Architecture of intraplate strike-slip fault zones in theYanchang Formation, Southern Ordos Basin, China: Characterizationand implications for their control on hydrocarbon enrichment

Abstract

Previous hydrocarbon exploration in the Ordos, Tarim, and Sichuan basins of China has indicated that strike-slip faults play an important role in controlling reservoir distribution. High hydrocarbon production within strike-slip fault zones in these basins indicates that the fault zones not only act as conduits or seals for hydrocarbon migration, but also provide space for hydrocarbon accumulation. 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 explored fault zone architecture in the southern Ordos Basin, inhibiting oil exploration and development. We explored faults in the Jinghe Oilfield in the southern Ordos Basin by integrating outcrops, wellbore cores, well logs, and 3D seismic data. We carried out fault segmentation, qualitative characterization of fault zone architecture, and quantitative characterization of the boundary between the damage zone and wall rock. The results showed that fault zone architecture is complicated by fault segmentation, architectural configuration, and damage zone asymmetry. Strike-slip faults can be divided into transtensional, strike-slip, and transpressional segments along the fault strike, with transtensional and strike-slip segments dominant in the Jinghe Oilfield. Each segment is further complicated by different configurations of gouge, breccia, and fracture zones along the fault dip. Compared with the strike-slip segments, transtensional and transpressional segments showed more complexity, with the fracture density and damage zone width of the hanging wall being greater than that of the footwall. Transtensional segments with braided and horsetail structures showed more complexity owing to the presence of multiple fault cores and damage zones around the main fault and its subsidiary faults. Quantitative analysis showed that the fault zone width was the greatest for transtensional segments, intermediate for transpressional segments, and the lowest for strike-slip segments. We determined a positive linear relationship between the relative widths of the fault core and fault zone. The cavities in breccia zones and fractures in damage zones provide conduits and storage space for hydrocarbon migration and accumulation. We conclude that damage zones in transtensional segments, particularly in the hanging wall, are primary potential targets for petroleum exploration and development.