James P. Rogers

New reservoir model from an old oil field: Garfield conglomerate pool, Pawnee County, Kansas

Garfield field, an old (1948) stratigraphic trap in central Kansas, produced 10 million bbl of oil from Cherokee (Desmoinesian, Pennsylvanian) conglomerate reservoirs and from Mississippian carbonate and sandstone reservoirs. The primary reservoir rock at Garfield conglomerate pool is the Pennsylvanian basal conglomerate (PBC), unique in that it accumulated as an unstratified alluvial-fan deposit, in a subtle structural depression that may be coincident with a microplate boundary. Each of these conditions is uncommon in the northern Mid-continent. The PBC at Garfield pool is composed of pebbles, cobbles, and boulders of reworked Mississippian chert and limestone in a matrix of porous, fine-grained sandstone and sandy shale. Microporosity in the chert cobbles has access to the borehole through the porous sandstone matrix. The sandstone matrix was the product of reworking of the original shaly sand matrix and older sandstones from the substrate during short-term Desmoinesian marine transgressions of the alluvial-fan surface. Ultimately, Marmaton transgressive onlap smoothed and buried the fan surface. A near-modern alluvial fan at Rocky Flats in central Colorado provides an ideal analog for the PBC deposit at Garfield pool. The Rocky Flats fan is nearly identical in scale, areal distribution, and thickness, and its constituents are remarkably similar to conglomerate zones, which produce hydrocarbons at Garfield pool. Future exploration in north-central Kansas and southern Nebraska should focus on the occurrence of conglomerate deposits in similar depositional settings where coincident with structural depressions.

Dry Creek Field, Nebraska: Subsurface Methods Case History: ABSTRACT

Dry Creek field produces oil from thin Lansing (Pennsylvanian) limestone beds at a depth of about 4,000 ft (1,200 m) in central Hitchcock County, Nebraska. This subtle oil accumulation was discovered in 1963 by good fortune and weakly quantified subsurface geology; it remains geologically ill-defined. Conventional subsurface studies have been of little value in explaining the oil accumulation at Dry Creek field. We have developed a method for evaluating thin Lansing reservoir beds that incorporates careful analysis of cuttings and logs of early vintage. This method facilitates mapping Lansing reservoir distribution, thereby permitting a better understanding of the subtle trap at Dry Creek. We believe this method can be used as an additional subsurface exploration tool in the northern Mid-Continent. Subsequent to original publication of the methods described in this study, additional wells have been drilled at Dry Creek field. Some of these wells supported our original interpretation of reservoir distribution at Dry Creek, while others did not. Recently, the geologic techniques developed at Dry Creek field have been used successfully in exploration drilling in Decatur County, Kansas. End_of_Article - Last_Page 1319------------

Dry Creek Field, Nebraska: Subsurface Methods Case History

Dry Creek field produces oil from thin Lansing (Pennsylvanian) limestone beds at a depth of about 4,000 ft (1,200 m) in central Hitchcock County, Nebraska. This subtle oil accumulation was discovered in 1963 by good fortune and weakly quantified subsurface geology; it remains geologically ill-defined. Conventional subsurface studies have been of little value in explaining the oil accumulation at Dry Creek field. We have developed a method for evaluating thin Lansing reservoir beds that incorporates careful analysis of cuttings and logs of early vintage. This method facilitates mapping Lansing reservoir distribution, thereby permitting a better understanding of the subtle trap at Dry Creek. We believe this method can be used as an additional subsurface exploration tool in the northern Mid-Continent.

Characteristics of Tidal Sedimentation in Phosphoria (Permian) Strata at Cottonwood Creek Field, Big Horn Basin, Wyoming: ABSTRACT

Phosphoria reservoir rocks at Cottonwood Creek field, in the Big Horn basin, exhibit many characteristics of sediments deposited by a confined current, such as those in channels in modern carbonate tidal flats. Such physical properties, observed in thin sections and hand samples, are marked in contrast to the appearance of the nonporous carbonate facies of the Phosphoria. Nonporous strata, updip and laterally adjacent to this large stratigraphic oil accumulation, have properties common to sediments of interchannel and supratidal environments observed in modern tidal flats. These observations, in conjunction with the problem of anhydrite pore-filling and fracture distribution, help to explain the distribution of the reservoir and trap at Cottonwood Creek field, and may hav application in exploration elsewhere in this province. End_of_Article - Last_Page 1393------------