Evidence for regional fluid migration in the Eagle Ford Formation, Austin Chalk, and Buda Limestone of south-central and west Texas, USA

Abstract

Calcite veins hosted by extension fractures and faults are common in the outcrop belt of the Austin Chalk – Eagle Ford – Buda stratigraphic section across south-central and west Texas, and contain aqueous and liquid hydrocarbon fluid inclusions. The hydrocarbon fluid inclusions in these thermally immature rocks indicate long-distance migration of oils from mature source rock down-dip and possibly from deeper units along faults. The widespread spatial distribution and extensive variety of oil types indicated by the fluid inclusion fluorescence suggest a protracted series of migration events – possibly from multiple source areas – rather than a single event. Oil inclusions in fault and fracture hosted calcite veins provide evidence of both strata-bound and cross-stratal oil migration along fault and fracture networks that are pervasive in the Austin Chalk, Eagle Ford, and Buda, as well as stratigraphically deeper units, in the subsurface and outcrop belt.Aqueous fluid inclusion microthermometry of the calcite veins provides evidence for significant exhumation of rocks in the Austin Chalk – Eagle Ford – Buda outcrop belt. Approximately 1.25–1.79 km of overburden was removed along the Balcones fault zone outcrop belt of south-central Texas, while in outcrop belt of the Devils River Uplift as much as 2.44–3.50 km of overburden was removed. In the productive region of the deeper basin down-dip from the outcrop belt, fluid inclusions in a deep core indicate that fluids were trapped during burial, but do not record maximum burial conditions.