Samuel T. Pees
Consulting Petroleum Geologist
Meadville, Pa.
Jon S. Fox
Samuel T. Pees & Associates
Meadville, Pa.
An approximate success rate of 30% in the Cambrian Rose Run formation along its subcrop belt beneath the Knox unconformity in Ohio has led to renewed interest in the Upper Cambrian beds in adjacent Northwest Pennsylvania.
The Rose Run formation in Ohio equates to the Upper Sandy member of the Upper Cambrian Gatesburg formation in Pennsylvania.
The main factors Pennsylvania operators are considering are:
- the chances of striking relatively high volume wells of about 1 bcf of gas plus oil as evidenced at a number of sites in Ohio,
- the correlation and extension of the reservoir's subcrop into a portion of Pennsylvania's northwestern counties (Fig. 1), and
- the opportunity to plug a well back to the Medina group if the Gatesburg formation is unsatisfactory.
The latter eliminates some of the economic risk and the first factor (high recoverable reserves) provides the incentive for projected deep drilling.
A prophetic article in 1966 by W. Wagner concerned the need for further Cambrian exploration in Northwest Pennsylvania. A number of wells had been drilled in 1964-65 to the Gatesburg formation in Crawford and Erie counties and resulted in shows and two producers.
Those wells prompted the article, but the plea for more drilling to this objective went largely unheeded for nearly 25 years. It is now once again a serious subject with explorationists.
HISTORY IN PENNSYLVANIA
Shows of gas and/or oil have been reported in every sedimentary group or formation in the Cambrian section in northwestern Pennsylvania with the exception of the Potsdam formation (Fig. 2).
Production has been realized in the Mines and Upper Sandy members of the Gatesburg formation, although primarily in the Upper Sandy.
During the years, 57 wells in Pennsylvania have reportedly penetrated Cambrian strata, and 21 of these have reported shows of hydrocarbons.
The first reported hydrocarbon show in the Gatesburg formation in Pennsylvania occurred in 1941 in Springfield Township, Erie County when the 1 Jay Childs was drilled by Ohio Oil Co. to 5,191 ft.
The Gatesburg formation was encountered at 5,179 ft. Shows of gas and oil were reported, but the well was plugged and abandoned.
Completion treatment in these beds was not as well known then as now. The relatively small degree of penetration of the Gatesburg formation, only 12 ft here, was to be relatively common in later drilling of this formation in northwest Pennsylvania.
Other shows of Gatesburg formation hydrocarbons, mostly gas, were found in Butler County (1 944), Lycoming County (1951), Union County (1955), Mercer County (1956), Warren County (1957), and McKean County (1962), until the first commercial production was achieved in February 1964 with the drilling of the 1 J. Scull well in Spring Township, Crawford County.
The run of events that led to this discovery in Beaver Township can be attributed to distant activity in 1961 in Morrow County in central Ohio with the discovery of commercial volumes of oil in the Upper Cambrian Copper Ridge formation dolomite against the Knox unconformity.
Drilling gradually spread east from there and reached the subcrop belt of the Rose Run sandstone. Drilling into that subcrop in Ohio led to several prolific discoveries.
An exploration assumption resulted in projecting the Rose Run subcrop into the then distant Beaver and Spring townships of Northwest Crawford County, Pa.
A multiwell program in 1964-65 in those townships by Transamerican Petroleum Corp. resulted in two producing wells and three other wells with significant shows of hydrocarbons.
Since the mid-1960s and into the 1980s only two poorly productive wells, one in McKean County and the other in Mercer County, have been realized in Cambrian strata in northwestern Pennsylvania. This number is unrepresentative of the potential that these strata hold.
The paucity of Cambrian wells may be largely attributed to the flurry of development of the Medina group sandstones as the dominant reservoirs in northwestern Pennsylvania because of their shallower depths and higher success rate.
During this same time frame, operators in Ohio pursued the Rose Run play in east-central and northeastern Ohio and are now filling in gaps.
The impressive potential of some Rose Run gas wells in Ohio, practically to the portal of Pennsylvania, has operators looking over the prospects in the Pennsylvania extension of the trend.
STRATIGRAPHIC RELATIONSHIPS
The major stratigraphic units encountered by 1 Kardosh in the subsurface of northwestern Pennsylvania from the Upper Ordovician Utica formation shale to Precambrian igneous and metamorphic basement are shown (Fig. 2).
The subcrop of the Rose Run sandstone against the Knox unconformity in Ohio has been traced through northwestern Pennsylvania by examination of sample descriptions and geophysical logs of wells that penetrated the Gatesburg formation. The extension of this subcrop belt is shown for the first time (Fig. 1).
Cambrian age strata can be traced from outcrops in central Pennsylvania into the subsurface of northwestern Pennsylvania. These rocks crop out as allochthonous thrust sheet blocks that have been displaced over younger (Silurian and Ordovician) strata in the Nittany arch belt.
To the north and west of the thrusted outcrop belt the Cambrian beds are situated at deeper basin levels and then gently rise to the northwest at a variable rate, which is generally about 50 ft/mile.
Thinning of these strata occurs due to basin margin on-lap and truncation near the Knox unconformity to the degree that 4,000 ft of Cambrian strata in central Pennsylvania are reduced to approximately 1,050 ft in northwestern Pennsylvania as evidenced by the section in the Kardosh well (Fig. 2).
Between the Kardosh well and the Lake Erie shore (City of Erie), Cambrian strata are further reduced to 835 ft.
Stratigraphic correlation of units in close proximity to the Lower Ordovician Knox unconformity, particularly below the unconformity, can be a challenging exercise.
Correlations are complicated by several factors that include probable faulting, facies changes associated with alternation and mixing of carbonate and siliciclastic depositional regimes, rough similarity of gamma ray motifs of beds that actually are geologically distinct, and the varying degree of erosion involved with the regional Knox unconformity.
A diagram shows log correlation from Ashtabula County, Ohio, through Crawford County into Erie County, a line of 48.5 miles (Fig. 3).
An important observation that pertains to the correlation of Gatesburg formation wells is that those which have a lithologic description of cuttings or cores, plus an adequate suite of logs (including a complex lithologic analysis log as shown in Fig. 3) can often be subdivided with more precision.
These wells can be tied together in correlation diagrams with more accuracy than wells with insufficient logs, no sample analyses, and only short penetration into the Gatesburg formation interval.
Paleotopographic relief of as much as 300 ft has been mentioned in a subcrop belt in central Ohio (Shearrow, 1987), and 150 ft was demonstrated in north central Ohio, where the unconformity engages the Copper Ridge dolomite (Dolly and Bush, 1972).
Relief approaching 100 ft was found in the Gatesburg Upper Sandy member subcrop belt in Northwest Pennsylvania during the course of this study, but erosional remnants of greater magnitude may be found by future drilling.
The Mines member comprising the Upper Gatesburg formation caps erosional remnants at places in the Upper Sandy member subcrop belt.
The primary controlling factor of development of paleotopographic relief appears to be fluvial-controlled erosion, some of which took advantage of pre-existing folded structure (anticlines, synclines) during the Lower Ordovician epeirogeny that was responsible for broad regional uplift, and ultimately the Knox unconformity itself.
A stratigraphic section and log diagram of the 1 L. Sensabaugh well in Beaver Township, Crawford County, Northwest Pennsylvania, were derived from detailed examination of the borehole cuttings by the authors (Fig. 4).
Full description is not included in this article.
The figure shows the several various lithologies that were encountered in the Shadow Lake formation, among which the most notable was a distinctive light to dark greenish-gray shale, which is usually pyritic and normally rests on the unconformity or on a thin Ordovician sandstone that comes and goes.
The main sandstones of the subcropping Upper Sandy member of the Gatesburg formation in this well were generally clear to white, fine to medium-grained, and usually subrounded to wellrounded.
TRAPS, RESERVOIR CHARACTERISTICS
Some investigators of producing strata in the Cambrian interval have published on the types of traps encountered in their areas of study (Wagner, 1966B; Ebright, 1970; Dolly and Busch, 1972; Janssens, 1973; Coogan and Maki, 1986, and others.)
Most traps explored to date have a close relationship to the major Knox unconformity. However there are some instances where the unconformity would have minor involvement, if any.
The latter situation would largely fit reservoir beds of the Gatesburg formation that are at some distance below the unconformity, say 150 ft or more, and are not a part of a local erosional remnant. Shows have been found in these deeper sections.
The Shadow Lake formation (Glenwood formation in Ohio) was deposited on the Knox unconformity surface and acts as a sealant in most places. It consists of dolomitized argillaceous limestones, regular limestone beds and shale. Pyritic shale often overlies the erosional surface forming a seal, but the carbonate beds, especially argillaceous ones, can be part of the Shadow Lake sealing mechanism, too (Figs. 3, 4).
The Shadow Lake formation is 15-40 ft thick in Northwest Pennsylvania. Its thickness is mostly a function of local relief of the surface upon which it was deposited. It thins and drapes over positive areas (buried hills) and in those places, has the attitude of supratenuous folding, which can be a trapping condition given an appropriate Shadow Lake formation reservoir bed.
A basal sandstone has been noted in the Shadow Lake formation in some wells and could be a small reservoir having affinity to the unconformity and employing an overlying shale or dense limestone for a seal.
The various trapping conditions in Gatesburg formation beds that are close to the unconformity include the following: buried hills sealed by Shadow Lake formation shales or other non-porous beds; anticlinal folds preserved from deep broaching; flank traps where reservoir beds were truncated at higher structural positions but locally preserved downdip; and a similar type of flank trap on a hill that was a negative structural feature (syncline) in the main but that would have a higher structural relief on the extremities as the beds rose into a pre-unconformity anticlinal fold.
Knox excavation and Shadow Lake formation sealing would cause and preserve this example.
Rather complex conditions may interact to create a trap, but the main genetic factors rely on paleotopographic configuration and paleostructural conditions (of which the latter partly controlled the former) as seen at the unconformity and the bed attitudes immediately below it.
Dolly and Busch (1972), using isopachous and structural methods in mapping an area in Morrow County, north central Ohio, noted cases of inversion of underlying structure versus relief of the unconformity. They found that a positive area like a ridge or hill would be a preserved remnant of an underlying syncline or structurally negative area.
The reservoir formation at the top of the Cambrian sequence in that area of study was the Copper Ridge dolomite, more or less equivalent to the Middle Dolomite member of the Gatesburg formation in Northwest Pennsylvania. Inverted topography such as this is possible at many places throughout the various Upper Cambrian subcrop belts.
Deep faulting in the Gatesburg formation is another phenomenon that may be more abundant than presently perceived in underdrilled Northwest Pennsylvania and Northeast Ohio. So far, in Northwest Pennsylvania, it is hard to be certain of Gatesburg formation or deeper faults, although clues are present to some extent.
If well density is close enough and the drilled area is of respectable size, the structural maps on reliable Cambrian beds will pick out dip reversals defining folds, abrupt variations in the rate of dip, and other anomalies, some of which could be marking faulted and fractured areas.
Deep deformation is expected, and basement could be involved. Structural indications of some of these matters have shown up as high in the section as the Upper Ordovician Queenston formation unconformity.
The Knox unconformity over a broad area is subject to linear and sinuous positive patterns marked by paths of reversals, a situation not uncommon on that irregular surface. Ridges, hills, valleys, folding, and probably faulting of varying magnitude are present in the Cambrian in Northwest Pennsylvania.
Seismic surveying is expected to be of more help in determining lines of displacement, and it should be a useful and practical exploration approach for Gatesburg features of various types.
In addition to the trap conditions discussed above, it is suspected that intraformational porosity barriers in certain Gatesburg beds may have caused stratigraphic-type traps and resulted in rather localized impoundment of hydrocarbons rather than pools of large extent.
The dolostones of the Gatesburg formation are believed to be of secondary origin.
Where Gatesburg strata were left as a remnant hill, percolating ground water and subaerial conditions during the erosional episode would have affected the beds to varying depths and would have created forms of porosity or enhanced what was already there.
In the case of the carbonates, vugs would have the opportunity to form. Vuggy porosity is known in the Copper Ridge dolomite at the unconformity in central Ohio.
Stratigraphic pinchout and thinning (unrelated to the unconformity) of Gatesburg formation beds in a northern direction in northwestern Pennsylvania may play a role in the creation of traps.
Updip pinchouts of certain beds may come to be a part of the overall play. This possibility will be better understood (or ruled out) when more subsurface control is realized by more drilling to this objective or deeper.
Landsat lineaments may be marking linear zones of interest for deep objectives like the dolostones of the Gatesburg formation. Some of these features can be expressions of recurrent faulting or stress zones where natural fracturing would be concentrated.
Brittle failure phenomena would be particularly evident in dolostones and tightly cemented sandstones both of which are plentiful in the Gatesburg formation. Natural fracturing can greatly enhance permeability. It can create a reservoir or improve the volumetrics of an existing one.
One Gatesburg exploration approach could be based on mapping viable lineaments, especially those that interact with other structural features, and then positioning wells to take advantage of possible fracture swarms at depth.
TESTS, PRODUCTION
At least four wells have produced hydrocarbons from the Gatesburg formation in northwestern Pennsylvania.
The first commercial production was achieved with the drilling of 1 J.H. Scull in Spring Township, Crawford County by Transamerican to 6,398 ft (Fig. 5).
The Gatesburg formation was encountered at 6,299 ft, and a natural flow of 7 MMcfd of gas was reported below the Knox unconformity in the Upper Sandy member. The well was perforated and acidized with a resultant flow of 2.59 MMcfd and some oil.
The Scull well produced 190 MMcf gas and 1,100 bbl of oil in 18 months and then was shut in 2 months. When reopened, it produced water and was consequently plugged. This well was the discovery well of the Scull pool within the large Conneaut field.
As is the case with all Gatesburg pools in Pennsylvania to date, it was a one well pool. Water saturation and coning appears to be a problem in some Rose Run/Upper Sandy wells.
Ohio has many cases of early water entry, but significant hydrocarbon production was realized in some of the cases,too.
Production from Cambrian strata after the Scull well was next achieved in May 1964 in Beaver Township, Crawford County by the 1 N. Voorhees drilled by Transamerican. Shows of gas were encountered in the Upper Sandy member, and the well was perforated at 6,006-14 ft and 6,016-21 ft.
An IPF of 1.64 MMcfd and some oil after acidizing from the Upper Sandy member was reported. It was the discovery well of the Beaver Center pool within the large Conneaut field.
In January 1975, in Lafayette Township, McKean County, the 2 Minard Run Tract, drilled by Minard Run Oil Co., had an IPF of 500 Mcfd, natural, reported from the Gatesburg formation. This well was the discovery well of the Minard Run pool within the giant Bradford field.
More recently the 7 Horvath was drilled by Atlas Resources in Lackawannock Township, Mercer County, to 8,869 ft and completed June 7, 1989. The Gatesburg formation was reached at 8,234 ft, and shows of gas were noted at 8,236 ft, 8,272 ft, and 8,312 ft.
Natural flow occurred, but the volume was not reported. After the well was perforated and acidized, an unknown volume of gas was said to be produced for a short time. The well was plugged back and completed uphole in the Medina group.
A possibly awkward situation occurs in Erie County, where Well 24388 in Waterford Township is used for brine disposal into the Upper Sandy Member of the Gatesburg formation. The authors do not know the lateral extent of water injection.
The 2 C. Reemsnyder (Permit No. 23768, Fig. 3) in Waterford Borough, shows separation between hydrocarbon weight and hydrocarbon volume curves from a complex lithology analysis log indicating poor to moderate shows of gas in the Upper Sandy member of the Gatesburg formation.
There are no reports of this well being tested in this zone, and water saturations are high.
The first Cambrian production in Ohio occurred in 1909 in Pleasant Township, Seneca County (Dolly and Bush, 1972).
Other sporadic, generally small discoveries occurred until June 1961, when the completion of 1 Orrie Myers in Morrow County in northcentral Ohio took place. Initial production of this well from vuggy Copper Ridge dolostone was 600 b/d of oil (Janssens, 1973).
The first production from Cambrian strata in New York was achieved with the drilling of 2 J.M. Niehaus in the Town of Sheridan, Chautauqua County, N.Y., completed Sept. 29, 1949. Heintz Oil & Gas Co. was the operator.
An initial flow of 2.2 MMcfd was encountered below the Knox unconformity, which later blew down to 80 Mcfd. Production is believed to be from the Upper Cambrian Tribes Hill formation dolostone, a temporal equivalent to the upper members of the Gatesburg formation.
This well still produces gas from Cambrian. It is also known that several pools produce gas from Cambrian strata in southwestern Ontario (T. Carter, 1990, written communication).
Northwest Pennsylvania is geographically close to 1 bcf class Rose Run (Gatesburg) wells. In adjacent Ashtabula County, Ohio, in New Lyme Township, the 3 Rhoa, drilled by Park-Ohio Industries Inc., produced more than 900 MMcf of gas and more than 1,200 bbl of oil from March 1982 through 1987 and has probably exceeded 1 bcf of gas and 1,500 bbl of oil by now.
This well and some others were drilled on the periphery of an elliptical anomaly detected on Landsat imagery (Pees and Palmquist, 1984).
Two other recently drilled wells in Rome Township, Ashtabula County, the 5 Bogdan unit (Permit No. 3842) and the 6 Bogdan unit (Permit No. 3859) have reported IPFs of 7 MMcfd after acidizing and also have the potential to be 1 bcf producers.
CONCLUSIONS
Another article in a separate journal treated on prospects other than the Medina group for Northwest Pennsylvania (Pees, 1989).
The Gatesburg formation's hat is now in the ring and presents a renewed challenge and the opportunity for high volumetrics from wells in propitious geological locations in Pennsylvania.
This study revealed that all members of that formation can be prospective in the right setting. Shows have also been found in the underlying Warrior formation and also above the Knox unconformity in the Shadow Lake formation in Northwest Pennsylvania.
The Upper Sandy (Rose Run) subcrop in Northwest Pennsylvania (Fig. 1) is an obvious first choice of trends for detailed mapping and seismic leading to a drilling program. The other Gatesburg formation subcrops should be considered, too.
Areas north of the Upper Sandy member subcrop would be in the subcrop of the Middle dolomite member (in Pennsylvania most of this would be located in Lake Erie) while south of it for a distance the Mines member should directly underlie the Knox unconformity. The prospective region for these members could be quite large, especially the Mines member.
A perusal of statistics compiled by the Ohio Department of Natural Resources (McCormac, 1990) revealed that 144 wells were drilled to the Cambrian in 1989, and 44 were completed as producers (some of those were highlight wells).
Of the rest, most were completed uphole in the Clinton formation (Medina group in Pennsylvania), and a few were deepened to the Mt. Simon formation for use as disposal wells. After that, there were very few dry holes to count.
This experience could progress along the subcrop into Pennsylvania when significant drilling programs are carried out.
The complicated nature of the Gatesburg formation play as well as recognition of its potential rewards behooves an operator to utilize as many geological methodologies as will serve to make the drilling strategy more accurate and profitable.
BIBLIOGRAPHY
Coogan, A.H., and Maki, M.U., Trapping configurations for Cambrian Rose Run production in Ohio: Soc. of Petroleum Engineers, paper no. 15922, 1986, pp. 71-77.
Dolly, Edward D. and Busch, Daniel A., Stratigraphic, structural, and geomorphologic factors control ling oil accumulation in Upper Cambrian strata of Central Ohio: AAPG Bull., Vol. 56, No. 12,1972, pp. 2,335-68.
Ebright, J.R., Ohio wells head for the cellar: OGJ, May 25, 1970, pp. 108-114.
Habib, H.E., and Trevail, R.A., Oil and gas exploration, drilling, and production summary, 1982: Ontario Ministry of Natural Resources Oil & Gas Paper No. 5, Appendix VI, 1984, pp. 183-216.
Janssens, A., Stratigraphy of the Cambrian and Lower Ordovician rocks in Ohio: Ohio Geological Survey Bulletin, 64, 1973, 197 pp.
McCormac, Michael P., 1989 Ohio oil and gas developments, ("The DeBrosse Report"), Ohio Department of Natural Resources, Division of Oil and Gas, 1990, 30 pp.
Pees, S.T. and Palmquist, J.C., Elliptical "morphotectonic" features on Landsat imagery in southwestern New York, northwestern Pennsylvania, and northeastern Ohio, AAPG Bull., Vol. 68, No. 12,1984, abs., p. 1,926.
Pees, S.T., More to Appalachia than the Medina, American Oil & Gas Reporter, Vol. 32, No. I 1 (Nov.), 1989, pp. 36-44.
Shearrow, George G., Maps and cross sections of the Cambrian and Lower Ordovician in Ohio: Ohio Geological Society, Columbus, Ohio, 1987, 31 pp.
Wagner, Walter, R., Stratigraphy of the Cambrian to Middle Ordovician rocks of central and western Pennsylvania: Pennsylvania Geological Survey General Geology Report 49, 1966A, 156 pp.
Wagner, Walter R., Pennsylvanians must delve into rocks of ancient age: OGJ, Dec. 12, 1966B, pp. 152-158.
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