ABSTRACT Organic-rich Mississippian carbonates and oil samples from wells completed in the Mississippian and Woodford zones in northern Oklahoma were sampled and geochemically assessed to evaluate charge history. Rock and oil samples were collected from the Cherokee platform and the Anadarko shelf. Samples were analyzed using gas chromatography and gas chromatography-mass spectrometry (GC-MS) techniques for quantitative analysis of diamondoids and saturate and aromatic biomarkers. Results indicate Mississippian hydrocarbon source rocks have generation potential and reached the early oil window thermal maturity. Extracted bitumen from Mississippian rocks and related oils show unique biomarker signatures such as the presence of extended tricyclic terpanes and high input of C 27 relative to C 28 and C 29 in regular and rearranged steranes. The extent of cracking, as measured by diamondoids, reveals a dramatic change in diamondoids concentration between areas east and west, respectively, of the Nemaha uplift. The higher concentration of diamondoids and biomarkers observed west of the Nemaha uplift indicates mixing of uncracked oil with cracked oil migrating out of the Anadarko Basin. This mix of uncracked and cracked oils west of the uplift suggests episodic hydrocarbon charge and a long-distance component to the migration model. In contrast, the Mississippian samples from east of the Nemaha uplift are depleted in diamondoids, suggesting limited migration distance and localized hydrocarbon generation under lower thermal stress.

Abstract The Meers fault in southwestern Oklahoma, with a prominent scarp resulting from late Holocene surface displacement, is the best-expressed late Quaternary surface fault known to occur in a “stable” continental interior (or mid-plate) region (i.e., regions far removed from areas of high tectonic rates). The Meers fault is part of a major fault system that has not been the locus of major tectonic activity since the Paleozoic, and although recent surface displacements have been sizable, average late Quaternary rates have been low, based on a lack of geomorphic expression indicating significant cumulative displacement. Activity of the Meers fault is unusual, because in mid-plate regions, few large historical earthquakes have occurred and recognized cases of late Quaternary surface faulting are very rare. Based on the extent of surface rupturing and amounts of displacement, the Meers fault appears capable of producing very large events (i.e., M > 7, or possibly even M > 7 ½). Recent events on the Meers fault produced surface displacements of a few to several meters. Such displacements are quite large, relative to the rupture length of about 40 km, and could result from a tendency for mid-plate or long-recurrence faults to rupture with higher stress drops than plate-margin or short-recurrence faults. Studies attempting to evaluate this possibility have produced conflicting results and may indicate this cannot be placed in as simple a context as plate-margin versus intraplate settings. A large earthquake on the Meers fault would produce strong ground motion throughout much of the south-central United States and could cause widespread damage. The existence of a potential source of large earthquakes in a region thought to be tectonically stable suggests that the seismic potential of this and other mid-plate regions may be underestimated.