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A comprehensive stable isotopic, trace-element, and petrographic examination of 10 cores, in the Jurassic upper Smackover Formation at Oaks field, Louisiana, suggests that early meteoric diagenesis was directly responsible for the present observed porosity distribution in the field.
An isotopic and geochemical model for near-surface meteoric diagenesis based on studies of Quaternary diagenesis is fundamental to understanding the diagenetic history of the Oaks field grainstones. Near-surface meteoric diagenesis develops a characteristic isotopic and chemical signature that is easily recognized when sampled closely in stratigraphic sequence. Rocks stabilized in high-flow freshwater phreatic lens show more negative meteoric ^dgr18O values, and low values of magnesium and strontium relative to the zones above and below. Furthermore, carbon isotopic values show a trend to lighter values upward toward the subaerial exposure surface, indicating partial cementation and recrystallization in the presence of isotopically light soil gas in the vadose zone.
Data from Oaks field are interpreted in light of this diagenetic model. Porous zones in Oaks field are characterized by more negative ^dgr18O values and lower magnesium and strontium content. The carbon data show a trend to lighter values toward the tops of the wells. These data suggest early mineralogic stabilization of the now-porous intervals in a high-flow, near-surface meteoric phreatic lens. Upon burial, those intervals that had been stabilized early were less susceptible to solution compaction than those intervals retaining significant proportions of unstable mineralogy. Porosity reduction through solution compaction occurred because the unstable minerals were more soluble under burial conditions than stable low-Mg calcite. These relationships are noted in all wells here porosity and permeability are well preserved.
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