LINKING COMPOSITIONAL DATA ANALYSIS WITH THERMODYNAMIC GEOCHEMICAL MODELING OF PERMIAN BASIN PRODUCED WATERS

Compositional data analysis (CoDa) of sedimentary basin brines can help reveal the hydrogeochemical history of hydrocarbon-producing formations. CoDa uses log-ratio transformations to convert compositional data into Euclidean space, a necessary condition for many mathematical models and basic data interpretations. This study demonstrates the novel approach of complementing CoDa with thermodynamic geochemical models for bivariate and multivariate subcompositions for a suite of brines within three producing formations from the Permian Basin, USA. Subcomposition analysis of basin brines using [Na, Cl, H2O] and [Ca, SO4, H2O] with corresponding equilibration models, plotted using isometric log-ratio (ilr) transformations, shows reaction with anhydrite and halite at depth. The [Na, Cl, Br] subcomposition often can discriminate between seawater evaporation and halite dissolution; the sparse Br data suggest both processes occur. Principal component analysis (PCA) of centered log-ratio (clr) data is useful for visualizing combinations of processes that are not readily apparent in the two-solute models. PCA of the Permian Basin brines reveals distinct groups of data, including samples likely affected by water flooding and others influenced by a combination of seawater evaporation or halite dissolution coupled with Ca and Mg ion exchange with Na on clays. These relationships support a hydrogeologic model in which meteoric inflow occurs primarily by vertical transport from overlying units, rather than within unit recharge from the uplifted western portion of the basin.