含水饱和度特征,其主要受控于晚侏罗世—早白垩世高温、高压场的变化,高成熟阶段干气的充注以及成藏后期气藏泄漏逸散等作用;⑦天然气以垂向近距离运聚成藏为主,源内、近源含气组合气源充足、含气饱和度高、气藏规模大,远源含气组合以次生气藏为主,气藏规模相对较小。

By the end of 2014, a giant Shenmu Gas Field had been found in the Ordos Basin with an explored gas-bearing area of 4 069 km^2 and proved geological gas reserves of 333.4 billion m^3. This paper aims to review the exploration history of this field and discusses its reservoir-forming mechanism and geological characteristics, which may guide the further discovery and exploration of such similar gas fields in this basin and other basins. The following research findings were concluded. （1） There are typical tight sand gas reservoirs in this field primarily with the payzones of the Upper Paleozoic Taiyuan Fm, and secondly with those of the Shanxi and Shihezi Fins. （2） Gas types are dominated by coal gas with an average methane content of 88% and no H2 S content. （3） The gas reservoirs were buried 1 700 2 800 deep underneath with multiple pressure systems and an average pressure coefficient of 0.87. （4） The reservoir strata are composed of fluvial delta facies sandstones with an average porosity of 7.8% and permeability of 0.63 roD, having high pressure sensibility and a strong water-locking effect because the pore throat radii are mostly less than 1μm. （5） There are different dynamics at various stages in the gas reservoir-forming process. The abnormal well-developed strata pressure was the main reservoir-forming force at the Early Cretaceous setting stage while the fluid expansibility became the main gas-migrating force at the uplift and denudation stage after the Early Cretaceous period. （6） Gas reservoirs with ultra-low water saturation are mainly controlled by many factors such as changes of high temperature and high pressure fields in the Late Jurassic and Early Creta ceous periods, the charging of dry gas at the highly mature stage, and the gas escape and dissipation at the post-reservoir-forming periods. （7） Natural gas migrated and accumulated vertically in a shortcutting path to form gas reservoirs. At such areas near the source rocks, large-scale gas reservoirs were easily found with plenty of gas sources and high gas saturation; but at those far from the source rocks, relatively small-scale and mostly secondary gas reservoirs were discovered.