CHARACTERIZING THE NATURAL FRACTURE SYSTEM OF THE EAGLE FORD FORMATION

The Eagle Ford Formation has attracted considerable industry attention as a self-sourced unconventional shale reservoir. The outcrops present along Highway 90 in Val Verde and Terrell counties, where it is locally called the Boquillas formation, provide considerable insight into the natural fracture system. The middle member, or producing interval, contains significant parasequences whose lithologic content varies upward from the base of the middle member to the base of the upper member. The upper member contains increasing proportions of limestones and represent a return to more a more oxygenated depositional environment.

This study approach is threefold. First, extension fracture orientation, frequency, and spacing, in relation to lithology variations, are used to determine paleo- and contemporary stress acting on the formation. Second, mineral-filled fractures are characterized, in order to determine their origin, spatial distribution, and age relationship with the regional extension fracture sets. Lastly, fracture spacing, and frequency are studied to determine the number and nature of fracturing units present. Comparing the results of this analysis with established stratigraphy result in a general fracture stratigraphic framework of the middle and upper members of the Eagle Ford Formation.

The paleo and contemporary stress regimes acting on the formation fall within the normal faulting regime category. Three fracturing episodes occurred, resulting in conjugate hybrid shear fracture sets and regional Mode I fractures. Spacing measurements obtained in competent beds were analyzed to determine maturity of the fracture system. Analysis showed that limestone beds of the middle member parasequences were more mature than marl beds.

The fracture stratigraphy of the Eagle Ford is divided into upper and lower fracturing units. Conjugate sets of mineral filled fractures present in the upper member indicate a different fracture stratigraphy from the middle member. The regional sets present throughout the middle and upper members show an increased frequency in the upper member, indicating that Mode I fracturing occurred with or closely following the shear fracturing.