INTEGRATED DIAGENETIC STUDY OF THE MISSISSIPPIAN BARNETT SHALE, FORT WORTH BASIN, TEXAS

An integrated diagenetic study using paleomagnetic, geochemical, and petrographic data from cores was conducted to better understand the nature and timing of diagenetic events in the Mississippian Barnett Shale, a source rock and the major unconventional gas reservoir in the Fort Worth Basin, Texas. Paleomagnetic analysis indicates the unit contains chemical remanent magnetizations (CRMs) with shallow inclinations and streaked southeasterly to southerly declinations. The streak could represent a mixing trend between two or more CRMs or discrete CRMs with overlapping data distributions. For example, some specimens contain a Pennsylvanian CRM that is interpreted to have formed from burial diagenetic processes. Specimens around vertical, northeasterly fractures contain a late Permian CRM that is interpreted to be related to fluid alteration.

Calcite, barite, celestine, albite, anhydrite, and cubic pyrite fill veins with the shale. Samples from the core which is closest to the Ouachita thrust zone and from some veins contain elevated ⁸⁷Sr/⁸⁶Sr values which is consistent with the interpretation that externally derived fluids altered the shale. This result should be treated with caution because some ⁸⁷Sr released from feldspars during the formation of authigenic illite could partly explain the elevated ⁸⁷Sr/⁸⁶Sr values. Most δ³⁴S values (e.g. -7.12‰) for vein sulfate in the Barnett are depleted relative to the Phanerozoic seawater sulfate field and suggest alteration by externally derived fluids. The depleted sulfates probably represent a mixture between ³⁴S-enriched (externally-derived evaporite sulfate and pore water sulfate) and ³⁴S-depleted (sulfide related to microbial sulfate reduction) components. A substantial fraction of sulfur in some vein sulfates would need to be derived from oxidized pyrite, which is a strong indication that the Barnett was altered by an externally-derived, oxygenated fluid. These results are consistent with previous studies which suggested that orogenic fluids entered the Barnett and caused localized alteration along the Ouachita thrust zone and adjacent to faults. This fluid alteration event is interpreted to have occurred in the late Permian.