Stratigraphic, Structural, and Geomorphologic Factors Controlling Oil Accumulation in Upper Cambrian Strata of Central Ohio

Abstract

The interrelations of stratigraphic, structural, and paleogeomorphic factors must be known if the erratic distribution of oil accumulation in the Cambrian sediments of Morrow County, Ohio, is to be understood. The Copper Ridge Dolomite is the highest Cambrian formation in Morrow and Marion Counties, Ohio. It is truncated by the post-Knox unconformity which is the Cambrian-Ordovician boundary. Three alphabetized zones of the Copper Ridge subcrop at the unconformity. Zones E and F consist of saccharoidal, porous, vuggy dolomite. Zone F, the stratigraphically highest zone, is present at the tops of the highest dolomite hills, whereas zone E subcrops on the flanks of the highest hills and caps the lower erosional remnants. Zone D is a fine-textured, nonporous, argillaceous dolomite and has the most extensive subcrop aerial distribution of the three zones. It is confined to the valley floors which separate the numerous hills. The pre-Knox unconformity structure consists of a closely spaced series of southeast-trending anticlines and synclines. Denudation of the structurally wrinkled zones of the Copper Ridge Dolomite caused the development of a trellis drainage system. The principal drainage courses were formed on anticlinal crests, and the divides which separated them are the synclines. The result is that the two highest zones of the Copper Ridge Dolomite are preserved as synclinal erosion remnants. The lower Chazy is the oldest Ordovician formation on the Knox unconformity in Marion and Morrow Counties. It consists of interbedded shale, siltstone, and argillaceous limestone which were deposited initially in the topographically low areas on the Knox unconformity. The Chazy surrounded the highest hills and buried the lower ones. From the weight of overburden, the lower Chazy was differentially compacted. The top of the middle Chazy is a good lithologic datum of reference. An isopach map of the interval between this datum and the base of the D zone of the Copper Ridge Dolomite illustrates the trends, widths, and positions of the pre-unconformity structures. Vertical exaggeration of the anticlines and synclines, however, is related directly to the amount of differential compaction o the lower Chazy sediments. These effects are eliminated by selecting a bentonite reference datum approximately 450 ft higher stratigraphically than the lower Chazy reference datum. Cambrian oil accumulation occurs only in the porous and permeable erosional remnants of zones E and F of the Copper Ridge Dolomite. The hydrocarbons probably originated in the lower Chazy shales. Northwestward lateral migration from these shales caused displacement of the original connate brines of the reservoirs. The regional tilt of the unconformity is eastward. Thus, the erosional remnants were filled progressively with oil in an updip direction. Lack of sufficient hydrocarbons and general absence of zones E and F on the west account for the scarcity of oil pools in Marion County.