Source, Maturity and Compartmentalization of Oil and Brines in the South Liberty Salt Dome, Texas Gulf Coast

Tat Banga, Regina Capuano, Adry Bissada, and Don Van Nieuwenhuise
University of Houston, Houston, TX

Understanding fluid flow around salt domes is challenging, as uplifted and faulted strata around salt piercement structures cause significant perturbation in terms of fluid flow and fluid-rock interactions. Chemical characterization of fluids (34 brine and 14 oil samples) and stratigraphic data from slightly over-pressured formations (Frio, Cockfield/Yegua and Cook Mountain) around South Liberty salt dome, Liberty County, Texas provides information on the fluid sources, nature of fluid-rock interaction and reservoir compartmentalization.

Whole-oil GC and GC-MS data indicate that the oils are of marginal marine origin, and have similar source and maturation history (temperature of expulsion around 130°C). Some samples have undergone post emplacement alterations such as evaporative fractionation and biodegradation, in reservoir compartments. The local brines are a product of mixing between local meteoric water and Gulf Coast geopressured brines, with mixing prevailing across formations. These Na-Cl type brines have undergone halite dissolution (TDS: 68-254 g/L). Like the local geopressured brines found away from the domes1, the South Liberty brines are buffered by calcite-disordered-dolomite equilibrium reactions. These fluids lie in the equilibrium stability fields of illite-albite-kaolinite suggesting these phases control K, Na concentrations, similar to the diagenetic reactions described for the Frio Fm. in similar reservoirs to the west1. This maintenance of equilibrium in these brines, despite dilution by up to 50% of meteoric water, suggests a long residence time sufficient to re-equilibrate with the rocks after mixing. Thus, the chemical changes in the water are products of long circulation rather than geologically recent or localized phenomena.