Stratigraphy and Petroleum Potential of Pre-Pennsylvanian Rocks, Palo Duro Basin, Texas Panhandle

Pre-Pennsylvanian rocks in the Palo Duro Basin include (1) basal transgressive marine Cambrian(?) sandstones deposited over Precambrian basement, (2) overlying Lower Ordovician dolomites of the Ellenburger Group that formed when shallow seas covered much of the North American continent, and(3) Mississippian limestones and dolomites deposited when the area was inundated again after middle Paleozoic uplift and erosion. A generally similar stratigraphic sequence exists in the adjacent Dalhart and Hardeman Basins. Mississippian deposits, the most widespread and best known pre-Pennsylvanian rocks, exhibit considerable facies and paleoenvironmental diversity throughout the Texas Panhandle. The lowermost Mississippian "Osage" contains cherty and shaly dolomites and limestones. In the eastern Palo Duro Basin and in the Hardeman Basin further to the east, these rocks are interbedded carbonate mudstones and limestone turbidites that were deposited below wave base in relatively deep, quiet water. Westward, the "Osage" includes progressively shallower water facies."Meramec" limestones are remarkably similar throughout the Texas Panhandle. These coarse-grained, light-colored, skeletal (bryozoan/echinoderm) grainstones record the establishment during the middle to late Meramecian of a widespread, shallow-water, carbonate sand shoal. However, before this shoal developed in the Hardeman Basin, numerous local carbonate buildups formed (Chappel Formation).The uppermost Mississippian "Chester" contains interbedded ooid grainstones and shales that attest to (1) the maintenance of shallow-water marine conditions and (2) the development of terrigenous clastic source areas associated with early phases of Late Carboniferous tectonic activity. Uppermost "Chester" shales (Barnett Formation) and limestones (Comyn Formation) in the Hardeman Basin to the east are not present in the Palo Duro Basin owing to facies change or erosion or both.All pre-Pennsylvanian units contain sufficient porosity and permeability, at least locally, to be hydrocarbon reservoirs. Potential structural and stratigraphic traps are plentiful throughout the area. Carbonate buildups are productive in the nearby Hardeman Basin; similar buildups may exist in at least the eastern part of the Palo Duro Basin. However, suitable top seals may be lacking in the Palo Duro Basin.Although the quality of organic matter contained in the pre-Pennsylvanian deposits in the Palo Duro Basin is good, there is probably too little organic carbon for these rocks to be hydrocarbon sources. The "Osage" of the eastern Palo Duro Basin contains the highest amounts of organic matter. The Barnett Formation, which contains organic- matter-rich shales in the Hardeman Basin to the east, does not extend into the Palo Duro Basin.Calculations of thermal maturity based on vitrinite reflectance indicate that although pre-Pennsylvanian rocks in the Palo Duro Basin are substantially less mature than those in the Hardeman Basin, most have attained at least the minimum degree of heating necessary to produce hydrocarbons. Thermal maturity in the area generally correlates with the present-day geothermal gradient, which increases toward the east.Petroleum potential of the pre-Pennsylvanian rocks of the Palo Duro Basin is relatively low. Future exploration in these rocks should concentrate on areas where source rock quality, maturity, and reservoir conditions are optimum. The extreme southern and eastern parts of the basin appear to offer the greatest promise.