鄂尔多斯盆地太原组致密灰岩天然气成藏地质特征与勘探潜力

2021年鄂尔多斯盆地榆探*H在太原组灰岩完钻喜获日产超50万m³的高产工业气流，勘探取得重大突破，同时还有700余口探井在太原组灰岩见到明显的含气显示，展示了盆地太原组灰岩良好的潜力，对于重新认识盆地太原组灰岩勘探领域具有极其重大的地质意义。对该区烃源岩特征、沉积环境、储集特征及成藏组合等进行综合研究表明：(1)太原组灰岩为陆表海碳酸盐沉积，发育生屑滩、生物丘有利沉积相带，沉积大面积分布的生屑粉晶灰岩、藻黏结灰岩，经准同生期短暂的大气淡水淋滤后，可形成有利储层；(2)太原组灰岩发育多类型储集空间，溶孔、亚微米级及纳米级晶间微孔、微裂缝等叠置组合发育，构成立体网络储集空间，平面上，横山—靖边—志丹生屑粉晶灰岩储层发育，厚4～10 m,佳县—子洲—清涧地区藻黏结灰岩储层发育，厚2～6 m;(3)发育“三明治”成藏模式，灰岩储层夹在主力煤系烃源岩之间，生成的天然气通过断裂、裂缝进入灰岩储层聚集，形成岩性气藏，横山—靖边地区的生屑滩和子洲—清涧的生物丘是太原组灰岩有利勘探区，面积约为1.5万km²,有望形成天然气增储上产的现实领域。

Accumulation characteristics and exploration potential of tight limestone gas in the Taiyuan Formation of the Ordos Basin

In 2021, a high-yield daily output of more than 5.0×10⁵ m³ industrial gas was obtained in well Yutan*H, signifying a major breakthrough in natural gas exploration in the limestone reservoir of Taiyuan Formation, Ordos Basin. Now, more than 700 exploratory wells have good gas shows in the reservoir, showing excellent exploration potential, which is of great significance for further studying the geological characteristics of the limestone of Taiyuan Formation. Based on comprehensive analysis of the source rock characteristics, sedimentary environment, reservoir characteristics and hydrocarbon accumulation assemblage in the area, we show that: (1) The limestone of Taiyuan Formation is an epicontinental carbonate deposit, in which favorable sedimentary facies belt composed of bioclastic beach and biological mound is developed, and the widespread bioclastic silty limestone and algal-rich limestone can form favorable reservoirs after short-term leaching by atmospheric freshwater in penecontemporaneous period. (2) The limestone of Taiyuan Formation develops several types of reservoir space, where solution pores, submicron to nano intergranular micropores and microfractures superimpose to form a three-dimensional reservoir space; meanwhile across the region, 4-10 m thick bioclastic silty limestone reservoir develops in the Hengshan-Jingbian-Zhidan area, and 2-6 m thick algal-rich limestone reservoir develops in the Jiaxian-Zizhou-Qingjian area. (3) The hydrocarbon accumulation mode has a “sandwich-like” structure, where the limestone reservoir is sandwiched between main coaly source rocks, and the generated natural gas enters limestone reservoir through faults and fractures to accumulate and form a lithologic gas reservoir. With a combined area of 1.5×10⁴ km², the bioclastic beach in Hengshan-Jingbian area and the biological mound in Zizhou-Qingjian area are the favorable exploration targets for the limestone of Taiyuan Formation, which is expected to bring the increase of natural gas reserve and production.