Characteristics of discrete and basin-centered parts of the Lower Silurian regional oil and gas accumulation, Appalachian basin: Preliminary results from a data set of 25 oil and gas fields

U.S. Geological Survey Open-File Report 98-216

DISCUSSION

The Clinton/Medina/Tuscarora basin-centered gas accumulation appears to differ significantly from the classic Cretaceous and lower Tertiary examples of the U.S. and Canadian Rocky Mountains and Great Plains (Masters, 1979, 1984; Law and Dickinson, 1985; Spencer, 1987). An important difference is the very wide transition zone that occurs between the Clinton/Medina/Tuscarora basin-centered, gas-bearing part of the accumulation and its updip discrete, oil- and gas-bearing part of the accumulation. Characteristics of this broad transition zone are: 1) low to modest water (brine) productivity, 2) a 50- to 75-mi-wide zone of oil-bearing strata in Ohio and northwestern Pennsylvania, 3) a general absence of hydrocarbon/water contacts, and 4) a continuous network of adjoining oil and gas fields. This zone of transition is so wide and the accompanying fields have coalesced to such a degree that the entire regional accumulation could be interpreted as a continuous-type accumulation.

One reason for the wide transition zone is the lack of an abrupt, regionally extensive depositional and (or) diagenetic facies change in the Clinton/Medina sandstone reservoirs. The sandstone bodies that constitute the reservoirs are so highly lenticular and complexly intertongued that they have similar properties, such as porosity and permeability, across a broad region. A general trend of eastward-diminishing porosity and permeability is noted across the regional accumulation but the rate of change is very gradual and, thus, no well-defined boundary exists between its discrete and basin-centered parts.

A second reason for the wide transition zone is that the regional accumulation has been substantially modified from its original basin-centered configuration. Among the probable post-generation/emplacement modifications to the regional accumulation are uplift and erosion, re-migration and slow leakage of constituent hydrocarbons, and mixing of several types of brine. A generalized model shown in figure 12 suggests how the Lower Silurian regional accumulation may have originated and how it was modified by post-entrapment processes. Important parts of the model include: 1) generation of oil and gas from a Middle Ordovician black shale source rock (Cole and others, 1987; Drozd and Cole, 1994; Ryder and others, 1998); 2) abnormally high fluid pressures caused by the transformation of oil to gas (Law and others, 1998), 3) updip displacement of moveable pore water by overpressured gas (Law and Dickinson, 1985), 4) post-orogenic uplift and erosion resulting in temperature decline, pressure loss, hydrocarbon remigration and loss, and basinward retreat of the updip limit of the basin-centered gas (Law and Dickinson, 1985), and 5) mixing of basin-centered and discrete parts of the accumulation with high-density brine derived from the Upper Silurian Salina Group.

The timing of mixing between Salina Group- and basin-center-derived brine is important for understanding the origin of the regional accumulation. The model shown in figure 12 suggests that brine from the Salina Group was introduced in Mesozoic time, much later than gas generation and entrapment in late Paleozoic time. B. E. Law (written communication, March 1998) favors pre-gas emplacement for the Salina Group-derived brine because of the unlikelihood that saline water has re-entered "Clinton" sands and Medina Group sandstone reservoirs during the underpressuring phase.

Because of its high degree of modification, one should perhaps also think about a paleo-configuration as well as a present-day configuration when classifying the accumulation as a whole. An understanding of the paleo-configuration of the accumulation would require reconstruction of the "zone of retreat" to determine the maximum updip extent of the original basin-centered gas. However, the most practical approach to classifying the Lower Silurian regional accumulation and its components, particularly for resource assessment purposes, is to work within the present-day framework established by Davis (1984), Zagorski (1991, 1996), and Ryder (1995). A tentative classification scheme that incorporates their contributions is shown in figure 13. This classification recognizes three parts of the regional accumulation: 1) an eastern, basin-centered part, 2) a western, quasi-discrete part, and 3) a wide, central hybrid part. This classification is imperfect because of the very subtle differences between its three components but it incorporates factors that are relevant to an understanding of the accumulation’s origin as well as to its recoverable energy resources.

Other than for historical and location purposes, the term field is meaningless as an assessment unit for this regional accumulation. In practice, designated fields within the regional accumulation are production "sweet spots" having high EURs per well. Because of the lenticular and intertonguing nature of the sandstone reservoirs, wells surrounding the sweet spots also are gas and oil productive but characteristically they have lower EURs per well. Development and exploration drilling around given sweet spots in the regional accumulation gradually evolve into an elaborate hydrocarbon mosaic whose constituent parts have differing ultimate production capabilities. This important characteristic of the Lower Silurian regional accumulation must be considered when assessing its potential for remaining recoverable gas and oil resources.

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