库车前陆冲断带多尺度裂缝成因及其储集意义

Genesis and reservoir significance of multi-scale natural fractures in Kuqa foreland thrust belt,Tarim Basin,NW China

以库车前陆冲断带深层白垩系巴什基奇克组砂岩储集层为例,利用地表露头、岩心、薄片和成像测井等资料,运用工业CT、激光共聚焦、阴极发光、电子探针、扫描电镜等技术手段,基于裂缝开度大小对裂缝进行分级分类,系统研究库车前陆冲断带砂岩储集层天然裂缝类型、特征、成因、期次及形成序列等,并分析不同尺度天然裂缝的储集意义。库车前陆冲断带深层致密砂岩储集层天然裂缝可分为4级:百微米级以上宏观构造裂缝(Ⅰ级)切割单砂体,形成优势运移通道,提高储集层渗透率;十微米级—百微米级微细构造伴生缝(Ⅱ级)切割基质颗粒,连通基质大孔隙,改善渗流性能;微米级粒缘显微成岩缝(Ⅲ级)连通中小孔隙,改善孔隙连通网络,提高天然气运移充注效率;纳米级基质裂隙(Ⅳ级)沟通粒内微孔隙,扩大储集空间,增加储量规模。Ⅰ级与Ⅱ级构造成因裂缝主要发育有3期,其中早期和中期裂缝主要为半充填—充填,是油气大规模充注前的上新世早期以前形成,晚期开启裂缝与油气大规模充注同期或略晚,形成于上新世末期之后。裂缝网络对孔隙度贡献率较低,但在平行裂缝走向上,裂缝可以提高渗透率2~3个数量级。

Natural fractures in the deep Cretaceous Bashijiqike Formation sandstone reservoirs of Kuqa foreland thrust belt, NW China, are classified according to fracture aperture based on the data of outcrops, cores, thin sections and imaging logging, using industrial CT scanning, laser scanning confocal microscope （LSCM）, cathodoluminescence （CL）, electron probe, and scanning electron microscopy （SEM）. The types, characteristics, genesis ages and formation sequence as well as reservoir significance of such natural fractures are examined. Four categories of fractures are classified. Category I （aperture〉100 ~tm） is macro structural fractures, which cut single sand body to form the dominant migration pathway, helping to increase the reservoir permeability. Category II （aperture=10-100 μm） is associated micro structural fractures, which cut matrix grains to connect large matrix pores and improve the seepage performance. Category III （aperture=1-10μm） is micro digenetic fractures at grain edges, which connect medium and small pores to improve the pore network connectivity and the gas migration and charging efficiency. Category IV （aperture〈1 μm） is nano-scale matrix fissures, which connect intragranular micro pores to expand the reservoir space, thereby increasing the reserves scale. Category I and Category II fractures were developed in three stages （early, middle and late）. The early- and middle-stage fractures, predominantly half-filled-filled fractures, were formed before early Pliocene when extensive oil and gas charging had not occurred. The late-stage opened fractures were formed after the Late Pliocene, they were at the same time as or later slightly than extensive oil and gas charge. The fracture network has low contribution to porosity, but it can improve the permeability by 2-3 orders of magnitudes in the parallel direction of fractures.