ABSTRACT

The search for hydrocarbons in the Lower Cretaceous Creybull Sandstone of the northern Bighorn Basin has been an ongoing process for almost a century. Exploration is complicated by difficulty in diagnosing the sedimentary environment of the hydrocarbon-bearing Himes Member (Moberly, 1960). This sand, which is generally called "The Creybull", has been catagortzed as a fluviatile meanderbelt facies (Shelton, 1968), a low sinuosity braided river deposit (Kvale and Vondra, 1986), and a shallow marine estaurine facies (Kvale, 1986). The economics are generally good, and hydrocarbon reserve estimates encourage further exploration efforts.

The difficult task of locating hydrocarbons in an uncertain depositional environment such as the Greybull requires the definition of certain facies parameters. These consist of: geometry, lithology, sedimentary structures, paleocurrent patterns, body fossils, and trace fossils (Selley, 1978). Fortunately there are excellent outcrops of the formation on both the east and west flanks of the northern Big Horn Basin as well as numerous well logs to aid in stratigraphic interpretation. The difficult exploration process is compounded, however, by the complicated block faulting on the flanks of the basin. The use of subsurface water resistivities (RWs), which is elsewhere a diagnostic tool in deliniating separate fault blocks, is essentially useless due to lack of pertinent data.

By analyzing stratigraphy and trapping mechanisms in existing fields such as the Greybull and Elk Basin Fields and using them as analogues, one can then apply these exploration models to areas of lesser control, and thus continue the search for the elusive Greybull.