Fault characteristics and their control on oil and gas accumulation in the southwestern Ordos Basin

Abstract

3D seismic data recently acquired from the Ordos Basin shows three sets of regularly distributed fault systems, which overrides previous understanding that no faults were developed in this basin. Seismic interpretation suggests that the faults in the southwestern Ordos Basin have three basic characteristics, namely extreme micro-scale, distinct vertical stratification, and regularity of planar distribution. These NS-, NW-, and NE-trending fault systems developed in the Meso-Neoproterozoic – Lower Ordovician strata. Of these, the NS-trending fault system mainly consists of consequent and antithetic faults which show clear syndepositional deformation. The fault systems in the Carboniferous – Middle-Lower Triassic strata are not clear on seismic reflection profiles. The NW- and NE-trending fault systems are developed in the Upper Triassic – Middle Jurassic strata. Of these, the NW-trending fault system appears as a negative flower structure in sectional view and in an en echelon pattern in plan-view; they show transtensional deformation. A NE-trending fault system that developed in the Lower Cretaceous – Cenozoic strata shows a Y-shaped structural style and tension-shear properties. A comprehensive analysis of the regional stress fields at different geologic times is essential to determine the development, distribution direction, and intensity of the activity of fault systems in the Ordos Basin. Current exploration suggests three aspects in which the faults within the Ordos Basin are crucial to oil and gas accumulation. Firstly, these faults serve as vertical barriers that cause the formation of two sets of relatively independent petroleum systems in the Paleozoic and Mesozoic strata respectively; this is the basis for the ‘upper oil and lower gas’ distribution pattern. Secondly, the vertical communication of these faults is favorable for oil and gas migration, thus contributing to the typical characteristics of multiple oil and gas fields within the basin, i.e. oil and gas reservoirs with multiple superimposed strata. Finally, these faults and their associated fractures improve the permeability of Mesozoic tight reservoirs, providing favorable conditions for oil enrichment in areas around the fault systems.