ABSTRACT

Abundant authigenic chamosite associated with higher-than-average porosities is present in a deep gas reservoir (Spiro sandstone) of the Arkoma Basin, east-central Oklahoma. Three types of chlorite can be distinguished petrographically, all of which appear texturally to have formed early: chlorite peloids, diffuse matrix, and grain coatings. X-ray diffraction, scanning electron microscopy, and electron microprobe studies show that most chlorites are composed of two distinct polytypes, Ib (ß = 90°) and IIb. A third structure, Ia polytype, was identified in only one sample. The relative percentage of the high-temperature IIb structure increases gradually with increasing thermal maturity, from <= 10% at 2.0% R_o up to >= 40% at 3.5% R SUB>o. IIb chlorite forms rather thick, blocky crystals distinct from the thin, pseudohexagonal plates typical of low-temperature Ib chlorite. Temperature estimates based on data on vitrinite reflectance and fluid inclusions suggest that IIb chlorite formed at burial temperatures >= 150-180°C. Higher contents of tetrahedral Al³⁺ and slightly higher Fe/[Fe+Mg] ratios in IIb chlorite are consistent with precipitation temperatures higher than those of the Ib structure. A higher-temperature origin for the IIb structure is also consistent with oxygen isotope data.