ABSTRACT

Surface and near-surface Permian sandstone has been drastically altered over the productive part of the structurally complex Velma oil field as a consequence of petroleum microseepage. Buried Permian sandstone along the northwest-southeast-trending anticline is cemented with abundant pyrite, ferroan calcite and ferroan dolomite. At the surface along the anticlinal crest, iron disulfide is scarce; isotopically anomalous carbonate-cemented sandstone is overlain by sandstone that is massively impregnated by hematite cement. Permian sandstone is normally reddish brown throughout southern Oklahoma, but along the anticlinal flank it has been bleached yellow and white owing to iron loss; some units contain abundant solid bitumen.

The mineralogy in the vertical section over the anticline follows the calculated stability relations for iron sulfide, iron carbonate, and iron oxide along a gradient from strongly reducing conditions at depth to oxidizing conditions at the surface. Reducing conditions were provided by seeping hydrocarbons from subsurface reservoirs of this multizone giant field. Production depths range from 120 to 2,975 m. The principal evidence that these alterations are seepage-induced is provided by reports of oil seeps in the early literature, by zones of solid bitumen cements, and by C¹³ PDB values for carbonate cements that range from -7.8 to -36.7 ppt.