AAPG Southwest Section

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Multiscale Natural fracture dimensions in the Woodford Shale


Understanding the natural fracture dimensions in ultra-low permeability shales is important because these parameters are related to the reservoir fluid flow capacity. There is a dearth of literature on the Woodford Shale fracture (primarily joints) dimensions, which include fracture apertures, heights, and lengths. Measurements from five outcrops in South-Central Oklahoma and derived thin sections were used to find the best-fit distributions and relationships between fracture apertures, heights, and lengths from outcrop to microscale.

Macrofracture (opening displacements > .05 mm) and microfracture (opening displacements < .05 mm) kinematic apertures were measured along scanlines oriented sub-perpendicular to the fracture traces on outcrops and under microscopes using thin sections, respectively. For large macrofractures (height > 1 m), height traces were measured on cliff faces. The cumulative frequency of the apertures and heights were plotted (separately) to find their best-fit distributions. Aperture vs. length measured along scanlines for some fractures were also plotted. In addition, macrofracture and microfracture spacings were recorded along scanlines.

Measured microfracture apertures range 0.001-0.05 mm. They primarily exhibit exponential, followed by lognormal distributions. Crack-seal textures are not evident in thin sections. Small macrofractures, i.e., the bed-bounded ones and those that transect a couple of beds (usually 1-10 cm in height) have apertures in the range of .05-1 mm and exhibit characteristic (exponential and lognormal) aperture distributions. Fracture aperture (mm) vs. length (m) plots exhibit power law (exponent range: 0.53-0.59) relationships. Microfractures show higher clustering (σspacing ÷ µspacing > 1) compared to the small macrofractures (σspacing ÷ µspacing < 1).

On the other hand, large macrofractures (i.e., height > 1 m), mostly striking in the E-W and NE-SW directions exhibit a power law and an exponential aperture distributions respectively. The apertures range 0.27-3.3 mm for the E-W and 0.27-10 mm for the NE-SW fractures. Both sets exhibit lognormal height distributions with maximum observed heights of 5 m (E-W set) and 1.8 m (NE-SW set). E-W set measured lengths range ~ 5-28 m and NE-SW set calculated lengths range ~ 5-10 m. Fracture length (m) vs. height (m) plots exhibit both a power law (exponent ~ 0.85) and a linear relationship (slope ~ 5.1) with similar R2 values (~ 0.75). Large macrofractures are likely more efficient fluid carriers compared to the microfractures and smaller macrofractures due to their wider opening displacements and transection of a larger number of beds.