The spatial characterization of stepovers along deeply-buried strike-slip faults and their influence on reservoir distribution in the central Tarim Basin, NW China

Abstract

Strike-slip faults imaged on seismic reflection data from the central Tarim Basin clearly illustrate the spatial and geometrical characteristics of deeply buried (>6.8 km depth) stepovers and their influence on reservoir distributions in fractured Middle Ordovician carbonates. Characterization of along- and across-strike seismic profiles demonstrates that restraining and releasing stepovers characteristically produce pop-up and pull-apart structures, respectively, defined by systems of high-angle subsidiary faults with down-dip reverse or normal displacements. Along-strike vertical displacement and damage zone width measurements reveal that restraining stepovers typically exhibit much wider deformation zones and higher densities of fractures compared to releasing stepovers. Core samples and well logs show that the fractured reservoirs in restraining stepovers comprise stylolite meshes and mosaic breccias. Damage zones in narrower releasing stepovers are more prone to the development of cavities, dilational breccias, and high-angle open fractures. Both restraining and releasing stepovers are strongly asymmetric in the across-strike direction, except in the central part of fault overlap zones where the maximum vertical displacements are symmetrical. Restraining stepovers can typically be subdivided into four sectors separated by helicoidal intermediate-scale linking faults: two in the hanging walls and two in the footwalls. Hanging walls are normally wider, intensely fractured and exhibit better well performance compared to footwalls. Strain compartmentalization with localized zones of upward and downward displacement exist both within, and adjacent to releasing stepovers. The localized extensional zones are where cavities and open fractures are most likely best developed. Seismic reflection profiles and well logging show that certain intraclastic grainstone and packstone horizons in the Middle Ordovician carbonate host rock sequences are more favorable for fracture development. This leads to vertical partitioning of local fractured reservoirs due to the development of down-dip stepovers or bends localized close to major lithological boundaries. Thus interactions between lithological layering and tectonic processes cause further three-dimensional complexity in the reservoirs.