Abstract

Depositional and diagenetic processes produced allogenic and authigenic clay features that contributed to the preservation of primary porosity in the Early Pennsylvanian Sells sandstone (Lower Atoka) of the Arkoma Basin.

A core through a distributary mouth bar facies of the Sells contains well sorted fine to medium grained sublith-arenites and quartzarenites. Clay rich zones within the medium grained quartzarenite act as permeability barriers and contributed to the preservation of porosity within the zones lacking substantial clay matrix. Clays within the clay rich zones are predominantly illite/mixed layer clays and authigenic chlorite (mostly chamosite). Allogenic features are present as soil cutans, grain coats, and intergranular clays. Soil cutans are preferentially oriented parallel (concentric) to the surfaces of sedimentary rock fragments and are generally associated with well developed "B" soil horizons. Other infiltrated clay features include disseminated intergranular clay matrix as well as clay coats and bridges between indi-vidual grains. Authigenic chlorite is present as radial crystals oriented perpendicular to grain surfaces. Porosity in the clay rich zones is essentially absent 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 mouth 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 the zones between clay barriers. Clay coats within these zones cushioned grain contacts and inhibited pressure solution and quartz cementation during compaction. Authigenic chlorite reduces existing primary and secondary porosity in porous zones, but has aided in preservation of primary porosity by blocking sites for nucleation of quartz cements.