Fracture attributes in reservoir-scale carbonate fault damage zones and implications for damage zone width and growth in the deep subsurface

Abstract

The width of reservoir-scale fault damage zone governs fluid flow of the tight reservoirs in subsurface petroliferous basins. Together with cores, thin sections, geophysical well logs, and production data from more than 60 wells, we describe fracture attributes and fault damage zones associated with a conjugate strike-slip fault system in tight Ordovician carbonate rock at depths of between 6500 m and 7500 m in the Tarim intracratonic basin in NW China. Fracture frequency, aperture, and oil production in wells reveals a power-law relationship with the distance from fault cores, with a variation in two orders of magnitude. The cumulative data of fracture frequency, aperture and oil production show multiple gradients in slope with the distance. Except for sampling bias, the varied gradients are possibly related to the overlaying multiple fault zones or multiple sections along a large fault zone, which are useful to constrain damage zone width. Together with fracture frequency, cumulative fracture frequency, aperture and oil production, four major damage zone boundaries are identified at ∼200, ∼420 m, ∼780 m and ∼1100 m. Therefore, a wide fault damage zone with a width of more than 2 km in the carbonate rocks could be inferred, which mostly depends on the interaction and superposition of secondary faults. In addition, there is a relatively high fracture frequency decay exponent (>1) with distance, which is possibly related to fault size, fault splays, background fracture frequency and the lithology. In turn, the growth of wide damage zone also resulted in a large scattered distribution of the fracture attributes with the distance from fault cores. Attributes of fractures of various scales from the deep subsurface provide powerful tools for assessing damage zone width and growth.