DIAGENETIC HISTORY OF LEONARDIAN BASINAL CARBONATES AND CLASTICS IN THE EASTERN MIDLAND BASIN, TEXAS: IMPLICATIONS FOR DIAGENESIS AND FLUID FLOW IN FINE-GRAINED ROCK

Fluid flow in fine-grained, low permeability rocks remains rather enigmatic. This diagenetic study of basinal interbedded and intermixed carbonates and clastics aims to expand what our discipline can learn about diagenetic processes affecting fine-grained rocks, such as if they act as relatively open or closed diagenetic systems. Rock core used in this study is comprised of lower Leonardian sediment gravity flow deposits (low density muddy sediment gravity flows; high density grainy sediment gravity flows; and clay-rich, silica-rich, and carbonate-rich hemipelagic sedimentation) from a well located at the toe-of-slope in the eastern Midland Basin, Texas.

The paragenesis includes: varying degrees of mechanical and chemical compaction; early and late calcite; baroque dolomite; silicification; multiple episodes of fracturing and fracture fill; gilsonite deposits; late dissolution; possible MVT sulfides; and possible displacive anhydrite. Heating and freezing measurements of primary fluid inclusions from the crystal interior of fracture-filling calcite indicate initial precipitation from an ambient temperature fluid (Th (mean) = 78.3 °C; Th (σ) = 3.3 °C) with constant high salinity (Tm_ice (mean) = -20.5 °C; Tm_ice (σ) = 1.9 °C). In contrast, measurements from primary inclusions in the crystal exterior reveal later precipitation occurred in a significantly hotter fluid (Th (mean) = 154.1 °C; Th (σ) = 3.1 °C) with highly variable salinity (Tm_ice (mean) = -15.8 °C; Tm_ice (σ) = 4.3 °C; Tm_ice(range) = -11.8 to -21.25 °C).

This change may reflect progression from a relatively closed diagenetic system to one that is more open, which may have been controlled by evolution of the fracture system: initial fracturing tapped into only adjacent rock and sourced fluids locally; further fracturing increased connectivity, tapped deeper fluid sources, and resulted in upward pumping of distally-sourced hot fluids. Highly variable salinity of these hot fluids may be caused by sourcing of fluids from separate stratigraphic zones that hosted fluids of different salinities.

After consideration of present geologic data, the source for these hot fluids is currently enigmatic. Further work includes stable and strontium isotope analyses, additional fluid inclusion analysis, and examination of recently acquired 3D seismic.