Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 250, part 1
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Petroleum Implications of Precambrian and Later Tectonism

The ultimate purpose of this study is to identify potential exploration targets that may be developed through study of the Precambrian rocks of Kansas, their tectonic setting, and their interaction with later events. There are several effects that pre-existing rocks and structure may have on subsequent petroleum-reservoir development and migration pathways. One is simply a topographic prominence being the focus of compaction of overlying sediments, creating thinning and draping. Similarly, marine transgression around a topographic prominence may create an apparent offlap of coeval sediments, reflecting the shoaling of water on the flanks of the prominence. Diagenetic reservoir enhancement may occur through the concentrations of fluids moving through the sedimentary body and along the basement/sediment surface, culminating at the top of the prominence.

If the topographic prominence is due to tectonic movement, recurrent movement will likely result in depositional thickness variations between high and low areas, and some younger rocks may be displaced by faults. This can create either permeability increases or barriers, may juxtapose reservoir rocks against sources, or, especially in carbonate rocks, create differential porosity zones. Other possibilities exist. Frequently, combinations of several of these possibilities co-exist and create complex reservoir settings. Migration of petroleum fluids can be controlled by pre-existing fracture sets (Gerhard et al., 1991, for instance).

One of the enigmas of Kansas petroleum production is the lack of significant production from the Salina basin (fig. 7). Exploration geology requires optimism, but optimism must be tempered with economic reality.

Figure 7--Generalized oil and gas map of Kansas. Kansas Geological Survey map.

In this case, although there has been significant exploration drilling, production has been confined to the margins of the Salina basin, peripheral to and along major blocks of the Central Kansas uplift and the Nemaha uplift, and in smaller fault-bounded structures. There are apparently two major oil sources: Ordovician and Devonian (Rich, 1931, 1933; Newell and Hatch, 1999) (see also Gussow, 1954), derived from Arbuckle and Simpson, and Chattanooga equivalent rocks, respectively. The Newell and Hatch study suggests local generation of oil may occur along major offsets such as the McPherson anticline, whereas the remainder of the Salina basin is likely too shallow to have caused any significant generation. The hypothesis presented herein is that the intersection of fractures of the Nemaha and Central Kansas uplift form a structural barrier to migration, so that oil moving from the south is shunted along the major fractures to fill reservoirs on and along the uplifts. These fractures create a migration shadow in the Salina basin (fig. 8) where oil is barred from the basin by the shunts.

Figure 8--Migration patterns and shadow map, showing approximate migration pathways for oils generated in the Ouachita and Anadarko basins, and local generation in the Forest City basin. Migration shadow is Salina basin, where migration was shunted away from the basin by faults into the Central Kansas uplift and the Nemaha Ridge.

Walters (1958) and Price (1980) both advanced the theory of long-distance migrations of oil from the Anadarko basin to central Kansas through Arbuckle rocks, during the mid-Permian. Their model calls for decreasing amounts of oil and gas to be emplaced northward as traps fill and correspondingly decrease the supply of petroleum for more northerly traps. The barriers to migration model suggested herein simply adds one more complication to the migration story and does not assume a restricted supply of petroleum from the Anadarko.

Exploration drilling requires reservoir rock, trap, and source. This hypothesis suggests that a source is missing in the Salina basin, and it is not economically feasible to continue exploration drilling in the basin barring demonstration of a potential source. If there is a source, it is likely to be local, barely mature, and of small volume (Newell and Hatch, 1999).

The concept of structural-migration shadows has been demonstrated elsewhere, such as in the Williston basin (Gerhard et al., 1991), where diffusion migration alone cannot explain the occurrence or lack of occurrence of petroleum (fig. 9).

Figure 9--Migration patterns in the Williston basin, North Dakota, Montana, and South Dakota, showing the influence of fractures and faults on migration. Oil is generated in the middle of the basin and migrates along fractures. This explains the linear accumulations and the lack of oil accumulations in portions of the basin intermediate in position between the source area and the known accumulations. From Gerhard et al. (1991).

Oil and gas migration along preferred pathways of high permeability, whether lithologic or structural, provides for long-distance migration, almost a prerequisite for significant accumulations of petroleum in shallow cratonic platform accumulations where depth of burial cannot provide the necessary heat and pressure to crack organic sedimentary content into petroleum. Newell and Hatch (1999) argue for juxtaposition of source beds and reservoir along faults on the McPherson "anticline," which they demonstrate to be an uplifted reverse-faulted block. They demonstrate that Ordovician oils are probably local, and of low relative maturity, coupled with Chattanooga oils that indicate maturity of source rock, and thus, longer-distance migration from south to north.

Thick accumulations of sediments lie south of Kansas, early Paleozoic accumulations in the tectonic troughs of eastern Oklahoma and western Arkansas, and younger thick deposits in the Anadarko basin in western and central Oklahoma, extending into southwestern Kansas. These "piles" have generated huge amounts of petroleum, from Ordovician rocks in the east (Arbuckle and Simpson Groups) and Devonian in the west (Chattanooga Shale and anoxic equivalents). Early ideas (Rich, 1931, 1933; Gussow, 1954) of long-distance migration have been recently validated (Walters, 1958; Price, 1980; Burruss and Hatch, 1989). Local generation may emplace significant amounts of oil, but the billions of barrels of oil produced in Kansas so far are likely to have migrated long distances from their source beds.

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Kansas Geological Survey
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