Paleogeomorphic mapping applied in Stagecoach gas field, N.Y.

Arthur J. Pyron
Pyron Consulting
Pottstown, Pa.

New York State Energy Research and Development Authority (Nyserda) and the author were partners on an 11 month demonstration project using Pyron Consulting's paleogeomorphic mapping technique to evaluate and identify stratigraphic traps in New York's Appalachian basin.

This work involves a subsurface mapping technique that uses sequence stratigraphic methods to identify potential new reservoirs. This technique has been successfully used on a variety of lithologies and reservoir types in both U.S. and non-U.S. basins and has revealed itself to be a precise indicator of economic accumulations of hydrocarbon reserves.

Having developed a mapping tool that can identify stratigraphic traps, this study attempted to address a larger question: Can subsurface mapping be used in combination with historic production data (or other relatively inexpensive data) to determine whether a given prospect is economically and technically viable?

Pyron Consulting developed its paleogeomorphic mapping technique as a tool that small independents could use to identify reservoirs where entrapment is associated with stratigraphic or diagenetic processes. The paleogeomorphic mapping technique can be used as a primary decision maker to select the optimum locations for well placement or lease acquisition, to guide the investment of capital, and to maximize the potential for success. In the event that a company requires seismic support data, then the paleogeomorphic mapping technique can be used to reduce proportionally that part of the exploration budget dedicated to purchasing seismic by limiting the purchase to the area of ultimate drilling.

The Appalachian basin

The Appalachian basin is a Paleozoic basin of approximately 100 million acres that extends over 10 states (Fig. 1 [74,681 bytes]). Historically, exploration in the basin is said to have begun in 1859 with the Drake well in Titusville, Pa.

The Appalachian basin hosts well over 500,000 oil and gas wells, average depth 3,700 ft. Production, primarily natural gas, comes from Pennsylvanian, Mississippian, and Devonian formations. Additional development occurs in Silurian, Ordovician, and Cambrian age rocks; these older Paleozoic formations generally form the basis for new exploration in the basin.

The basin's regional structure has been affected by the thrusting which formed the Appalachian Mountain range, known informally as the Eastern Overthrust. This thrust faulting is important because it creates zones of fractured porosity and localized structural traps. Secondary porosity formation is essential to the economics of production, especially in the eastern part of the basin, because naturally occurring interstitial porosity is rare, especially in clean silicilastic sediments like the Oriskany sand.

Source rocks for hydrocarbon generation fall in two sequences. The younger sequence includes marine shales of Middle Devonian to Early Mississippian age. The older sequence consists of marine shales and nodular limestones of Middle to Late Ordovician age. In addition, interbedded limestones, coal beds, and siltstones might provide regional sources of hydrocarbon for individual reservoirs.

Devonian rocks

Pyron Consulting recognizes three general groupings of Devonian rocks: the Genesee Group, the Hamilton Group, and the Helderberg-Onondaga Group.

The Genesee Group consists of a thick sequence of organically derived shales. In many studies, the Genesee is considered one of the more significant tongues of deposition that comprise the Catskill delta. Locally eroded, the sequence is not completely present in Tioga County, N.Y.

At the base of this sequence lies the Tully limestone. Based on the analysis by Heckel,¹ the Tully represents a significant unconformity between Genesee and Hamilton sequences. In addition, the Tully also represents a significant lateral facies change from a clastic equivalent farther east, to a carbonate over much of the Appalachian basin.

Many observers consider the Hamilton Group to be the precursor of the Catskill delta. It is composed of a thick sequence of black and gray organic shales that apparently represents a cyclical depositional cycle, as well as a perceived upward coarsening of sediments within a given cycle.²

Within this sequence of rock, two formations, the Cherry Valley limestone and the Marcellus shale, have been identified as easily recognized markers within the Hamilton Group and have been used for structural isopach mapping.

The last Devonian interval of significance in this investigation is the Helderberg Onondaga grouping. This interval includes a basal carbonate member (the Helderberg Group), a middle arenaceous sandstone member (the Oriskany sandstone), and an upper carbonate member (the Onondaga limestone Group).

These rocks were deposited in a nearshore marginal marine transition zone. They are of importance in this study because the Helderberg and the Oriskany form significant reservoirs in the Appalachian basin. As a result, there is a great deal of subsurface data on these intervals, especially as a result of exploratory drilling.

The Devonian rocks of Tioga County unconformably overlie Silurian evaporites of the Salina Group. A Silurian age "salt basin" formed to the south of Tioga County and allowed deposition of gypsum, anhydrite, and halite. Rickard³ noted the similarity and uniformity of deposition of the evaporitic sequence in both the Appalachian and Michigan basins, with the exception that the barrier reefs found in the Michigan basin are not found rimming the Appalachian basin.

Stagecoach field model

Stagecoach field is in southeastern Tioga County, N.Y. It was discovered in 1987 with the drilling of the Belden & Blake (now Quaker State) 1 Fyock well. The field was confirmed with the drilling of the Belden & Blake (Quaker State) 1 Racht well.

Operation of the productive wells in the field, and additional developmental drilling, was transferred to Belden & Blake Corp. in the early 1990s.

Currently, the field area has 12 productive wells and 15 dry holes. As of early 1996, cumulative production was 7.88 bcf of natural gas, with more than half that coming from two wells, the Belden & Blake W. Widell 1 (2,295.36 MMcf) and the Belden & Blake E. Campbell 1 (1,792.65 MMcf).

The well index (Table 1) and well location map (Fig. 2 [53,315 bytes]) provide detailed information on Stagecoach field wells.

The state agency lists Stagecoach production as being from the Upper Helderberg formation. Discussion with the field operators suggests that they believe production is associated with a significant fracture that intersects the Oriskany sandstone.

Of even greater interest is that these wells are producing dry natural gas with little associated water. Based on review of New York Department of Environmental Conservation files, it appears that many wells are producing through natural flow with no completion or induced fracturing treatments. A type log for the field, the W. Widell 1 (Fig. 3 [47,446 bytes]), shows very clearly the pronounced fracture at the top of the Helderberg section.

Pyron Consulting began assembling subsurface and isopach maps on select intervals based on well log interpretation and interpolation. An example of a structure map for one of the horizons is found in Fig. 4 [56,466 bytes]. A complete selection of structural and isopach maps is included in the final report, which Nyserda's Albany office distributes.

After completion of the various isopach and structural maps, a paleogeomorphic or synchronous high map was constructed for the field area (Fig. 5 [58,948 bytes]). The paleogeomorphic map interval top was the Cherry Valley limestone, which many geologists believe represents an unconformity surface. The base of the interval is the top of Silurian. In those wells in which the Silurian was not encountered, interpolation of the projected top of the Silurian was completed by correlation between wells. This interval is referred to as the First Derivative Interval, and it represents the type of interval that would be mapped under the paleogeomorphic mapping method.

The subsurface mapping exercise in Stagecoach field provides an interesting evaluation of subsurface conditions in the field area. To evaluate these maps, the following evaluation parameters were established:

The datum points for a particular interval represent possible conditions in the subsurface.
The interpolated maps provide a reasonable interpretation of subsurface conditions and relate well to the production history of the wells.
When all data for the area are integrated, the synergized history can be represented by the cumulative, per well production.
Based on the derived mapping, suggestions can be made that will either highlight new development opportunities or provide a logic for development of an exploration model that can be used elsewhere in the basin.

Using these evaluation parameters, several interesting interpretations can be made based on the subsurface data:

• Without exception, the structural maps created on various datums were neither sensitive nor reliable enough to relate to cumulative production. As a result, these maps were not reliable indicators of production. The structure map based on the Top of Devonian datum was interesting because it apparently shows the location of fault blocks under the field boundaries. Given this interpretation (which has not been verified either by seismic investigations or evaluation of published structure maps), the relationship of faulting to reservoir quality is significant to production in this field.

• The isopach maps which were prepared are slightly more reliable indicators of production but still not sensitive enough to equate to cumulative production histories. Of greater interest are the isopach maps showing thickness of the Oriskany sandstone and the thickness map of the Tully limestone. In the former, the interpreted thickness of the Oriskany sand increases to greater than 90 ft along the center of the field. (It is important to recognize that sand thickness is not an indicator alone of reservoir quality; the presence of fractures is also very important). The Tully limestone isopach map (Fig. 6 [44,156 bytes]) shows thinning of this interval over the top of the field.

• The paleogeomorphic map based on the First Derivative Interval is the most sensitive map interval produced during the subsurface mapping program. This map identifies interval thinning typical of a paleogeomorphic high. Thinning of the mapped interval is synonymous with hydrocarbon accumulation and production (Fig. 7 [46,255 bytes]).

Based upon integration of the subsurface mapping, production data, and other subsurface information, it is apparent that several of the subsurface maps that were created can be directly applied to hydrocarbon occurrences in Stagecoach field. Using the criteria established above, it appears that the First Derivative Interval, a paleogeomorphic map, is an indicator of the location of economic accumulations of natural gas. Those wells near the 600 ft thickness interval on this map show the best production, most probably because fractures intersected by the well bore tap the prime reservoir, which lies within the 600 ft interval.

As verification of this analysis, the production history of select wells in the field provides a good basis for appraising the validity of application. The Belden & Blake (Quaker State) 1 Widell well (cumulative production 2.295 bcf) has a First Derivative Interval thickness of 644 ft, while the Belden & Blake (Quaker State) 1 Campbell well (cumulative production 1.793 bcf) has a First Derivative Interval thickness of 645 ft. By comparison, the Belden & Blake (Quaker State) 1 Fyock (cumulative production 8.9 MMcf) has a First Derivative Interval thickness of 718 ft. This corroborates the assumption that wells located closer to the paleogeomorphic thin (i.e., thinning of the interval) with well established fractures will host more economic production than similarly fractured wells located in thicker paleogeomorphic intervals.

A second interval that may be an indicator of production at depth is the Tully limestone thickness. There appears to be a correlation between thinning of the Tully interval and production in Stagecoach field, with significant production being associated with the 75 ft thickness interval.

Fig. 6 shows the relationship between production and the Tully isopach interval. Thinning of the Tully limestone may be related to the presence of a paleo-structure under the surface of deposition. In other areas, chemically precipitated sedimentary rocks (i.e., limestone, dolomites, anhydrites, and even bedded salts) have often formed a marker bed overlying structures that have hosted hydrocarbon accumulations. If a paleo-structure was formed simultaneously with deposition of a limestone, then the limestone would be thinner over the structure. Presuming that there was no post limestone deposition erosion, then the thinning of the limestone bed is diagnostic of the underlying paleo-structure, whether or not that structure exists now.

In the Stagecoach field example, thinning of the Tully limestone can be correlated with thickening of the underlying Oriskany sandstone. Genetically, this thickening of the Oriskany sandstone looks like a "channel" deposit. In this usage, channel is used not to imply a fluvially generated geological formation but is used to represent a linear clastic deposit whose origin may vary from deltaic deposit to strand deposit to marginal marine deposits. An interpretation of the breadth of the "channel" is provided in Fig. 7.

Based upon this mapping, it appears that several infill positions still remain to be developed in Stagecoach field. More importantly, the model developed here could be used on a regional basis to develop additional exploration targets in the county.

Additional studies not presented here verified the application of the paleogeomorphic technique by identifying additional exploration leads for additional development.

Conclusions

The purpose of this investigation was the demonstration of the effectiveness of the paleogeomorphic mapping technique in locating subtle stratigraphic traps in Appalachian basin province of New York State.

The sensitivity of the technique directly relates to the subsurface conditions that define a hydrocarbon trap (e.g., porosity, permeability, seals, and source material), and this in turn relates to how the mapping interval is chosen, as well as the localized sequence stratigraphy (i.e., depositional environment). The success rate associated with this mapping method promotes the more effective use of exploration capital. This allows the cost of exploration to be significantly reduced.

Of perhaps greater importance than the physical results of this investigation is that it was completed on a limited budget with easily accessible, low cost data. If this study were done in an industry exploration setting, the next step would be the directed acquisition of geophysical data, including seismic. By first applying the paleogeomorphic method, the cost of geophysical data would be reduced because its acquisition would be directed towards areas with the best potential. Not only would the data acquisition be more pertinent, but acquisition would be cost effective. In the new world of exploration, the value of a tool of this type is obvious.

References

Heckel, Philip H., Stratigraphy, petrography, and depositional environment of the Tully limestone (Devonian) in New York State and adjacent region, dissertation, Rice University, Houston, 1966, 448 p.
Brett, Carlton E., and Landing, Ed, Dynamic stratigraphy and depositional environments of the Hamilton Group (Middle Devonian) in New York State, Part 2, New York State Museum, Bull. 469, 1991.
Rickard, Lawrence V., Stratigraphy of the Upper Silurian Salina Group, New York, Pennsylvania, Ohio, and Ontario, New York State Museum and Science Service, Map and Chart Series 12, 1969, 57 p. plus 14 plates.