致密砂岩储层构造裂缝特征及地质建模——以塔里木盆地库车坳陷大北12气藏为例

Characteristics and geological modeling of structural fractures in tight sandstone reservoir:taking Dabei-12 gas reservoir in Kuqa Depression,Tarim Basin as an example

以塔里木盆地库车坳陷大北12气藏为例,基于岩心、薄片、成像测井等资料表征了构造裂缝的类型与产状,分析了构造裂缝的影响因素及形成期次,分别采用随机和确定性建模方法建立了小尺度和大尺度裂缝模型,二者叠加得到总裂缝模型。结果表明:①在燕山运动和喜马拉雅运动近NS向的构造挤压背景下形成的大北12背斜型气藏,其目的层白垩系巴什基奇克组致密砂岩的宏观裂缝以近NS向的直立和高角度裂缝为主,剪切裂缝和张性裂缝均有发育;微观构造裂缝也可沟通基质孔隙,提高储层的连通性。②在岩性相同的前提下,构造裂缝密度随单层厚度增大而减小,且减小幅度也随之降低;单层平均厚度相近时,构造裂缝密度与距断层距离呈良好的负相关;细砂岩中的裂缝密度最高。③大北12气藏发育3期构造裂缝,其中形成于喜马拉雅中期和晚期的第2、3期有效裂缝是天然气高效富集成藏的关键因素。④背斜型油气藏核部的构造裂缝密度低,但张开度、渗透率高,通常具有较高的单井产能;翼部的构造裂缝密度高,但张开度和渗透率低,可采用压裂措施提高产能。该成果可为裂缝属性等效模型建立及气藏数值模拟提供数据基础,服务于气藏开发。

The characterization and geological modeling are of great significance to exploration and development of fractured tight sandstone gas reservoir.Taking the Dabei-12 gas reservoir in Kuqa Depression of Tarim Basin as an example,the types and occurrence,influence factors and formation sequence of structural fractures were characterized based on core,rock thin section,imaging logging and other data.Then,the small scale and large scale fracture model of the Dabei-12 gas reservoir were established by stochastic and deterministic fracture modeling method respectively,and a total fracture model was established by superimposition of the two models.The results show that:(1)The Dabei-12 gas reservoir,an anticlinal gas reservoir which formed under the background of NS tectonic compression due to the Yanshanian movement and Himalayan movement,is located in the Bozi-Dabei area of Kelasu structural belt,Kuqa Depression.The target layer of the Dabei-12 gas reservoir is the tight sandstone of Lower Cretaceous Bashijiqike Formation.The macroscopic fractures in the Dabei-12 gas reservoir are mainly vertical or with high dip angle,and shearing and tensile fractures are both developed to some extent.The microscopic fractures can also connect matrix pores and therefore enhance the connectivity of reservoir.(2)Under the geological background of Dabei-12 gas reservoir,the development extent of structural fractures is related to average thickness of single layer in condition of same lithology,and distance to fault and lithology in condition of same or similar thickness of single layer.Taking fine sandstone in the Dabei-12 gas reservoir as an example,the density of structural fractures in fine sandstone decreases with the increase of thickness of single layer,and so does the decreasing amplitude.In condition of similar thickness of single layer,the density of structural fractures shows good negative correlation with the distance to faults.As to different lithology in the same condition,the fine sandstone has the highest density of structural fractures,the coarse sandstone and medium sandstone take the second place,while siltstone and finer lithology have low density.(3)There are 3 periods of structural fractures in the Dabei-12 gas reservoir,which are the Late Yanshanian period(Late Cretaceous-Paleogene),the Middle Himalayan period(Neogene Kangcun period),and the Late Himalayan period(Neogene Kuqa period-Quaternary)respectively.The second and third period fractures formed at the Middle and Late Himalayan periods are the critical factor of efficient accumulation of natural gas.(4)The hinge of an anticlinal reservoir has structural fractures with low density but high aperture and permeability,and therefore has always high production.The wings of a faulted-anticlinal reservoir have structural fractures with high density but low aperture and permeability,and fracturing measures should be taken to improve production.This study can provide data foundation for the establishment of fracture property equivalent model and gas reservoir numerical simulation,and then serve for the development of gas reservoir.