Deeper study of Precambrian warranted in western Ohio

July 29, 2002
The casual observer generally rejects the possibility of hydrocarbons occurring in the Precambrian basement rocks of Ohio and surrounding region. However, it appears that a major paradigm shift must occur as we continue to search for deeper resources.

OHIO PRECAMBRIAN-2

The casual observer generally rejects the possibility of hydrocarbons occurring in the Precambrian basement rocks of Ohio and surrounding region. However, it appears that a major paradigm shift must occur as we continue to search for deeper resources.

This section of the article will discuss the poorly understood and sparsely drilled Precambrian rocks that without a doubt have numerous undrilled prospects and may contain previously unidentified source rocks either in the Precambrian or Cambrian.

The tectonic setting of the region was presented in part 1 (OGJ, July 22, 2002, pp. 34).

Hydrocarbon possibilities

Typically, wells drilled through the Cambrian Mount Simon sandstone in Ohio and surrounding areas penetrate only a few tens of feet of highly weathered Precambrian igneous or metamorphic rock before reaching total depth. However, of the 207 Ohio Precambrian tests, 11 wells encountered Precambrian sedimentary rocks: red sandstone of the Middle Run formation and an unnamed black carbonate (see Fig. 1, part 1).

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Ten Precambrian test wells drilled in Indiana and Kentucky also encountered red sandstone below the Mount Simon sandstone (see Fig. 1, part 1). Available well log and core data indicate that the Middle Run is a poor quality reservoir rock.17 However, a Middle Run gas show at 3,120 ft was reported in the Yewey well in Mercer County, Ohio (well 2 on Fig. 1, part 1).

The most encouraging example of potential Middle Run reservoir rock at this time is in the Brooks well in Hart County, Kentucky, where a 120 ft-thick sandstone unit within the Middle Run was encountered at 8,090 ft (well 4 on Fig. 1, part 1). Sample examination of this interval indicates friable, poorly cemented sandstone.16 Electric logs through this interval are inconclusive regarding reservoir porosity, but more significant is the blow of gas from this sandstone. An operator was testing the Middle Run in the Brooks well at this writing. Source rock has yet to be encountered to account for gas shows from the Yewey or Brooks wells.

The Friend well in Clark County, Ohio (well 3 on Fig.1, part 1), penetrated a 700 ft-thick sequence of black carbonate rock and volcanic rhyolite ash flows11 below the Mount Simon and also is of exploration interest. The black carbonate has not been named, and little is known about reservoir quality and source potential.

The Friend well had a show of oil below the Mount Simon in the volcanics at 4,370 ft; this show is very significant to understanding the Precambrian petroleum potential of western Ohio.17 The stratigraphic/structural relationship of these pre-Mount Simon carbonates to the Middle Run is currently unknown.

Hydrocarbons are found in Precambrian rocks worldwide but generally not in great quantities. These strata have not been extensively probed in the subsurface beneath Paleozoic cover, especially in Ohio.

Black, organic-rich Precambrian shales as old as Archean (>2 billion years) are known to exist worldwide.25 Significant hydrocarbon production related to Precambrian source rocks has been reported from Siberia,25 Australia,26 Oman,27 and China.28

The Precambrian Keeweenawan Midcontinent Rift System (MRS) has considerable potential source rock from the Nonesuch shale of the Oronto Group of northern Michigan,29 the Rice formation of Kansas,30 and Unit C shales of Iowa.31

Thermal maturity of possible Precambrian source rocks in this region is not discussed for this article. The worldwide occurrence of Precambrian source rocks supports the concept that organic-rich shale similar to the Nonesuch shale and equivalents may exist in the ECRB. Furthermore, the possibility exists that marginal-marine to marine organic-rich shales and carbonates were deposited following the ECRB as the foreland basins developed.

The present study and earlier work32 suggest that similar possibilities exist across Ohio and into the Midcontinent area of North America, although Precambrian source rocks have yet to be discovered in the eastern Midcontinent.

Favorable targets

Figs. 5A to F show the seismic interpretations of several potential hydrocarbon traps across the Ohio COCORP OH-1 line (corresponding to lettered boxes of Fig. 2, part 1). These traps extend eastward from the ECRB to the western part of the GF, where the foreland basins are deepest.

Fig. 5A is centered at v.p. 325 and shows the thickest part of Middle Run sediments occurring east of a major basin-bounding normal fault. Little or no eastward thickening of pre-Middle Run strata indicates that normal faulting was initiated at about the start of Middle Run deposition. Faulting may have continued throughout Middle Run deposition, but there is no evidence indicating that it continued during deposition of overlying GP foreland basin-fill sediments. Favorable hydrocarbon traps also may occur throughout the Middle Run sedimentary sequence along the major normal faults and at possible facies changes on the downthrown side of faults.

Fig. 5B is centered at v.p. 515 and reveals a pronounced anticline in Middle Run sediments and a marked angular unconformity between the Middle Run and younger foreland basin sediments. Potential hydrocarbon traps occur on both flanks of the truncated Middle Run anticline and within the Middle Run sequence. Other possible traps exist on both flanks of the Middle Run anticline where GP foreland basin-fill sediments pinch out at an angular unconformity.

Fig. 5C lies at v.p. 780 near the eastern border of the ECRB where basin sediments dip westward. Potential hydrocarbon traps exist here where the Middle Run pinches out on an anticlinal rollover.

Prospects also exist above a pinchout in a GP foreland basin-fill anticline, truncated at the Precambrian-Cambrian unconformity. In addition, lenticular-shaped prospective traps occur beneath the pinchout.

These lens-shaped areas are interpreted as carbonate buildups, which onlap the high side of marked deepening ECRB.

The relationship of black carbonate from the Friend well in Clark County, Ohio, to these onlapping, lens-shaped areas is open for speculation. An alternative interpretation for these features would be igneous intrusive laccoliths.

Fig. 5D shows another prospect at v.p. 1020 east of the ECRB where Grenville thrusting has generated an anticline, truncated by the Precambrian-Cambrian unconformity.

This thrust-generated anticline extends downward into GP foreland basin-fill and has youngest foreland basin sediments pinching out at the unconformity on both flanks of the anticline.

Significant here is the fact that this Precambrian structure exerted long-standing influence on later sedimentation as the base of the Paleozoic shows a marked 500-millisecond (500-600-ft) dropoff west of this structure. Fig. 5E illustrates another Grenville thrust trap at v.p. 1130 similar to that beneath v.p. 1020.

On Fig. 5F, structure centered around v.p. 1270 reveals a pronounced Grenville basement high with numerous thrust faults on the forelimb into basin-fill. Other possible traps include pinchouts of basin-fill and erosional truncations against this thrusted Grenville basement high.

Conclusions

Detailed sequence stratigraphy of the Precambrian along the COCORP OH-1 seismic line shows that the ECRB predates the Grenville Orogeny and has a complex history of extensional tectonics, infill of clastic and volcaniclastic sediments, and structural deformation by Grenville orogenesis.

The upper part of ECRB fill consists of Middle Run sedimentary rocks, with overlapping foreland basin sediments from eroded Grenville orogenic highlands.

Numerous small Precambrian Grenville foreland basins developed across Ohio in association with the Grenville Orogeny.

Significant hydrocarbon possibilities exist in Precambrian sedimentary rocks of ECRB and GP foreland basin strata. Although the Precambrian of western Ohio has been sparsely drilled, oil and gas shows and known favorable source and reservoir rocks throughout the Midcontinent area warrant consideration for exploratory drilling of the prospects highlighted in this article as well as others along and adjacent to the COCORP OH-1 and OH-2 seismic lines.

Acknowledgments

The authors thank Thomas McGovern of Lauren Geophysical, Denver, for providing industry reprocessed COCORP OH-1and OH-2 lines; John Wicks for the interval velocity contour profile from the reprocessed stack data; Lisa Van Doren for computer drafting artwork; and Dennis Hull, Lawrence Wickstrom, and Merrianne Hackathorn for reviews.

References

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The authors

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Stuart L. Dean ([email protected]) is professor of geology, emeritus, at the University of Toledo, Toledo, with 35 years of teaching and research experience. His major interests and published works lie in structural geology and tectonics. He is a Fellow of the Geological Society of America and a cooperating research geologist with the West Virginia Geological and Economic Survey and the Ohio Division of Geological Survey. He has BS, MS, and PhD degrees in geology from West Virginia University.

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Mark T. Baranoski ([email protected]) is geologist with the Ohio Division of Geological Survey, Columbus, with over 20 years exploration and research experience. He worked for Gulf Oil in the Rocky Mountain region and consulted in the Michigan basin. His major interests include Precambrian tectonics and basin analyses. He has BS and MS degrees in geology from the University of Toledo.