A CHRONICLE OF NATURAL FRACTURE PROPAGATION USING FLUID INCLUSIONS

Growth rates of natural partially cemented fractures under deep burial conditions have been enigmatic. Based on the comparison of fracture surface markings with experimental fracture growth it is commonly inferred that barren joints grow by rapid fracture propagation. Recent reconstructions of partially cemented natural fractures from tight-gas sandstones suggested that fractures widen in aperture over tens of millions of years. The duration of fracture opening was obtained by reconstructing the fracture opening history through textural mapping of quartz fracture cement bridges and by detailed microthermometry of fluid inclusions trapped in these cement bridges. These bridges formed by repeated fracture opening and cementation (crack-seal), trapping fluid inclusions during fracture opening and thus recording the thermal history during fracture opening. The timing and rate of fracture opening can then be reconstructed by relating fluid inclusion temperatures to the known burial history of the formation.

Here, we apply the same technique to test if fractures propagate over similar time intervals. For a single fracture in the Travis Peak Formation, East Texas, we sampled multiple crack-seal cement bridges from the fracture center to the fracture tip. The fracture propagation history was obtained for each cement bridge by detailed fluid inclusion microthermometry combined with high-resolution scanning electron microscopy-based cathodoluminescence (SEM-CL) imaging, then correlating the opening history of each bridge along the fracture length from center to tip. Fluid inclusions record both decreasing and increasing temperature trends, corresponding to prograde burial and subsequent exhumation, respectively. We found that the oldest fracture cement is at the fracture center, indicating start of fracture growth at ~48 Ma, with continued fracture opening until close to the present day. Progressively shorter and younger opening histories were observed toward the fracture tip indicating that the fracture propagated over 48 m.y. Reconstruction of the fracture opening history thus suggests that the fracture propagated concurrently with aperture widening over prolonged geologic time periods.