Geochemical Interpretation Pitfalls in the Utica, Point Pleasant, and Trenton Formation (Upper Ordovician) Tight Petroleum Play, Eastern Ohio, USA

C. D. Laughrey¹, T. E. Ruble², L. H. Wickstrom³, J. Kostelnik¹, R. J. Drozd², L. R. Heim⁴, and G. A. Alcorn^5
1Weatherford Laboratories, Golden, CO, [email protected],
²Weatherford Laboratories, Houston, TX,
³ODNR Division of Geologic Survey, Columbus, OH,
⁴Ardent Resources, Pittsburgh, PA,
⁵Point Pleasant Energy LLC, Houston, TX

Organic-rich mixed-carbonate/siliciclastic mudstones in the Upper Ordovician Utica, Point Pleasant, and Trenton Formations are important source rocks in the northern Appalachian basin. These rocks generated most of the petroleum produced from various Cambrian through Silurian reservoirs in the basin, including hydrocarbons produced from lower Paleozoic reservoirs on the basin flank in southwestern Ontario. These rocks are the target of unconventional tight-oil and shale-gas exploration and development, particularly in eastern Ohio. This activity is dictated by geology and geochemical screening parameters routinely employed to map and constrain the generative potential, kerogen type/expelled product, and thermal maturity of source rocks. These parameters include TOC, the quantities S1, S2, S3, and T_max from programmed pyrolysis, HI, OI, PI, and S2/S3 calculated from these measurements, and optical maturation parameters from organic petrology.

Careful scrutiny of data derived from these screening methods, and comparisons with data from more advanced geochemical tools such as stable isotope, light hydrocarbon, and biomarker analyses, reveal potential pitfalls in interpreting the geochemistry of these rocks. These pitfalls include underestimation of present day and original TOC due, respectively, to sample dilution and erroneous HI values. The HI error originates with suppression of programmed pyrolysis yields due to mineral matrix interference, organic matter variations, and contamination by high-molecular weight bitumen components. The latter problem is common in these rocks and results in bimodal S2 peaks and anomalously low Tmax values. Calculated vitrinite reflectance values from T_max are usually fallacious, as are many directly measured reflectance values made on various macerals and solid bitumen. Additionally PI values are often misleading due to expulsion and migration because ±75% of the petroleum generation process is complete in these intervals and expulsion efficiency is relatively high.

Other pitfalls occur with comparative sedimentology efforts in vogue in the play, especially in comparisons of the Point Pleasant and Cretaceous Eagle Ford tight oil play. Deposition, porosity, and storage mechanisms in these rocks have little in common and a comparison of these systems is egregious.

AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012