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, I b (beta = 90 degrees ) and II b . A third structure, I a polytype, was identified in only one sample. The relative percentage of the high-temperature II b structure increases gradually with increasing thermal maturity, from <= 10% at 2.0% R o up to >= 40% at 3.5% R o . II b chlorite forms rather thick, blocky crystals distinct from the thin, pseudohexagonal plates typical of low-temperature I b chlorite. Temperature estimates based on data on vitrinite reflectance and fluid inclusions suggest that II b chlorite formed at burial temperatures >= 150-180 degrees C. Higher contents of tetrahedral Al (super 3+) and slightly higher Fe/[Fe+Mg] ratios in II b chlorite are consistent with precipitation temperatures higher than those of the I b structure. A higher-temperature origin for the II b structure is also consistent with oxygen isotope data.