The Paleogene Wilcox sedimentary system, onshore Texas, USA: a potential play for carbon storage

The Wilcox system contains substantial hydrocarbon reserves in the Gulf of Mexico basin, as well as an important aquifer onshore. Many studies exist assessing the hydrocarbon and water resources; however, there is limited knowledge on Wilcox’s suitability for geological carbon storage.

Onshore Texas, the Wilcox is a sand-prone sedimentary system with thick sandstones and good-to-moderate reservoir quality. Lower and Upper Wilcox are favorable for CO2 storage, while Middle Wilcox is mud-prone and seals the Lower Wilcox section. Upper Wilcox/Carrizo is confined by Reklaw Formation mudstones.

Extensive well-log correlation and mapping along a 590 km transect defined the regional distribution of Wilcox reservoir units. 2D seismic interpretation helped to identify major structural elements. Sedimentological analysis using cores and well-logs helped in understanding the aerial distribution of depositional facies. Estimation of reservoir properties was obtained by petrophysical evaluation. Integration of all data favored CO2 storage capacity estimations in areas with less structural complexity and few legacy wells.

Important variability in reservoir thicknesses is observed. Western area has moderate sandstone content in Lower Wilcox but sandier in Upper Wilcox. The central area of the transect contains the thickest sandstones, especially in Lower Wilcox. The eastern area shows interbedded sandstones–shales, which contrasts with the abundant blocky sandstone packages from central area. Growth faulting significantly increased accommodation around the paleo-shelf and shelf-margin.

Reservoir quality can be variable, with 16–26% porosities and 5–80 md permeabilities, respectively, in western area. In central area, porosities and permeabilities range from 12% to 20% and 1 to 120 md, respectively. In general, the depositional facies control the reservoir quality, since diagenesis is relatively low. CO2 storage capacity estimations were conducted in areas with good sandstone presence, not complex structurally, with low number of legacy wells. CO2 storage capacity is more promising in central and east areas of the studied transect.