塔里木盆地塔河地区中-下奥陶统碳酸盐岩储层天然裂缝发育特征及主控因素

Characteristics and main controlling factors of natural fractures in the Lower-to-Middle Ordovician carbonate reservoirs in Tahe area,Northern Tarim Basin

利用野外露头、岩心、测井和铸体薄片资料,对塔里木盆地塔河地区中-下奥陶统碳酸盐岩储层天然裂缝发育特征及其影响因素进行研究。塔河地区中-下奥陶统碳酸盐岩储层天然裂缝主要分为构造裂缝、成岩裂缝和复合成因裂缝3种类型。其中,构造裂缝以剪切裂缝为主,其次为张裂缝;成岩裂缝主要为水平层理缝,其次是成岩缝合线;复合成因裂缝主要包括构造-成岩裂缝、构造-表生裂缝和岩溶裂缝。构造剪切裂缝是该区的主要裂缝类型,发育有北东-南西向、北西-南东向、近东-西向和近南-北向4组,它们在不同层位的发育程度存在明显的差异性;其中,北东-南西向裂缝为层位的优势裂缝发育方向。裂缝的倾角主要在70°以上,以高角度裂缝为主。裂缝的纵向延伸长度受岩石力学层控制,裂缝在岩石力学层内发育,并终止于力学层界面上。裂缝密度在纵向上和平面上存在明显的非均质性,主要受沉积作用、构造作用和岩溶作用等地质因素控制。随着塑性矿物含量与岩石层厚的增加,裂缝密度呈逐渐降低的变化规律。断层与褶皱构造对裂缝的发育程度有重要影响,使得不同构造部位的裂缝密度存在明显的差异;距断层面与褶皱轴面的距离越远,裂缝密度也逐渐降低。岩溶作用影响溶洞上部岩层中的裂缝发育程度;在溶洞上部岩层中,主要发育近直立裂缝、斜交裂缝和近水平裂缝3种产状类型的裂缝系统。

The characteristics of and major factors controlling the natural fracture development in the Middle-to-Lower Ordovician carbonate reservoirs in Tahe area are studied through observations of outcrops,cores and casting thin sections,as well as logging data analysis.The natural fractures in the study area come in three types:tectonic fractures,diagenetic fractures and composite fractures.Specifically,the tectonic fractures are mainly shearing fractures,followed by tensional fractures;the diagenetic fractures are mainly bedding-parallel fractures,followed by diagenetic stylolites;the composite fractures mainly include structural-diagenetic fractures,structural-epigenetic fractures and karst fractures.The tectonic shearing fractures are dominant in the study area,developed in four groups trending NE-SW,NW-SE,nearly EW and nearly NS,respectively.However,they differ remarkably in different horizons in terms of development degree,and occur preferentially along the NE-SW direction in these horizons.High-angle fractures are dominant with a dip angle of over 70°.The longitudinal propagation length of the fractures is controlled by the rock mechanical stratigraphy,within which the fractures are developed,and on the interface of which the fractures terminate.Fracture density shows obvious heterogeneity both vertically and horizontally,mainly controlled by geological factors including sedimentation,tectonics and karstification.Along with the increase of plastic mineral contents and rock layer thickness,the fracture density gradually decreases.Faults and folds play a vital role in fracture development,leading to evident variation in fracture density at various structural locations.The fracture density tends to gradually decrease along with the increasing of distance to fault plane and fold axial plane.Besides,karstification affects the degree of fracture development in the upper strata of karst cavens,where there are mainly three types of fracture systems,i.e.nearly vertical fractures,obliquely-crossing fractures and nearly horizontal fractures.