Fracture patterns and their origin in the Upper Devonian Antrim Shale gas reservoir of the Michigan basin: A review

Robert T. Ryder

Open-File Report 96-23

MINERALOGY OF FRACTURED INTERVALS

X-ray diffraction analysis indicates that the major mineral constituents of the Antrim and Ellsworth Shales are quartz, carbonate (calcite and dolomite), muscovite, and illite; secondary mineral constituents are pyrite, kerogen, feldspar (potassium and plagioclase varieties), kaolinite, and chlorite (Dellapenna, 1991; Manger and Curtis, 1991; Dellapenna and Harrison, 1993). Of the strata that constitute the Antrim Shale and the Ellsworth Shale, the black shale members of the Antrim Shale have the highest quartz content (40-60%) and the lowest carbonate content (0-5%). In contrast, the gray shale units in the Antrim and Ellsworth Shales have less quartz (30-40%) and significantly more carbonate minerals (15-30%) than the black shale units. Total organic carbon content in weight percent (TOC) ranges from about 5 to 14 in the black shale units and generally is less than 1 in the gray shale units. Dellapenna (1991), Manger and Curtis (1991), and Dellapenna and Harrison (1993) suggest that the quartz-rich black shale units are brittle and, thus, should be more heavily fractured than the ductile calcite-rich gray shale units. A similar conclusion was reached by Nelson (1985) but, in addition to quartz, he considers feldspar and dolomite to be brittle constituents of the black shale units.

The most variable mineral constituents in the Antrim Shale samples collected at the Paxton quarry are quartz, clay, carbonate, and organic matter (Richards, Walter, and others, 1994). Both frequency and aperature width of fractures show a direct relationship to TOC and carbonate content and an inverse relationship to clay content. These relationships apply to both the northeast-southwest and northwest-southeast fracture sets. Judging from the lower fracture frequency (higher fracture spacing) of the northwest-southeast fracture set and the direct relationship between TOC and mean aperture width, Richards, Walter, and others (1994) suggest that the northwest-southeast fracture set has been generated by a natural hydraulic mechanism during peak thermal maturation of Antrim Shale organic matter.

Stable carbon and oxygen isotopic compositions of concretions, calcite fracture fillings, and host rock were measured in outcrop and core samples by Richards, Walter, and others (1994). Compositions are tightly clustered around mean values of d13C ~-10 and d18O ~-10 for concretions and mean values of d13C ~-5 and d18O ~-5 for host rocks. In contrast, isotopic compositions are widely scattered for the fracture fillings: d13C values range from about +26 to -42 and d18O values range from about -3 to -10. Richards, Walter, and others (1994) suggest that the calcite cement was precipitated from late Pleistocene glacial water that percolated through the fractures.

Created by the EERT WWW Staff.