Abstract

Ohio's first oil boom occurred from 1890 to 1905 when the Trenton Dolomite field of northwestern Ohio was the largest oil field in the world. Now, in 1964, Ohio is again the scene of another oil boom, this time the target is the Cambrian dolomite of central Ohio. Monthly production in Morrow County increased from 36,835 barrels in January, 1963, to 452,871 barrels in December, 1963.

Stratigraphically, Ohio is composed of Paleozoic sedimentary rocks resting upon Precambrian metamorphic rocks, with a cover of Pleistocene glacial drift over the northwestern two-thirds of the state. All Paleozoic systems are represented, which, in general, thicken toward the southeast. The regional structure of Ohio is dominated by the Cincinnati Arch, a northward slightly plunging axis in the western part of the state. A gentle eastward regional dip into the Appalachian basin is common, except (1) in northeastern Ohio where dip is to the south, (2) in northwestern Ohio where dip is northwestward around the Michigan basin, and (3) in southwestern Ohio where dip is northward around the Cincinnati dome.

Oil and gas has been produced in Ohio since 1860. The productive areas of the state are in general (1) the Shallow Sand Field of eastern Ohio, which produces from stratigraphic traps and local structures in rocks of Pennsylvanian, Mississippian, and Devonian age, (2) the "Clinton" (Albion) Field of east-central Ohio, which produces from stratigraphic porosity traps in Silurian dolomites and sandstones, (3) the Trenton Field of northwestern Ohio, which produces from dolomitized Ordovician limestones, and (4) the Central Cambrian Field, which is producing from stratigraphic traps in dolomites below the Knox (post-Beekmantown) unconformity. Some overlapping of producing zones occurs between various fields. The Trenton field is now practically plugged out and many older pools in the "Clinton" and Shallow Sand fields are abandoned.

Present interest is primarily focused upon the petroleum potential of rocks of the Sauk Sequence (Knox unconformity to basement complex). Formations of the Sauk Sequence are, in ascending order, (1) Mt. Simon (basal) Sandstone, (2) Shady Dolomtie, (3) Rome Formation, (4) Conasauga Shale, (5) Maynardville Dolomite, (6) Copper Ridge Dolomite, and (7) Chepultepec Dolomite. The lower four formations, Mt. Simon Sandstone to Conasauga Shale inclusive, contain much clastic material and are placed in the Montevallo Supergroup. The upper three formations, Maynardville Dolomite to Che-pultepec Dolomite inclusive, consist mainly of dolomite and comprise the Knox Dolomite Supergroup.

Sedimentation studies indicate that rocks of the Sauk Sequence were deposited in a transgressing sea which advanced from the southeast. The stratigraphic succession of Cambrian and Ordovician rocks of the Appalachian basin is clearly evident across Ohio. In northeastern Indiana, however, a transition occurs between the Appalachian basin carbonate facies and the Upper Mississippi Valley clastic facies of Wisconsin and Minnesota.

Cross sections reveal that the Sauk Sequence is truncated northward beneath the Knox unconformity. Petroleum production is related to stratigraphic traps below the unconformity. Chepultepec (Beekmantown) rocks have produced small quantities of oil and gas to the south in Kentucky. Copper Ridge (Trempealeauan) Dolomite is producing in central Ohio. Shady Dolomite is the reservoir rock of the Clearville pool in southern Ontario, and the wedge-edge of the Mt. Simon Sandstone is productive in the Gobles pool of central Ontario.

An isopach map of the Sauk Sequence shows a narrow, north-south area of thin Sauk in central and southern Ohio, over which lower beds are relatively thin or absent. This is interpreted as a Precambrian buried ridge known as the Waverly Arch. A lesser ridge may be present in eastern Ohio. In central-northern Ohio, isopach studies indicate a Precambrian platform in the vicinity of Lake Erie.

Oil accumulations of Morrow County are in stratigraphic traps of the erosional remnant type. Many are buried hills of local areal extent (100 to 300 acres), but most have high relief (100 to 200 feet) with pay sections up to 150 feet or more in thickness. Angle of west slope appears to be the critical trapping factor in remnant reservoirs. The Lower Chazy Dolomite ("Glenwood") and part of the Middle Chazy Limestone are generally missing by non-deposition. Commonly, secondary dolomitization of the Middle Chazy Limestone has occurred above the unconformity in these pinnacle type remnants, making the top of the Cambrian difficult to find.

Accumulation in the Marengo area of Bennington Township is apparently in a buried ridge of low relief, with 20 to 30 feet of overlying Lower Chazy Dolomite present. Pay thicknesses in remnants of the buried ridge type commonly range from 5 to 20 feet.

Erosional remnants are gas-solution type reservoirs with a possible moderate water drive. Porosity commonly varies from 6 to 20 percent and permeability from 1 to several hundred millidarcies. Water saturation is commonly 18 to 25 percent. Initial gas-oil ratios of 300 to 400 cubic feet per barrel increase gradually with production. Primary reserves are conservatively estimated at 140 barrels per acre foot, based upon 25 percent recovery of 560 barrels per acre foot in place.

Statigraphic traps due to truncation, sand pinch-outs, permeability barriers, and erosional remnants below the Knox unconformity may be present in all parts of Ohio. Lack of information concerning structure in the Sauk Sequence does not rule out the possibility of structural accumulations. To the present time, the great bulk of Cambrian production has been from erosional remnants in Morrow County. Extensive exploration in Ohio is expected to continue for at least several years.