An application of outcrop analogues to understanding the origin and abundance of natural fractures in the Woodford Shale

Abstract

Subsurface natural fractures in shales are vital for fluid transport pre- and post-hydraulic fracturing in naturally fractured reservoirs such as the Woodford Shale. However, the dominant fracture sets that control the fluid flow in the subsurface are mostly unseen, and their intensity variation is unknown due to non-availability of image logs in most wells. In this paper, we have assessed the possibility of the existence of subsurface natural fractures in the Woodford Shale by understanding their generation history from outcrop and thin section studies. Additionally, we have addressed their relative abundance based on the bed thickness and composition.Out of the several fracture sets identified in the Woodford Shale, two joint sets (E-W and NE-SW) were interpreted as the oldest sets based on crosscutting and fill. These sets date back to before the Mid-Virgilian Arbuckle Orogeny and likely have different generation timings. The relatively silica- and carbonate-rich beds primarily contain the E-W fractures and the highly clay- and organic-rich beds mainly contain the NE-SW fractures. The E-W and NE-SW sets origins (both not related to local structural bending) are likely more numerous in the flat subsurface compared to fractures sets whose origins are related to structural bending. These two fracture sets probably also control the fluid flow in the subsurface. Younger fracture sets show more influence of local folding and are overrepresented in the outcrops with tilted beds. Therefore, they do not likely control relatively flat subsurface fluid flow. Although some fractures (fold or non-fold related) only have one type of cement or bitumen fill, others have two types of fills, i.e., bitumen along with another cement, or two types of cement (non-bitumen) indicating that these fractures underwent more than one stage of opening. Also, a negative correlation between fracture intensity and bed thickness, and a positive relationship between fracture intensity and quartz/carbonate content exist in the studied location.