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Fracture Characteristics of the Wolfcamp Formation, Delaware Basin, TX


A recently collected suite of core and image logs provides the foundation for a new understanding of natural fractures within the Permian Wolfcamp Formation. Two main fracture types dominate, high-angle (vertical to sub-vertical) veins and bed-parallel (horizontal) veins. The high-angle veins have no clear facies association, while the horizontal veins are primarily found in argillaceous mudstones. A relatively insignificant number of small displacement faults (reverse and normal), vertical stylolites, bedding-plane slip surfaces, and septarian veins are also present. Most of the veins are not associated with faults, and thus represent a background set of regional fractures that are persistent to varying degrees across the basin. High-angle veins range in kinematic aperture from <0.004 – 0.3 in. (<0.1 – 8 mm), have apparent lengths up to 68 in. (1.73 m), are commonly coarse-calcite filled (with minor anhydrite, dolomite, euhedral quartz), and are closed to partially open with patchy cement. In contrast, horizontal veins are much thicker and have no porosity. They range in kinematic aperture from 0.02 – 0.79 in. (0.5 – 20 mm), are filled with antitaxial fibrous calcite, and an early phase of quartz and dolomite, and often contain dark medial lines, host rock inclusions, and echelon geometries. Volumetric fractions of high-angle veins range from 0.09 – 0.21%. Horizontal vein frequencies range from 0.10 – 0.17 veins/foot with vertical extensional strains ranging from 0.12 – 0.14%. P32 densities were also calculated for high-angle veins in each core which provide a metric for fracture abundance in DFN modeling. Measured values range from 0.83 – 3.19 m-1, with a mean of 2.01 m-1. Image log observations show a primary population of high-angle veins oriented ~290-300°, along with a secondary population oriented ~045-060°. Homogenization temperatures of primary fluid inclusions trapped in the veins, combined with 1-D burial and thermal history models, suggest that the pervasive ESE striking set formed during maximum burial in the latest Cretaceous/early Paleogene, which happens to coincide with peak oil generation. In contrast, the lack of primary hydrocarbon filled fluid inclusions and low homogenization temperatures found in horizontal veins suggest they formed during rapid burial in the Permian, and are consistent with cross-cutting relations observed in core.