299-1 Differentiating Fluvial Versus Marine Sediments in a High Temperature Setting Using Clay and Groundwater Chemistry

Session: Geochemical Studies of Sediments (Posters)

Presenting Author:

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Abstract:

A new methodology exists in which the cation composition of clays is determined from the chemical composition of water based on the instantaneous cation exchange equilibrium between pore water and clay minerals. This has been used to evaluate the environments of deposition of the sediments in the Lissie, Goliad, and Wilcox formations of the Texas Gulf Coast at shallow depths (Capuano and Jones 2020, 2023), but has not been used on deeper systems at elevated temperatures. In this study this methodology is evaluated on deeper, higher temperature sediments, to determine whether the relative abundance of mole fractions of cations on clays can be used to identify the environment of deposition of clay-rich sediments.

Exchange constants needed for this study were not available at elevated temperatures, so as part of this study an internally consistent thermodynamic database of heats of reaction for the relevant exchange reactions was compiled. From these data exchange constants up to 154℃ were calculated and used to calculate the clay-cations from water samples collected from 78 oil wells from depths of 2624 to 4740 meters from the Frio Formation and overlying units of the Texas Gulf Coast. Although the Frio Formation is stratigraphically and mineralogically similar to the shallow Lissie, Goliad, and Wilcox formations, it has been subjected to deeper burial and higher temperatures.

Preliminary results show that although the clay-cation compositions of deeper sediments no longer reflect their depositional environment alone due to long term water flushing, they can be used to gain insight into the hydrodynamics of the system. Flushing was found to occur as a two-step process; Mg is exchanged out of the clay for Na and then Na is exchanged out of the clay for Ca as exchange constants change with increased temperature. The wide variation of clay-cation compositions at depth is likely due to diagenetic feldspar dissolution and hydrodynamic influences. Additionally, the clay-cation compositions allow the identification of compartmentalization, suggesting that the history of exchange can be used to understand the hydrodynamics of the system.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8055

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