Kansas-Type Cyclothems and Porosity Development in Middle Pennsylvanian Marmaton Group, Dirks Field, Logan County, Kansas: ABSTRACT

Abstract

Seven depositional units are recognized in the Texaco 2 Dirks core (4,374 to 4,419.5 ft, 1,333 to 1,347 m) from the upper part of the Middle Pennsylvanian (Desmoinesian) Marmaton Group, Dirks field, Logan County, Kansas. These units make up two Kansas-type cyclothems, the Altamont and Lenapah, and record deposition in response to fluctuating sea level and differing terrigenous influx on the broad, epeiric shelf that was the northern extension of the Hugoton embayment. The units correspond to Heckel's basic, Kansas-type cyclothem members, which are (transgressive) middle limestone, (transgressive) core shale, (regressive) upper limestone, and (regressive) outside shale. The Altamont cyclothem lacks the core shale and perhaps the middle limestone, both of which are present n outcrops in eastern Kansas. The overlying Lenapah cyclothem contains all of the basic members of Heckel's Kansas-type cyclothem. The (transgressive) middle limestone of the Lenapah cyclothem is a relatively thin, locally burrowed, bioclast wackestone containing Osagia-coated bioclasts and a relatively diverse marine biota. Overlying this is calcareous, olive-gray to olive-black, fissile core shale containing phosphorite(?) nodules and local brachiopods. Outside (regressive) shales in the Dirks 2 core are greenish to brownish gray. Although generally unfossiliferous, brachiopods and crinoids are present in the upper part of one of the outside shales. Evidence of plant roots is present locally, and nodular, unfossiliferous limestone with calcite-filled desiccation fractures occurs in the outside shale of the Lenapah cyclothem. Porosity development in the Altamont and Lenapah cyclothems of the Dirks 2 is restricted to the upper limestone members. The upper limestone of the Lenapah cyclothem, the Idenbro limestone member, is a bioclast packstone and grainstone grading upward into cross-stratified, ooid grainstone. This unit contains intergranular and secondary moldic (principally oomoldic) porosity. Portions of this regressive limestone are slightly oil stained, and porosity is poor to locally fair. The upper limestone member of the Altamont cyclothem, the Worland limestone member, is strongly oil stained and is a pay zone in this well and in the study area. This unit consists of bioclast wackestone overlain by chaetetid-coral bafflestone, and above that, dolomite containing angular limestone clasts, irregula -shaped vugs, and sediment-filled fractures. The Worland limestone displays intragranular, secondary moldic, solution-enlarged moldic, vuggy(?), and fracture porosity. Porosity and permeability are fair to good locally and are thought to reflect, for the most part, meteoric diagenesis related to subaerial exposure on a subtle Altamont high on the sea floor. A cross section through the study area shows a slight thinning of strata overlying the Altamont Limestone in wells where Worland porosity is well developed (i.e., Worland pays) relative to wells where the Worland limestone is tight. This thinning may indicate an Altamont high, the crest of which may have been situated just northeast of the present-day structure. End_of_Article - Last_Page 1323------------