INTEGRATING DIAGENETIC DATA TO DEVELOP MODELS FOR PREFERENTIAL HYDROTHERMAL FLOW BELOW STRATIGRAPHIC DISCONTINUITIES

Study of reservoir and non-reservoir rocks using an integrated petrographic and geochemical approach can provide new insights into fluid flow and porosity evolution. Mineral paragenesis, CL, UV, or BSE cement stratigraphy, fluid inclusion homogenization temperature and salinity, and isotopic tracers provide the record of temperature and fluid composition necessary in formulation of new conceptual models for predicting the distribution of porosity in the subsurface.

An example of such an innovation is in the Cambrian-Ordovician through Pennsylvanian section of the Midcontinent, USA, where hydrothermal systems progressed in character over time, initially with megaquartz precipitation and dissolution during migration of ~80-90°C, low-salinity connate fluids (3.1-6.0 wt. %) sourced from the Anadarko basin. Later fluid flow out of the basin was at 93-144°C, 15-23 wt. %, and precipitated baroque dolomite. Regional trends in ⁸⁷Sr/⁸⁶Sr, fluid inclusion temperatures, δ¹⁸O and δ¹³C show that hydrothermal fluid flow was advective. The δ¹⁸O data are highly correlated with ⁸⁷Sr/⁸⁶Sr, showing radiogenic values associated with depleted δ¹⁸O. The δ¹⁸O is most depleted higher in the section, supporting increasing temperatures and hydrothermal fluid flow upward, toward the top of the Mississippian. Later hydrothermal fluids precipitated calcite at 93.0-104°C from solutions with salinity of 15-16 wt. %. The calcite has more radiogenic ⁸⁷Sr/⁸⁶Sr and more spatial variability in δ¹⁸O than baroque dolomite, suggesting a change from regional advective hydrothermal flow to more locally controlled hydrothermal flow.

This study shows that for hydrothermal aquifers, with hydrogeology allowing for regionally advective and vertical flow, the highest temperature hydrothermal fluid flow is concentrated toward the tops of aquifers, just below lower permeability stratigraphic discontinuities. This is caused by the low density of high-temperature fluids preferentially migrating to the top of the aquifer, and then migrating laterally. The results of this study are broadly applicable to the subsurface, and offer an explanation for late porosity enhancement below unconformities and other stratigraphic discontinuities.