The generation and continued existence of overpressure in the Delaware Basin, Texas

Abstract

The Delaware Basin, part of the larger Permian Basin, contains important hydrocarbon plays. Permian strata contain 71% of in-place oil and 53% of in-place gas, with the remainder hosted in the Lower Palaeozoic. Excessive pore fluid pressures (up to 〜 8000 psi above hydrostatic) are found within Early Permian and Pennsylvanian strata, which account for 30-35% of the hydrocarbon producing zones. This study has utilised an array of basin analysis techniques to analyse the pressure history of the Delaware basin. To fully appreciate the geopressure history in the Delaware Basin, a rigorous quantitative approach has been applied using advanced thermochronology techniques. This has enabled for the first time an accurate burial history curve to be established for the basin. The results show that maximum burial occurred in the basin at 55 Ma as a consequence of an additional 6890 ft of Mesozoic and Cenozoic sediment. The basin then underwent two major tectonic uplift events during the Cenozoic. The Laramide orogeny (55-50 Ma) uplifted and eroded off 3890 ft of sediment, then during the Eocene and Oligocene the basin subsided and accumulated a further 600 ft of sediment. The Basin and Range event (25-10 Ma) then uplifted and tilted the basin further, eroding off 3600 ft of sediment from the centre of the basin. The new burial history curve has been integrated with wireline logs and basin modelling software to evaluate the mechanism of overpressure generation and its maintenance through geological time. This study has shown that the main mechanism for overpressure generation in the Delaware Basin was disequilibrium compaction. Analysis of the sonic log using the Equivalent Depth Method and the Eaton Ratio Method, combined with velocity I density cross-plots, indicate that compaction is driven by vertical loading, and undercompaction seen in the basin is a consequence of the sediments' inability to dewater. Basin modelling shows that it was the rapid deposition of the Permian sediments that enabled disequilibrium compaction to generate overpressure. These techniques have also shown that a secondary cause of overpressure due to unloading mechanisms (e.g. gas generation or expansion with uplift, lateral transfer and hydrocarbon buoyancy) may be occurring within localised horizons below the Wolfcampian Series. Overpressure has been maintained within the basin for more than 250 Myr. Basin modelling and wireline logs have shown that numerous intercalated tight limestones (Mississippian to Late Permian) acted as pressure seals to maintain overpressure. In addition, low permeability mudstones (l0 (^-6) mD) have contributed to the inability of the Delaware Basin to reach pressure equilibrium.