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Diagenetic History of Lower Leonardian Strata in the Eastern Midland Basin: Implications for Fluid Flow and Predicting Reservoir Quality


Fluid flow and diagenesis in fine-grained reservoirs remains enigmatic. We studied the diagenetic history of lower Leonardian, toe-of-slope strata in the eastern Midland Basin (Howard County, Texas) composed of pelagic components, fine-grained clastics, and carbonate sediment gravity flow deposits (high and low density). These carbonates host highly variable macroporosity (1 to 15%) and distribution of diagenetic phases has impacted porosity location. The paragenesis includes: early dissolution; early and late calcite cement; early and late dolomite cement; displacive anhydrite; mechanical and chemical compaction; petroleum migration; late silicification; pyrite; episodes of fracturing; bitumen; and late dissolution. Extant porosity is the result of preserved primary porosity (dominantly intraparticle) and secondary porosity (vuggy, moldic, and fracture). Analysis of primary, aqueous, two-phase fluid inclusions within calcite cement in an opening-mode vein indicate initial precipitation from an ambient burial temperature, locally sourced fluid (mean Th = 78.3 °C). High salinity (mean Tmice = -20.5 °C) suggests that deep burial ambient water in the reservoir was evaporated seawater delivered through reflux in the Late Permian (e.g. Stueber et al. 1998). In contrast, later calcite precipitated from an anomalously hot, deeply sourced, hydrothermal fluid (mean Th = 154.1 °C). Highly variable salinity (range Tmice = -11.8 to -21.25 °C) supports injection and mixing of old meteoric water from reservoirs below a Devonian aquitard (e.g. Engle et al. 2016) with ambient brine. There are two plausible sources for these anomalously hot fluids: (1) faults cutting kilometers into and injecting fluids from underlying basement; or (2) the deep Val Verde Basin to the south where a meteoric pressure head in the elevated Marathon mountains could have driven migration of hot fluids through a permeable unit north into the Midland Basin. The transition from locally derived fluids to hydrothermal fluids may represent progression from a relatively closed to open diagenetic system. This finding of a hydrothermal system has important exploration implications, as thermal maturation and porosity-altering diagenetic events may be localized near certain fracture systems.