Opening-mode fracturing and cementation during hydrocarbon generation in shale: An example from the Barnett Shale, Delaware Basin, West Texas

Relative timing of fracturing is a key input for predictive fracture models, but timing information for fractures is commonly diffi-cult to obtain. In this study, we used crosscutting relations and fluid inclusion assemblage temperatures from fracture cements from a few well-documented sampled fractures, combined with a one-dimensional burial history model, to establish timing for three generations of opening-mode fractures in a Barnett Shale core from the southern part of the Delaware Basin, Pecos County, West Texas. A burial history model is presented for the cored well and matched to measured vitrinite reflectance in sam-ples from the core, and bottomhole temperature in the well. The earliest fractures (group 1) likely formed due to early fluid-expulsion events (ca. 300 Ma) and were folded during host-rock compaction. Later group 2 fractures are sealed with fibrous barite containing primary, liquid hydrocarbon inclusions (mean homogenization temperature [Th] =-9 degrees C) and aqueous fluid inclusions (mean Th = 108.1 degrees C). Group 2 fractures likely formed in response to fluid overpressure associated with crack-ing of type II kerogen to oil. Group 3 vertical fractures are up to 2 m in height with kinematic apertures ranging from less than 0.05 to 1.4 mm, partly open, and strike dominantly 010 degrees-020 degrees. Sequentially trapped aqueous fluid inclusions in fracture-spanning quartz cement bridges (mean Th = 110 degrees C in crack-seal texture and 128 degrees C in post-crack-seal fracture cement) record fracture opening under increasing temperature, inferred to reflect increas-ing burial, with continued overpressuring during the Triassic to Late Cretaceous. Some group 3 fractures may have continued to fill during Cenozoic uplift.

Automated summary

In this study, the timing of three generations of fractures in Barnett Shale was determined using crosscutting relations, fluid inclusion assemblage temperatures, and a burial history model. The results provide important insights into the formation and evolution of fractures.