鄂尔多斯盆地中东部奥陶系盐下天然气成藏特征及勘探方向

Characteristics of natural gas accumulation and exploration target of the Ordovician subsalt in the central and eastern Ordos Basin

鄂尔多斯盆地中东部地区奥陶系马家沟组发育巨厚的盐岩、膏岩沉积,封盖性能好,膏盐岩之下的碳酸盐岩一直是备受关注的重要勘探领域。在对区域构造沉积背景及沉积相带分布研究的基础上,以马家沟组马五段膏盐岩盖层之下的马四段碳酸盐岩为目标,主要开展了储层及成藏特征研究。研究认为:马四段白云岩分布主要受控于古隆起控制下的台内分异作用,发育滩相晶间孔型白云岩储层,马三段等层段的溶孔型白云岩储层也具有较好的储集性能;加里东期的风化剥蚀作用造成马四段白云岩与上古生界煤系烃源岩、奥陶系盐下海相烃源岩形成了空间配置关系,天然气成藏具有双源供烃的特点,形成岩性相变控制下的白云岩岩性气藏。在对不同区域生储盖配置关系及成藏控制因素的差异性对比的基础上,综合分析认为盆地东部盐下马四段盐岩底辟隆起带和中部乌审旗古隆起控制下的岩性相变带是盐下勘探的有利目标,深层的马三段、马二段也具有较好的勘探潜力。

The large-scale salt and gypsum rocks are deposited of the Ordovician Majiagou Formation in central and eastern Ordos Basin,with great thickness and superior sealing capacity.The underlying carbonates have always been a major field for natural gas exploration.Based on the analysis of regional tectonic sedimentary settings and sedimentary facies distribution,the characteristics of carbonate reservoir and gas accumulation of the target layer-the 4^(th) member of Majiagou Formation(Ma 4 member)are studied,which is underlaid by subsalt caprock of the 5^(th) member of Majiagou Formation(Ma 5 member).The results show that,controlled by the intra-platform sedimentary differentiation in the paleo uplift,the intercrystal pore type dolomite reservoir of beach facies is developed of Ma 4 member.In addition,the dissolution pore type dolomite reservoir of the 3^(rd) member of Majiagou Formation(Ma 3 member)also has good reservoir performance.The weathering and denudation during the Caledonian resulted in a good spatial configuration between the dolomite reservoir of Ma 4 member and the source rocks of coal series in the Upper Paleozoic and marine source rocks of the Ordovician subsalt.Therefore,the gas reservoir has the characteristics of dual-source hydrocarbon supply and lithologic gas reservoir controlled by lithologic facies change.The detailed analysis of source rock–reservoir–caprock assemblages and the comparison of controlling factors for gas accumulation in different regions indicate that the salt diapir uplift zone of Ma 4 member of the Ordovician subsalt in the eastern basin and the lithologic facies change zone controlled by Wushenqi paleo uplift in the central basin are favorable targets for gas exploration.In addition,Ma 3 member and the 2^(nd) member of Majiagou Formation(Ma 2 member)in deep formation also have promising prospects.