Abstract

During latest Mississippian through earliest Permian time a variety of basinal facies in the Marathon Thrust Belt were thrust successively northwestward over the equivalent rocks of the late Paleozoic carbonate shelf. Regional balanced cross sections are used to illustrate the structural complexity of the belt and trace thrust systems from outcrops into the subsurface. A discussion of the sequential development of the thrust belt and adjacent foreland is also provided.

Oil and gas production from thrusted basinal facies is currently limited to a leading edge imbricate thrust zone. These traps have proven to be structurally complex, limited in areal extent, and virtually impossible to image seismically.

Two study areas are presented in detail. At the northeast corner of the Marathon Uplift, four thrust sheets are draped over a large subthrust basement-involved fault block. One well on the crest of this structure recovered fresh water from both thrusted rocks and subthrust strata, whereas numerous other wells in the area have encountered salt water and/or hydrocarbons from the same zones.

Along the western portion of the Marathon Uplift, seismic profiles reveal the presence of potential detatched upper Pennsylvanian Gaptank carbonate buildups beneath the Dugout Creek Thrust. Structural and sedimentological data are presented to support this interpretation. These possible buildups provide a new exploration play for the Marathon Thrust Belt.

The many geophysical and non-geophysical pitfalls confronting an explorationist in the Marathon Thrust Belt are discussed in detail. These make the belt a truly challenging prospective frontier area.