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Factors that Control Early Diagenetic Clay Formation: Implications for Pre-Compactional Porosity Preservation in Conventional and Unconventional Reservoirs


Early diagenetic clays form coatings on sand grains stabilize grain-to-grain contacts & preserve pre-compactional pore space in conventional sandstone reservoirs. Macquaker et al. & others have also shown authigenic kaolinite forms within the shell interstices, preserving pre-compactional shelter porosity. Factors that control authigenic clay precipitation in the near subsurface, however, are poorly constrained, limiting the ability to effectively predict clay distribution in & their impact on reservoir quality during diagenesis. This study determines the influence of temperature, Al-complexation, & microbial processes on early diagenetic clay formation & models clay minerals formation in sedimentary systems. We conducted a series of laboratory experiments using a model low pH (<4) hydrothermal solution containing 38 mM Fe & SO4 or Cl, 0.1-15 mM Al, 0-0.11 mM Ca & F or oxalate, & 6-8 mM Si at 25-80oC. In “high temperature” (80oC) systems, Al-complexation by sulfate, a ligand with high affinity for Al, enhances kaolinite & poorly-ordered mixed-layer smectite formation within 3 days. When fluoride is present, few crystalline smectites form & amorphous silica is favored. When oxalate, an organic ligand, is present nontronite precipitation is favored. Microbial cells, whose charged surfaces facilitate mineral formation in low temperature (<50oC) systems, have no impact in high temperature systems. In low temperature experiments (50-25oC), microbial cell surfaces facilitate smectite formation over kaolinite in oxidizing environments by providing nucleation sites for early iron oxides that are transformed to smectite with aging (7-21 days). Microbial metabolic processes may facilitate kaolinite formation in circum-neutral (pH5-8) anaerobic bottom sediments where reduced S & Fe are precipitated as pyrite, leaving Al & Si available. Early diagenetic clay formation across a variety of pHs is temperature & solution composition dependent. Microorganisms (cell surfaces & metabolisms) control clay mineral formation in low temperature systems by providing nucleation sites for clay minerals & through metabolically influencing solution chemistry. In contrast, as the rocks are progressively buried, aluminum complexation by inorganic & organic ligands controls the clay formation. These processes may work in concert to preserve porosity & be used to model the formation & distribution of early diagenetic clays in conventional & unconventional reservoirs.