Abstract: Effects of Chamosite on Porosity in the Spiro Sandstone of the Wilburton, Red Oak, and Kinta Fields, Arkoma Basin, Oklahoma

Forrest B. Hess

The lower Atokan Spiro Sandstone was deposited in a shallow marine setting. It is a productive reservoir in the Arkoma basin of Eastern Oklahoma and Western Arkansas. The Spiro is found below 15,000 feet in the southern part of the basin and approximately 4,000 feet on the northern flank. Outcrops are located along the Choctaw Fault as a result of the thrust faulting. The Spiro Sandstone is predominately a fine-grained to medium-grained quartz arenite with interbedded calcareous zones and some fossiliferous limestones. Many repeats of the Spiro are encountered during drilling due to the numerous thrust faults. Chamosite, an iron-rich chlorite, is often found in the subthrusted units and occurs as an early diagenetic clay. In the Spiro Sandstone, chamosite is observed as a syndepositio al coating of grains and as pellets formed at the site of deposition. The presence of allogenic chamositic coatings on quartz grains in the Spiro Sandstone is believed to be an important factor in the preservation of primary porosity. Chamosite grain coatings prevent the stress-induced pressure solution of quartz and the subsequent formation of quartz overgrowths. Formation of dolomite at the expense of chamosite was also observed in thin section. With the replacement of chamosite by dolomite, calcite cement precipitates and the porosity is subsequently destroyed. When chamosite coatings are absent, pressure solution of quartz grains is common and quartz overgrowths become very common. Therefore, the chamosite-rich zones in the Spiro Sandstone are the productive reservoirs, and when cham site is absent the Spiro reservoirs are tightly cemented and non-productive.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994