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

INTRODUCTION

Two fundamentally different types of hydrocarbon plays were recognized in the regional, Lower Silurian oil and gas accumulation of the Appalachian basin for the U.S.Geological Survey (USGS) 1995 National Assessment of United States Oil and Gas Resources (Ryder, 1995; Ryder and others, 1996; fig.1).

The western part of the accumulation in east-central Ohio and northwesternmost Pennsylvania was defined as a play with conventional reservoirs and discrete fields, whereas an eastern part of the accumulation in eastern Ohio, northwestern Pennsylvania, western New York, northwestern West Virginia, and northeasternmost Kentucky was defined as a group of plays with unconventional reservoirs and regionally continuous zones of gas accumulation (fig. 1). The plays in the western and eastern parts of the regional accumulation were categorized, respectively, by the USGS as discrete (conventional) and continuous-type (unconventional) accumulations (Schmoker, 1995a,b). Major reservoirs in the Lower Silurian sandstone sequence consist of the "Clinton" sands and Medina sand in Ohio and adjoining West Virginia and their respective equivalents, the Grimsby Sandstone and Whirlpool Sandstone of the Medina Group, in Pennsylvania and New York. These sandstone units were deposited in shallow marine to estuarine environments. A third hydrocarbon play in the sequence, the Tuscarora Sandstone of west-central Pennsylvania, south-central New York, and west-central West Virginia is an eastern and more proximal facies of the Lower Silurian sandstone depositional system with a moderate fluvial component (Ryder, 1995; fig. 1). Although part of the regional Lower Silurian hydrocarbon accumulation, the Tuscarora Sandstone gas play with its generally small scattered gas fields is not discussed any further in this report.

General attributes assigned to the western (discrete/conventional) play in the USGS 1995 National Oil and Gas Assessment are: 1) discrete fields controlled by well-defined stratigraphic and (or) structural traps, 2) oil- and (or) gas productive regions surrounded by nonproductive regions, 3) sandstone reservoirs with good to moderate porosity and permeability, 4) well-defined oil- and gas-water contacts, 5) normal (hydrostatic) fluid pressures, 6) hydrocarbon production dominated by oil and associated gas, and 7) moderate to high water yields. Using a play analysis approach, where field size is the basic unit of assessment, the USGS estimated that, because of the densely spaced drilling in the play since the late 1870s, there are no remaining undiscovered fields of 1 million barrels of oil or greater (or 6 billion cu ft of gas or greater) except beneath Lake Erie. Thus, most of the remaining hydrocarbon resources in the western play were considered to be additions (growth) to existing reserves (Gautier and others, 1995; 1996).

The eastern (continuous-type/unconventional) plays were adopted from a deep basin/basin-centered model for the Clinton/Medina gas accumulation proposed by Davis (1984), Zagorski (1988, 1991), and Law and Spencer (1993). In this report, the term basin-centered accumulation is used. Following Rose and others (1986) and Law and Spencer (1993), a basin-centered gas accumulation is a regionally extensive and commonly very thick zone of gas saturation that occurs in low-permeability rocks in the central, deeper part of a sedimentary basin. Foreland basin examples, such as the Lower Silurian of the Appalachian basin, reside in the thicker, more deeply buried part of the characteristic cratonward-tapered sedimentary wedge that lies updip of the thrust-faulted margin of the basin. Basin-centered gas accumulation is a variety of continuous-type accumulation (Schmoker, 1995a,b) and is synonymous with deep-basin gas accumulation (Masters, 1979; 1984). Tight gas (low-permeability) reservoirs described by Dutton and others (1993) are a necessary but not a sufficient condition of basin-centered gas accumulation.

General attributes assigned to the eastern (continuous-type/unconvential) plays in the USGS 1995 National Assessment ("Clinton" sands and Medina Group sandstone part of basin-centered accumulation of this report) are: 1) regionally extensive accumulations of nonassociated gas without stratigraphic or structural control on entrapment, 2) gas shows and (or) production after hydrofracturing in most wells drilled, 3) sandstone reservoirs with low porosity and permeability, 4) an absence of gas-water contacts, 5) rocks with high water saturation are situated updip of the gas accumulation and serve as the regional trap, 6) abnormally low formation pressures, and 7) low water yields. Because of the recency of drilling and production involving the eastern continuous-type plays (late 1970s to present), this part of the regional Lower Silurian accumulation is considered by the USGS to be an emerging source of natural gas. Using a new methodology (Schmoker, 1995b) that assumed a regionally extensive gas-saturated reservoir and used estimated ultimate recovery (EUR) of gas per well as a basic unit of assessment, the continuous-type plays were estimated to have, at a mean value, about 30 trillion cu ft (TCF) of technically recoverable gas (Gautier and others, 1995; 1996).

Primary objectives of this report are to compile a reliable set of oil and gas field data from the Lower Silurian regional accumulation and to develop criteria for subdividing it into discrete and basin-centered parts. Comprehensive field studies required for in-depth analysis of the Lower Silurian regional accumulation are sparse. However, numerous attributes of selected fields are available in the literature (Laughrey, 1984; Keltch and others, 1990), unpublished theses (Seibert, 1987; Zagorski, 1991), and published data sets (McCormac and others, 1996). At least two important elements of the second objective will be addressed. First, many of the criteria applied to the Lower Silurian regional accumulation in the 1995 National Assessment (Ryder, 1995) were based on Rocky Mountain basin analogues and, thus, need to be more rigorously tested with reliable Appalachian basin data. Secondly, the tentative boundary drawn by Ryder (1995) between identified discrete and continuous-type (basin-centered in this report) parts of the regional hydrocarbon accumulation needs to be validated. This boundary is presently located at the approximate downdip (eastern) limit of oil production (see fig. 2; Wandrey and others, 1997). Both elements identify a need to better define the nature and origin of a very complex hydrocarbon system.

Field-specific data collected for this investigation — representing a variety of geoscience disciplines—and conclusions reached from them are expected to increase the accuracy of future assessments of remaining recoverable natural gas and oil in the Lower Silurian regional accumulation. Assessment accuracy is expected to increase because of: 1) the better definition of the discrete and basin-centered parts of the accumulation each of which requires a different assessment methodology, and 2) a better understanding of the internal variability of the accumulation such as the distribution of production "sweet spots". Moreover, criteria developed for characterizing these Appalachian basin examples may be applicable to other domestic and foreign examples of regional hydrocarbon accumulations.

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