Fracture sets, timing, and size distributions in the Cretaceous Frontier Formation, Greater Green River Basin, Wyoming
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Fractures influence permeability but sampling subsurface fractures is difficult in vertical wells. Horizontal cores are special cases allowing fracture abundance, distribution, and aperture size populations to be measured. Four horizontal cores (41.5 m) in Cretaceous Frontier Formation, eastern Greater Green River Basin, Wyoming, sample litharenites to sublitharenites (average 87.4% quartz, 2.1% feldspar, 10.6% lithics) deposited in upper and lower shoreface marine environments. Low porosity (3-10%) results from compaction and quartz, calcite, and kaolinite cement. Younger north-striking Set 2 fractures cross cut older east-striking Set 1 fractures, and both are likely regional fractures predating local folding. Both sets contain quartz, calcite, and kaolinite cement with local remnant porosity. Fluid inclusion assemblage temperatures were sequenced using quartz crack-seal cement textures compared to thermal history, and indicate Set 1 opened at 140-160°C during burial in Eocene time. Set 2 fractures opened at maximum burial, and continued to open during uplift from local basement-involved fold-fault. Subsequently some Set 1 fractures reactivated as faults. Fault-related kaolinite deposits locally occlude fracture porosity. Extensive SEM-CL micro-imaging demonstrates that transgranular microfracture populations are rare. Although only 48% of macrofracture aperture sizes could be measured accurately, aperture size ranges appear to be narrow, with apertures of 0.62-1.75 mm. Spacing ranges from 0.01 mm to meters with moderate clustering. Lower macrofracture abundance in the upper shore face (2.39 fractures/m) compared to the lower shore face (4.12 fractures/m) corresponds to lower subcritical crack index (SCI) and fracture toughness of the upper shoreface. Upper shoreface sandstones have lower average SCI (46) and fracture toughness (1.6 Mpa√m) than upper shoreface sandstones (54 and 2.2 Mpa√m, respectively). Presence of crack-seal quartz in both sets, together with extremely sparse microfractures, indicates thin, sparse, intermittently-bridging synkinematic quartz likely provided insufficient within-fracture bonding to partition deformation into microfracture populations during fracture development. High initial gas production in these wells correlate with observations of persistent fracture porosity in fractures through burial and uplift, locally-large apertures, and extremely long regional fracture lengths (>500 m) in Frontier Formation outcrops. Results suggest that for the Frontier Formation in this setting, productive fractures are not necessarily localized near folds and faults.