An integrated paleomagnetic and diagenetic investigation of the Barnett shale and underlying Ellenburger Group carbonates, Fort Worth Basin, Texas

The Ordovician Ellenburger Group carbonates are extensively karsted and brecciated throughout portions of the western half of the Fort Worth Basin, Texas, where it underlies the Mississippian Barnett Shale gas reservoir and source rock. An integrated geochemical/petrographic, paleomagnetic and rock magnetic study was conducted to better understand the nature and timing of diagenetic events in the unit. Samples from three scribe oriented conventional drill cores of the uppermost Ellenburger Group carbonates were analyzed for their diagenetic and paleomagnetic properties. Thermal demagnetization of samples from both units reveals a low-temperature steeply downward viscous remanent magnetization (VRM) as well as several components that are removed at higher temperatures (200-540°C). The higher temperature components reside in magnetite and are interpreted as chemical remanent magnetizations (CRMs) based on low burial temperatures. The specimen directions are streaked from an easterly and shallow direction to a southerly and shallow direction.The modern VRM was used to orient the CRM data for one of the wells and to test the scribe orienting method. The results confirm that the streak of directions is real. The streak disappears when the directions are grouped by diagenetic facies. Specimens from clasts in the karst breccia facies contain a CRM with easterly declinations and shallow inclinations that fails a conglomerate test and has an Ordovician pole. A mixed dolomite-limestone with shale filled fracture facies contains a pole which falls off of but close to the Late Mississippian-Early Pennsylvanian part of the apparent polar wander path. A group of facies (crystalline dolomite, wavy bedded to argillaceous dolomite, mottled, burrowed dolomite with fine grained breccia facies, and clastic-rich peritidal carbonates) contains a Late Permian-Early Triassic CRM. Dolomites with vug-fill solution-reprecipitation features contain a Late Triassic-Jurassic CRM. The results from these facies suggest a possible tectonic counterclockwise rotation of the basin by 15 to 20°. The Ordovician CRM is interpreted as related to relatively early diagenetic processes, perhaps associated with Ordovician to early Silurian dolomitization. The Late Mississippian-Early Pennsylvanian CRM is interpreted to represent a mid-Pennsylvanian burial diagenetic event coinciding with burial of the unit to the oil window. The Late Permian-Early Triassic CRM is interpreted as forming from externally derived hydrothermal fluids that may have migrated from the Ouachita thrust zone or along deep basement faults. The Late Triassic-Jurassic CRM may be related to gravity driven fluid migration occurring during basin relaxation and extension during the uplift of the Llano and breakup of Pangea.