Architecture of intraplate strike-slip fault zones in the Yanchang Formation, Southern Ordos Basin, China: Characterization and implications for their control on hydrocarbon enrichment

Abstract

Commercial discoveries in the Ordos, Tarim, and Sichuan Basins of China have aroused widespread interest in strike-slip fault-controlled reservoirs. The productivity of different wells, however, can vary within one strike-slip fault zone, suggesting that variability in fault zone architecture controls hydrocarbon enrichment. To date, very few studies have finely and quantitatively explored fault zone architecture in the southern Ordos Basin, inhibiting hydrocarbon development. With the goal of understanding the architecture variability and scale of strike-slip fault zones in the subsurface, we carried out a refined and quantitative study of faults in the Jinghe Oilfield in the southern Ordos Basin by integrating outcrops, wellbore cores, well logs, and 3D seismic data. We quantitatively constrained the subsurface damage zone boundaries according to the slope changes in the cumulative CFI (comprehensive fracture index log) curves at logging scale and the cumulative FSI (fault shape index seismic attribute) curves at seismic scale. We proposed a model highlighting the effects of fault segmentation, architectural configuration, and damage zone asymmetry on the variability of fault zone architecture. Results showed that strike-slip faults can be divided into transtensional, pure strike-slip, and transpressional segments along the fault strike, with transtensional and pure strike-slip segments dominant in the Jinghe Oilfield. These segments exhibited six types of geometric structures, forming complex architectures with multi-fault cores and multi-damage zones surrounding the main and secondary faults. Each segment was further complicated by three configurations of gouge, breccia, and damage zones along the fault dip. The damage zone asymmetry further increased the architecture variability, with the fracture density and damage zone width of the hanging wall being greater than those of the footwall. Quantitative analysis showed that the fault zone width was the greatest for transtensional segments, intermediate for transpressional segments, and the lowest for pure strike-slip segments. And there was a positive linear correlation between the fault core width and the fault zone width (including the fault core and damage zone). Fault segmentation and damage zone asymmetry affected the hydrocarbon enrichment along the fault strike and dip within one strike-slip fault zone. We conclude that damage zones in transtensional segments, particularly in hanging walls, are primary targets for hydrocarbon development in the southern Ordos Basin.