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Fracture Growth Processes in Sandstone Inferred by Textural and Fluid Inclusion Investigations of Crack-Seal Fracture Cements


Opening-mode fractures are widespread in sedimentary rocks even in slightly deformed and flat-lying sequences. Opening-mode fractures in sandstone are often partially or completely cemented in deep reservoirs (>1 km depth). Such fractures in sandstones develop by progressive opening over a long period of time. The history of fracture opening can be recorded in crack-seal textures within quartz cement bridges, where each crack-seal increment represents a stage of fracture opening. We can distinguish several mechanisms by which opening-mode fractures can form. Fractures may widen in aperture at the same time as they lengthen by tip propagation. Alternatively, fractures may propagate to a certain length followed by an increase in aperture without further propagation. This second process can be envisioned for fractures with high ratios of aperture to length, common in deep gas sandstone reservoirs. Discerning the mechanism of fracture growth is essential to modeling and predicting fracture network geometry and its effects on hydrocarbon migration and production. Crack-seal cement textures were imaged using a scanning electron microscope with a cathodoluminescence detector. We examined three partially cemented Travis Peak fractures in core of the SFOT-1 well, located in the East Texas Basin. Fracture crack-seal increments decrease both in number and thickness toward the tips. Such trends are observed for both fracture length and height dimensions. The timing and rate of fracture opening can be reconstructed by relating the temperatures obtained from fluid inclusions trapped in the crack-seal cements to the known burial history. Fluid inclusions record both decreasing and increasing temperature trends in one isolated fracture, and increasing temperature trends in two conjoining en échelon fractures, corresponding to prograde burial and subsequent exhumation, respectively. We found that the oldest fracture cements are at the fracture center, indicating start of fracture growth at ∼48 Ma, with continued fracture opening until close to the present day in the isolated fracture, and to ∼36 Ma in the en échelon fractures. Progressively shorter and younger opening histories were observed toward the fracture tip indicating that the fracture propagated over long periods of time. Reconstruction of the fracture opening history thus suggests that the fractures propagated concurrently with aperture widening over prolonged geologic time periods.