Abstract: Effects of Depositional and Authigenic Clays on Porosity Development, White Oak Field, Arkoma Basin

C. Dianne Phillips

Depositional and diagenetic processes have produced allogenic, infiltrated, and authigenic clay features that have contributed to the preservation of primary porosity in the early Pennsylvanian Sells Sandstone of the Arkoma Basin.

A core through a distributary mouth bar facies of the Sells contains well sorted, fine to medium grained sublitharenites and quartzarenites. Two clay rich zones within the medium grained quartzarenite act as permeability barriers and have contributed to the preservation of porosity within these zones. Clays within these zones are predominantly illite/mixed layer clays and authigenic chlorite (mostly chamosite). Allogenic features are present as inherited clay rinds or as soil cutans. Thin clay rinds oriented parallel to the surface of quartz grains are indicative of inherited clay features. Soil cutans are preferentially oriented parallel (concentric in appearance) to the surfaces of sedimentary grains. Cutans are generally indicative of well developed "B" soil horizons. Infiltrated clay features include disseminated intergranular clay matrix as well as clay coats and bridges between individual grains. Authigenic chlorite is present as radial crystals oriented tangent al to grain surfaces and nucleating from grain coats, rinds, and cutans oriented parallel to the grain surfaces. Porosity in these clay rich zones is essentially absent and is in sharp contrast to intervals where porosity ranges from 10 to 15 percent. Clay rich zones reflect alternating sand and clay deposition within the Sells point bar. Shortly after deposition and during very early diagenesis, clay and sand layers were open to diffusive and advective exchanges. Clay minerals were readily translocated to grain surfaces and intergranular areas. Further burial, compaction, and dissolution of framework grains served to further reduce porosity in the clay rich zones and produce partitions to fluid migration into the zones free of substantial clay matrix. Primary porosity was preserved in t e clay free zones between clay barriers.

AAPG Search and Discovery Article #90957©1995 AAPG Mid-Continent Section Meeting, Tulsa, Oklahoma