致密砂岩储层微观孔隙特征评价——以中国吐哈盆地为例

Analyses of micro-pore structural characteristics of tight sandstone reservoirs:A case study in Turpan-Hami Basin,northwestern China

致密砂岩微观孔隙特征评价是致密油气地质评价的核心研究工作。基于恒速压汞、铸体薄片、X-射线衍射等测试分析,对吐哈盆地水西沟群致密砂岩开展微观孔隙特征评价工作。研究表明:靶区致密储层岩石类型以岩屑长石砂岩和长石岩屑砂岩为主,不稳定矿物含量高是导致微观孔隙结构复杂的主要原因;致密储层的物性与矿物组成有密切关系,石英由于其脆性而易形成微裂缝,对储层的渗透率贡献明显,对孔隙度影响不大;黏土矿物由于其可塑性及遇水膨胀的特性,对孔隙度和渗透率影响明显,均呈负相关;长石由于其不稳定性,在溶蚀作用下形成高岭石,但在致密储层复杂孔喉体系的背景下,流体的迂回程度高,造成高岭石多在原地沉淀,堵塞孔隙和喉道,与孔渗存在明显的负相关;基于毛管压力曲线,可把致密储层的孔喉结构划为3类,分别为粗喉道所控制的高孔区、中喉道控制的中孔区和细喉道控制的低孔区;同常规储层相比,致密储层具有排替压力高、进汞饱和度低、喉道半径小、孔喉比高、结构系数高等特点。该评价方法和思路,为吐哈盆地,乃至我国其他盆地的致密油气勘探提供科学依据。

Evaluation of micro-pore structural characteristics is one of the key issues needed to be addressed in tight oil and gas geology evaluation.This article discussed a study of the kind based on data gathered through experiments and technologies like rate-controlled mercury penetration,casting thin sections,X-ray diffractions,upon samples from tight sandstone reservoirs in a study area of the Shuixigou Group in Turpan-Harmi Basin,northwestern China.The result showed that the tight rock types are mostly lithic feldspar sandstone and feldspar lithic sandstone.The main reason of the complicated micro-pore structure and system is high content of unstable minerals.The physical properties of the reservoirs were linked closely to the composition of rock minerals.Brittle quartz was easily crashed to form micro-fissures and intergranular pores,thus affecting reservoir permeability（but not porosity）.Clay was soft and swellable with water.The content of clay had an obvious influence upon reservoir porosity and permeability,whose correlation is negative.Feldspar was unstable and easily to be dissolved to form kaolinite,which were trapped by detoured pore network and precipitated in-situ to block pores and throats.The content of feldspar also had a good negative relationship with reservoir permeability and porosity.Pore-throat structure of tight sandstone reservoirs may present itself as the following three scenarios based on curves plotted upon data from rate-controlled mercury penetration experiments from study area：large-pore system controlled by fat throats,pores of medium size controlled by medium-sized throats and tiny pores controlled by tiny throats.Compared with conventional reservoirs,the tight sandstone reservoirs had higher expulsion pressure and lower mercury-injection saturation,half as much throat radius,and obviously higher structural coefficient.The evaluation method proposed in the article may serve as a scientific ground for future tight oil and gas exploration in Turpan-Hami as well as in other basins in China.