鄂尔多斯盆地上三叠统延长组特低渗透砂岩储层裂缝特征及成因

Characteristics and Origin of Fractures in the Extra Low-Permeability Sandstone Reservoirs of the Upper Triassic Yanchang Formation in the Ordos Basin

根据地表露头、岩心、薄片和实验等资料,对鄂尔多斯盆地上三叠统延长组裂缝的分布特征及其成因进行了分析。鄂尔多斯盆地延长组为一西倾的平缓单斜构造,其上发育两组正交型式的构造裂缝系统,在盆地中部的靖安—安塞地区主要为东西向和南北向两组正向正交裂缝,而在盆地西南部的陇东地区主要为北西—南东向和北东—南西向两组斜向正交裂缝,它们主要在侏罗纪末期和白垩纪末期—古近纪形成。侏罗纪末期,在北西西—南东东方向水平构造挤压为主的应力作用下,可以形成东西向和北西—南东向共轭剪切裂缝;白垩纪末期—古近纪,在北北东—南南西方向水平挤压应力场作用下,可以形成南北向和北东—南西向共轭剪切裂缝。但由于受沉积和成岩作用造成的岩层强烈非均质性的影响,同一时期的两组共轭剪切裂缝的发育程度在不同沉积体系不一致,其中一组裂缝受到抑制,发育程度较差。因此,在每个构造时期主要发育有一组单剪裂缝,在两期构造作用下,主要表现为两组正交的裂缝系统。

According to outcrops, cores, slices and experiment data, the distribution and origin of fractures in the extra low-permeability sandstone reservoir of the Upper Triassic Yangchang Formation in the Ordos Basin are analyzed. The Yanchang Formation in the Ordos Basin is a gently monocline structure inclining to the west, where developed two sets of systemically orthogonal tectonic fractures. In the Jingan-Ansai area of the central Ordos Basin, there mainly are E-W and S-N orthogonal fractures, and NE-SW and NW-SE orthogonal fractures in the Longdong area in the southwestern Ordos Basin. Fractures formed at the end of the Jurassic and the end of Cretaceous-Early Tertiary. At the end of the Jurassic, the E-W and NW-SE conjugate shear fractures would be formed by the horizontal compression stress of WNW-ESE orientation. At the end of Cretaceous-Early Tertiary, the S-N and NE-SW conjugate shear fractures would be formed by the horizontal compression stress of NNE-SSW orientation. But by the influences of the strong terrane anisotropism made by sedimentary and diagenesis, the development of two sets of conjugate shear fractures at the same time was different in the dissimilar sedimentary system, and one set of conjugate shear fractures would develop well and the other was restrained. Therefore, the conjugate shear fractures generally showed one set in every tectonic period, and the fractures formed in two tectonic periods are two sets of orthogonal fractures.