--> Sedimentology, Stratigraphy, Organic Content and Reservoir Characteristics of Utica Shale Cores, Central Ohio

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Sedimentology, Stratigraphy, Organic Content and Reservoir Characteristics of Utica Shale Cores, Central Ohio


Five long cores of the Utica-Point Pleasant-Trenton interval were studied in detail. Thin sections were described semi-quantitatively and used to aid in cm-scale core description. TOC and carbonate content were analyzed down to a one foot resolution in organic-rich intervals. The basal Kope and uppermost Utica Shale consist of interbedded gray and black shale. The black shale beds have TOC up to 3%. Carbonate content is generally <10% in this interval. Below that is an interval of black shale with TOC up to 3.5% and carbonate content generally around 25%. At the base of the Utica is an interval consisting of gray shale and coarse limestone storm beds with abundant bryozoans. This interval is sometimes called upper Point Pleasant but it is organic-poor with TOC generally <1% and carbonate content of 25-50%. There is an apparent unconformity at the top of this interval. The organic-rich limestone interval that is the main target for drilling is composed of three formations: Point Pleasant, Lexington and Logana. The organic-rich Point Pleasant has abundant carbonate storm beds. TOC values are up to 5% and carbonate content is generally in the 40-60% range. The Lexington Limestone has higher carbonate content and some well-developed limestone beds at the top where there is evidence for an erosional unconformity. There is very high TOC immediately above and below this apparent unconformity with values up to 4.5%. The Lexington generally has carbonate content around 70%. The underlying Logana Formation has TOC up to 5%. The upper Logana has few limestone beds, but the lower part has some laterally extensive brachiopod rudstone beds. The organic-rich beds have abundant ostracods, which may be planktonic. There is good evidence for storms throughout the main organic-rich reservoir interval and many of the organic-rich intervals appear to be burrowed. There are benthic fossils within the organic-rich facies that are likely to be in situ. This suggests that the organic-rich strata were deposited in a relatively shallow water environment that was affected by frequent storms. Water conditions were therefore not permanently anoxic but may have been seasonally anoxic. No visible porosity was found in any of the thin sections studied. Porosity in the Utica is likely to be primarily organic porosity formed during maturation of hydrocarbons. This makes it different than some other liquids plays which generally have a component of matrix porosity.