Characteristics and formation process of contractional deformation bands in oil-bearing sandstones in the hinge of a fold: A case study of the Youshashan anticline, western Qaidam Basin, China

Abstract

The deformation bands in oil-bearing sandstones in a thrust fault-related fold were studied. The outcrop is exposed in the hinge of the Youshashan anticline. The deformation bands are compactional shear bands (CSBs), which are characterized by low net shear displacement (S) and compaction (C) ratios and by conjugate networks. The kinematics of the deformation bands are the result of the combined effects of shear and compaction, in which shear played a dominant role and was accompanied by additional compaction. CSBs are identified in two sets, including D1 CSBs that were cut by D2 CSBs. The CSBs formed under a contractional regime due to regional shortening and are oriented approximately in the NE-SW direction. The time of formation of the D1 CSBs (NNE-SSW-striking and E-W-striking) is consistent with or slightly later than the formation of the Youshashan anticline. At the beginning of the anticline formation, the D1 CSBs formed under a strike-slip regime due to the deeper burial depths (approximately 2.5 km). With the evolution of the anticline, the burial depths gradually become shallower. The D2 CSBs formed a NW-SE-striking direction under a thrust-fault regime. The porosities of the contractional D1 and D2 CSBs that developed in oil-bearing sandstones are 14–19.7% and 11–18% lower than those of the host rock, respectively. The permeability reduction for the D1 and D2 CSBs are between 1 and 2 orders of magnitude. Our findings suggest that the stress state changed during the formation uplift in the contractional regime, from a strike-slip stress state to a thrust-fault stress state, forming CSBs with low S/C ratios. The CSBs have low porosity and small pore space for hydrocarbon storage, and thus, the oil-bearing properties of the CSBs are less favorable than those of the host rock. Nevertheless, the oil saturations in the CSBs are 15.7–47.9% higher than those in the unformed oil-bearing sandstones.