PROPAGATION RATE AND TIMING OF NATURAL FRACTURES IN DEEP RESERVOIRS

Knowledge of the opening and sealing history of natural fractures is essential for understanding the location of open subsurface conduits. Although cement deposits sealing fractures are amenable to analysis, natural fracture propagation rates are unknown. Recent fracture cement reconstructions tied to burial history through temperatures from fluid-inclusion assemblages show that some fractures in tight-gas sandstones widen over tens of millions of years. Here, we extend the textural and fluid-inclusion approach to show for the first time how natural fractures grow in height and length. For three partly open fractures in Cretaceous Travis Peak Formation, East Texas, fracture propagation history was obtained for local fracture-spanning quartz bridges by fluid inclusion microthermometry combined with cement deposit sequence obtained using high-resolution scanning electron microscopy-based cathodoluminescence (SEM-CL) imaging. We correlated opening history along the fracture length from center to tip. Fluid inclusions in one isolated fracture record increasing then decreasing temperature trends, and in two conjoining en échelon fractures an increasing temperature trend, interpreted to mark prograde burial and subsequent exhumation, respectively. The oldest fracture cements are at the fracture center, which for this burial history indicates 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 two en échelon fractures. Progressively shorter and younger opening histories were observed toward the fracture tip indicating that the fracture propagated over millions of years. Reconstruction of the fracture opening history suggests that the fractures propagated concurrently with aperture widening. Results are compatible with persistent, disseminated open fracture conduits having poor connectivity in deep reservoirs.