横山气田是鄂尔多斯盆地首个致密灰岩气田,提交储量超2 000×108 m3,是长庆油田增储上产现实的接替领域.基于岩心、铸体薄片、扫描电镜的观察及物性、测井、试气等资料,开展沉积相、储层特征及成岩作用等综合研究,分析有利储层形成机理及展布规律.结果表明:①太原组灰岩储层岩石类型主要包括生屑粉晶灰岩、藻黏结灰岩、生屑泥晶灰岩,储集空间主要为溶孔、残余生物体腔孔、晶间孔及微裂缝.溶蚀孔、纳米—亚微米级晶间微孔和裂缝构成立体网络储集空间,这是形成太原组致密灰岩气藏的关键.②太原组灰岩储层主要形成于准同生期间歇性暴露的生屑滩.生屑滩是储层形成的物质基础,决定了储集空间类型及平面分布,准同生岩溶作用有利于溶蚀孔的形成,能有效改善储层的储集性能,裂缝对改善致密灰岩储层渗流能力具有重要作用.③受控于海平面升降旋回与古地貌,太原组灰岩有利储层纵向上优势发育在斜道段,平面上具有"斜坡发育、环凹沿高"式发育规律.研究成果有助于推动整个华北地区太原组生物灰岩领域的再认识,对于国内古老碳酸盐岩层系的油气勘探开发有重要的启示.

In 2021,high production gas flow was obtained in tight limestone of Lower Permian Taiyuan Formation in Well A,a risk exploration well.Through integrated exploration and development,Hengshan gas field,the first limestone gas field in Ordos Basin,was discovered with a submitted reserve of over 200 billion cubic meters.It is a replacement field of reality for increasing reserves and production in the Changqing Oilfield.Based on data of cores,casting thin sections,scanning electron microscopy,physical properties,logging,and gas testing,comprehensive research on reservoir characteristics such as sedimentary facies,petrology,reservoir space and diagenesis is conducted to analyze the formation mechanism and the distribution pattern of favorable reservoirs.The results show that:(1)The lithology of limestone reservoir is mainly composed of bioclastic very finely crystalline limestone,algal bound limestone and bioclastic micritic limestone.The storage space is mainly composed of dissolution pores,residual organism cavities,intergranular pores and microcracks.Dissolution pores,nano-submicron scale intergranular micropores and fractures form a three-dimensional network storage space,which is the key to form a tight limestone gas reservoir in the Taiyuan Formation.(2)The limestone reservoir is mainly formed in the intermittent exposure of bioclastic shoals under the influence of karst during the quasi contemporaneous period.The bioclastic shoal are the material basis for reservoir formation,determining the type and planar distribution of reservoir space.Quasi syngenetic karstification is conducive to the formation of dissolution pores and can effectively improve the reservoir storage performance.Cracks play a crucial role in improving the permeability of tight limestone reservoirs.(3)Controlled by sea level cycles and paleogeomorphology,the favorable limestone reservoirs of Taiyuan Formation are vertically developed in Xiedao Member,and horizontally developed with a pattern of"around depression and along highland",especially with larger reservoir thickness at the slope due to lateral stacking under the background of regression.This study will help to promote a further understanding of biogenic limestone of Taiyuan Formation in North China,and have important implications for the exploration and development of ancient carbonate rock in China.