ABSTRACT: Depositional Systems, Diagenesis, and Hydrocarbon Entrapment in Upper Atokan Caddo Clastic Interval of Northwestern Fort Worth Basin

Alan Ammentorp, Arthur W. Cleaves

The upper Atokan Caddo conglomerate and sandstone interval was deposited by a wave-reworked braid delta system. This system prograded southeastwardly from the uplifted Electra arch into the adjacent northwestern extremity of the still-actively subsiding Fort Worth basin. Subsurface net sandstone isolith, sandstone percentage, clastic ratio, and interval isopach maps prepared for the interval between the top of the Pregnant shale and the base of the Strawn Caddo limestone demonstrate the presence of three updip, linear net sandstone trends that represent coastal plain braided streams and a total of six downdip, cuspate, strike-elongate delta lobes. Marine reworked channel-mouth bar sandstone and conglomeratic braid distributary channels comprise the principal reservoir fac es in the upper Atokan stratigraphic traps of southern Montague County and adjacent areas to the west and south.

The Caddo terrigenous clastic facies had a plutonic igneous source involving uplifted crystalline basement and not an orogenic provenance from the Ouachita fold belt. Porosity development is controlled by the absence of early forming quartz overgrowths and calcite cementation, accompanied by an absence of pore-occluding kaolinite and chlorite. Optimum conditions exist for the development of porous reservoir rock when enlarged intergranular porosity, derived from the dissolution of detrital matrix, and secondary moldic porosity, resulting from the dissolution of potassium feldspar, together lack the pore-filling clays and preserve an effective network of interconnecting pores.

AAPG Search and Discovery Article #90096©1990 AAPG Southwest Section Meeting, Wichita Falls, Texas, March 11-13, 1990