AUTHIGENIC CHLORITES IN SANDSTONES AS INDICATORS OF HIGH-TEMPERATURE DIAGENESIS, ARKOMA FORELAND BASIN, USA

Abundant authigenic chamosite associated with higher-than-average poro sities is present in a deep gas reservoir (Spiro sandstone) of the Ark oma Basin, east-central Oklahoma. Three types of chlorite can be disti nguished petrographically, all of which appear texturally to have form ed early: chlorite peloids, diffuse matrix, and grain coatings. X-ray diffraction, scanning electron microscopy, and electron microprobe stu dies show that most chlorites are composed of two distinct polytypes, Ib (beta = 90-degrees) and IIb. A third structure, Ia polytype, was id entified in only one sample. The relative percentage of the high-tempe rature IIb structure increases gradually with increasing thermal matur ity, from less-than-or-equal-to 10% at 2.0% R(o) up to greater-than-or -equal-to 40% at 3.5% R(o). IIb chlorite forms rather thick, blocky cr ystals distinct from the thin, pseudohexagonal plates typical of low-t emperature Ib chlorite. Temperature estimates based on data on vitrini te reflectance and fluid inclusions suggest that IIb chlorite formed a t burial temperatures greater-than-or-equal-to 150-180-degrees-C. High er 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 II b structure is also consistent with oxygen isotope data.