Using Outcrops to Understand and Explain Subsurface Geological Phenomenon: Examples from the Upper Devonian (Famennian) Cleveland Shale Outcrop Belt, Greater Cleveland Metro Area, Ohio
Subsurface reservoir evaluation technology continues to advance in the oil and gas industry. Still, there are many aspects of geology that are difficult to communicate to non-geologists and geologists. Outcrops present an excellent opportunity to characterize the behavior of sedimentary successions in three dimensions that are not readily observable in core and cuttings. The dominantly black, organic-rich, Cleveland Member in Ohio display rhythmic bands of resistant, organic-rich shale beds that alternate with recessive, differentially clay-rich layers. This banding is often dramatic in outcrop, but may be cryptic from a core or log perspective. Weathering profiles in stream cuts demonstrate how these facies react to seasonal changes in water saturation. Organic carbon-rich, siliceous facies demonstrate a resistance to weathering, while less organic, clay-rich facies readily degrade when exposed to water. These observations may help characterize the subsurface behavior of mudstones when introduced to water during the drilling and completion phases. Further, the Cleveland Member readily displays the relationship of fractures to organic carbon content. While absent in organic-lean facies, jointing is common in organic-rich facies. Joints occur when pressure induced by hydrocarbon generation exceeds the tensile strength of the rock. These observations in the Cleveland Shale suggest that reservoir pressure within a shale unit is not consistent, but varies as a function of hydrocarbon generation and expulsion on a bedding scale. Finally, outcrops of the Chagrin-Cleveland shale succession in the greater metro area of Cleveland, OH provide an excellent view of subtle, yet important, spatial/temporal changes associated with the top-Chagrin- base-Cleveland Member regional “shale-on-shale” unconformity, not readily discernable in the subsurface; herein, we observe southeastward onlap of Cleveland Member divisions onto this unconformity in respective upward-order: gray basal Cleveland unit, followed by a black shale division, succeeded by the gray mudstone and siltstone-dominated Penitentiary Glen Shale marker bed. Following southeastward, upslope, pinch-out of the Penitentiary Glen bed, the black, upper part of the Cleveland Member comes to rest directly atop the Chagrin. Such an onlap process is discernable with good core and log control, but direct outcrop observation and correlation within the relevant interval is critical as ground truth perspective.
AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019