Depositional Systems, Petrography, and Petroleum Geology of a Caddo Conglomerate (Atokan) Wave-reworked Braid Delta in North-central Texas
Abstract
The Fort Worth Basin is a wedge-shaped foreland basin whose primary sediment fill consists of Ordovician and Mississippian carbonates and Middle (Atoka and Strawn Series) and Upper Pennsylvanian terrigenous clastic rocks. The Atokan and Strawn clastic units were deposited on the basin's structurally stable northern shelf and contain significant quantities of natural gas and liquid hydrocarbons. The Caddo conglomerates and sandstones are the second most productive interval in the Atoka Series but its depositional model and diagenetic characteristics are poorly understood by exploration geologists. The principle sources of data employed for this study were more than 700 electric logs from which stratigraphic cross sections and subsurface maps were constructed. Production trends as determined from published data were correlated to the structural contour, net sand isolith, sandstone percentage, clastic ratio, and interval isopach maps generated by this study. Two cores from the study area were examined to determine the controls on porosity and permeability. The Caddo conglomerate and sandstones were deposited by a hybrid high-destructive wave-dominated and braid delta complex that prograded from northwest to southeast. The Atokan fluvial-deltaic facies initiated the deltaic sedimentation for the basin. Porosity and permeability development in the Caddo clastic units are controlled by the absence of early-forming authigenic quartz overgrowths and calcite cementation, along with the absence of pore-filling kaolinite and chlorite. Optimum conditions exist when enlarged intergranular porosity (dissolution of detrital matrix) and secondary moldic porosity (dissolution of feldspars) are void of the pore-filling clays and allow the preservation of an effective interconnecting network of pores. Caddo production trends correlate very well to the sandstone trends indicated on the net sandstone maps. Pure stratigraphic trapping is the major mechanism for hydrocarbon emplacement throughout the study area. The Middle Pennsylvanian, upper Atoka Caddo clastics of the Bend Group are known prolific hydrocarbon producers within the study area. The narrow, up-dip, braided fluvial channels and the wave-dominated braid delta sequences of the Caddo clastics are therefore a prime exploratory; development target of the petroleum industry. Through more complete detailed mapping and better understanding of the geometry and the porosity development of these and like clastics, a more successful hydrocarbon exploration program can be anticipated.
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