Petrographic and isotopic data from an 8,500-m thick section of the Great Valley sequence indicate that widespread calcite cement in sandstones and mudstones was precipitated during at least two distinct stages that are linked to organic and clay-mineral diagenesis.

The range of ^dgr¹³C values for calcite in mudstone is -11.6 to +1.0 ppt PDB, in sandstones is -9,7 to +4.5 ppt PDB, and in veins is -16.7 to -2.2 ppt PDB. The ^dgr¹³C values are heavier in progressively older and, presumably, more deeply buried strata. A strong shift to heavier ¹³C/¹²C ratios of calcite in mudstones and sandstones corresponds with an abrupt 20 ppt shift to more enriched ^dgrD values of OH-hydrogen in diagenetic smectite and a 10A clay-mineral from mudstones. Clay D/H ratios range between -69 to -49 ppt SMOW. The stratigraphic position of the shift corresponds to a modeled burial temperature estimate of about 80 to 100°C. This is interpreted to be the burial temperature for late-stage dehydration and conversion of smect te to a 10A clay-mineral phase.

Theoretical considerations indicate that a shallow burial phase of predominantly pore-filling calcite formed in association with bacterial production of methane (< 80°C), or with migrated thermogenic gas. A second stage of calcite, found in deeper strata, and mostly of replacement origin, formed from deep formation waters (> 80°C) that contained thermogenically produced low C₂+ gases. Release of dry gas at increasingly elevated temperatures was characterized by continuous ¹³C-enrichment in CH₄. Depleted calcite carbon in the most basal strata was derived from CH₄-rich basement fluids. Estimation of ^dgrD values of formation waters indicates that CH₄-H₂O equilibrium was not attained.

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