AN INTEGRATED PALEOMAGNETIC AND DIAGENETIC INVESTIGATION OF THE BARNETT SHALE AND UNDERLYING ELLENBURGER GROUP CARBONATES, FORT WORTH BASIN, TEXAS

Abstract

An integrated geochemical/petrographic, paleomagnetic and rock magnetic study was conducted to better understand the nature and timing of diagenetic events in both the Mississippian Barnett Shale and underlying Ordovician Ellenburger Group carbonates. The Ellenburger is extensively karsted and brecciated throughout portions of the western half of the Fort Worth Basin, Texas, where it underlies the Barnett, the primary source rock and the major unconventional gas reservoir in the Fort Worth basin, Texas. Previous workers have hypothesized that orogenic hydrothermal fluids, originating from the nearby Ouachita thrust front, locally affected the thermal maturity of the organic-rich shale near fluid conduits. The extent of fluid alteration in both Barnett and Ellenburger, as well as the timing and origin of the fluids, are poorly understood.

Samples from five scribe oriented conventional drill cores in the Barnett Shale and Ellenburger Group carbonates were analyzed for their diagenetic and paleomagnetic properties. 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 and were not caused by rotation of the core.

The Ellenburger Group paleomagnetic streak of data 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.

The Barnett Shale streak of paleomagnetic data does not reveal an apparent control on the CRMs by lithofacies. Fracture orientation, however, appears to be correlated to the paleomagnetic directions. Fractures associated with bed-parallel stratiform mineralization contain a CRM interpreted to the mid-Pennsylvanian and the fracture fills are mostly non-radiogenic. Sub-vertical fractures with northeasterly azimuths are found in rock that contains a similar mid-late Pennsylvanian CRM and the fracture fills are non-radiogenic. Both bed-parallel and sub-vertical fractures are interpreted to have formed from early burial diagenetic fluids. Vertical northwesterly fractures are associated with rock that contains a late Pennsylvanian to early Permian CRM. These rocks are interpreted to have been altered primarily by internally derived fluids based on their relatively low levels of radiogenic strontium. The CRM is interpreted to have formed from burial diagenetic processes, such as clay transformations or hydrocarbon generation and migration. A set of vertical, northeasterly fractures contains a late Permian CRM. These fractures commonly display evidence of reactivation and are filled by multiple generations of calcite, silica, sulfides and sulfates, and may contain open vugs and incomplete cementation. The fluids that precipitated these late fracture fills were largely radiogenic. This late CRM is interpreted as related to gravity driven orogenic fluids sourced from the uplifted Ouachita front. The paleomagnetic data suggests the possibility of a counterclockwise block rotation of up to 20° in late Pennsylvanian time in the northern half of the basin. Vertical magnetic susceptibility data shows similarities between some cores and suggests that MS data can be used to correlate the Barnett in some parts of the basin. The early and late diagenetic alteration of the Barnett resulted in the addition of some minerals (e.g., silica, sulfates, and authigenic albite) that could have changed the mechanical behavior of the mudstones by making them more or less rigid.