ORDOVICIAN OIL POTENTIAL IN NEW YORK DETAILED

Joseph E. Robinson
Syracuse University
Syracuse, N.Y.

Oil and gas have been produced in New York state since the 19th century, but oil production has been confined to Devonian reservoirs with only natural gas from the Silurian and Ordovician section (Fig. 1).

Gas from the Silurian Medina formation is present throughout much of the western part of the state, with the Ordovician Queenston and Trenton gas prevalent in the central and northern section.

In addition the Lower Ordovician and Cambrian carbonates and sandstones frequently contain gas. In fact, gas is so common that most of the wells drilled in western and central New York encounter either gas shows or commercial gas.1

Conversely, commercial oil has not been found in the deeper section. Until a very recent test, there has not been a documented Ordovician oil show. However, there are very few deep tests.

In some parts of the state there is only one deep well for several hundred square miles. Even with the limited number of wells, there have been a number of stories in which drillers described shows of light colored liquid oil or condensate.

At least one of the wells in the area just south of Lake Ontario was reported to have produced an oily sludge. Unfortunately no samples of oil or oily material from any of the older wells were retained, so no direct historic evidence of oil or oil shows is available.

In the northern section where most of the deep tests have been drilled, fracture porosity in the middle Ordovician Trenton limestone forms the main gas reservoir.

In addition there have been a number of gas shows and some production from the underlying Black River limestone and Theresa-Potsdam sandstones. In fact, high pressures in the Trenton reservoirs suggest the gas column may continue into the lower formations, possibly as deep as the Theresa-Potsdam.

Deep New York state gas is sweet, 98-99% methane, and has been used commercially without treatment. Gas production records do not indicate the presence of oil or condensate.

ORDOVICIAN SHOW

However, there now is a definitive indication that oil may be present within the Ordovician of New York.

A recent test, the UrbanSnow Gas Co. 1 Kermit & Emily Keysor, drilled to basement in northern Cayuga County (Fig. 2), encountered a very unusual interval in the basal Theresa-Potsdam sandstones of Upper Cambrian-Lower Ordovician age.

Instead of the anticipated 210 ft of clean white medium to coarse grained quartz sandstone that comprises the normal Theresa-Potsdam section in this location, the Keysor well intersected a 145 ft interval of a dark colored mixture of organic material, silt sized quartz particles, and shale with minor amounts of coarse, well rounded sand grains before encountering 65 ft of normal sandstone (Fig. 3).

The organic rich silt had a very strong petroliferous odor, and geochemical analysis of the samples indicated the presence of a paraffin rich crude oil. These samples appear to be the first direct evidence of petroleum potential in the Ordovician of New York state.

The Lower Ordovician-Cambrian section of New York state is a sequence of sandstones and dolomites that thin northward both depositionally and by erosion of the Knox unconformity surf ace.

The result is a wedge that pinches out along the south shore of Lake Ontario. The unconformity surface is overlain by the organic rich, shaly limestones of the Middle Ordovician Black River and Trenton limestones.

Near the subcrop, the dolomite that usually delineates the base of the Theresa is missing, and the entire preKnox sedimentary section becomes a uniform, clean white quartz sandstone with occasional minor dolomitic cement.

In this area the Theresa is virtually indistinguishable from the Potsdam. Geophysical logs and samples show the section as a continuous sand section with occasional tight zones in the middle portion.

The sandstone consists of rounded and frosted quartz grains averaging 0.5 mm in diameter and is poorly cemented so that it decomposes in the hands. The friable portions are lightly cemented with calcite, although some chips show quartz cement.

The section has variable porosity with the best porosity at the base and at the top of the formation. Some sections show porosities of 1015%, while in other areas the entire section may be tight.

Microscopic examination of samples show evidence of overgrowths that suggest much of the present porosity is secondary. Gas appears most often in the upper porous section, while the lower porous section tends to be water filled.

KEYSOR WELL

The 1 Keysor well was drilled to test possible reservoirs from the Trenton limestone to the Theresa-Pots- dam sandstone.

In this area the sandstone directly underlies the Knox unconformity and is a clean white, commonly porous quartz sandstone with occasional minor dolomitic cement.

Figs. 4A & B contrast the normal Lower Paleozoic section as logged in a nearby well, the Urban-Snow Gas Co. 1 Hunter, with the Keysor well.

In the Hunter well, located approximately one half mile southwest of the Keysor, the Theresa-Potsdam is a continuous sandstone, while the upper 145 ft of the same section in the Keysor well is a black organic silt. The total Theresa-Potsdam section in the Keysor well was 215 ft.

The well was drilled to the Knox unconformity with fluid, and intermediate casing was set. Drilling then proceeded with air in the expectation of intersecting a normal and potentially gas bearing Theresa-Potsdam sand section.

However, instead of the anticipated white sand, the cuttings appeared as a cloud of black odorous silt. This was a surprise for the drill crew, and it was some time before samples were taken.

At first the samples were difficult to collect from the air discharge because particles were very small and required a fine mesh screen, Fortunately enough material was obtained to provide a number of samples for geochemical analysis.

Drilling in this silt layer continued without appreciable change for 145 ft before the normal Potsdam sandstone was encountered. Some 70 ft of normal clean white sandstone was penetrated before the Precambrian basement was encountered at the depth anticipated in the original prognosis.

Only the upper Theresa-Potsdam section was anomalous, and it appeared that the upper portion of the sandstone had been replaced with an organic siltstone.

SAMPLE ANALYSIS

Three laboratories analyzed the black silt samples.

Determinations included carbon analysis, Rock-Eval analysis, TOC analysis, and mass spectrometer analysis. All reports agree that the samples indicate the presence of a paraffin rich crude oil.

The samples consisted of a fine grained mixture of siltstone and shale with minor amounts of coarse, well rounded sand grains.

Total organic carbon ranged from 3.6-4.8%. The carbon analysis sample had a calcium carbonate equivalent of 44.15%, although it was probable that there was some free carbonate, limestone, or dolomite in the sample.

After washing with solvent to remove the hydrocarbons, the sample was found to contain 1.25% organic carbon. The organic matter was considered to be gas prone and in the oil window. Organic solvent removed 3.60% organic carbon.

Rock-Eval pyrolysis of the unextracted sample indicated that 1.56% of the sample was free hydrocarbons, 15,630 ppm for Peak 1 and 7,800 ppm for Peak 2 (Fig. 5).

The pyrogram suggests that both these values probably represent free hydrocarbons. Peak 2 is poorly defined, and Tmax was well below the temperature usually associated with oil generation.

No vitrinite was present for TAI measurements. However in one report a value of 1.2 was estimated from the amorphous kerogen. There is general agreement in the reports that there is no evidence of the kerogen being overmature.

Excerpts from the reports point out that the gas chromatograms of a thermal extract and solvent extract indicate the presence of a paraffin-rich crude oil with carbon numbers peaking at n-C18 and declining rapidly to n-C25 (Fig. 6). There appears to be no odd or even dominance.

Pristane and phytane are approximately one third the abundance of their neighboring paraffins. Pristane is somewhat dominant over phytane, suggesting mild oxidizing conditions in the depositional environment.

Mass spectral analysis (Fig. 7) ranged from 118 peaks on 1 min exposure to 160 peaks on 3 hr exposure. The analysis suggests the presence of substantial hydrocarbons in the sample and indicates a petroliferous character to the cuttings.

Biomarker analysis of the subject oil indicates it is a natural product, likely allochthonous, which was generated from source rocks containing clay minerals, possibly a limy shale or a shaly limestone.

The oil does not appear to be a contaminant, and its biomarker composition compares well with other North American pre-Devonian oils. There are features that are found in Ordovician oils from both the Michigan and Williston basins.

All information obtained about the samples from the silt interval indicates the presence of organic materials that clearly show that the rocks have reached a maturity that is within the oil window and contain evidence of a paraffin rich crude oil, possibly the first definitive evidence of potential Ordovician reservoirs in New York state.

INTERPRETATION

The replacement of the normal, clean quartz sandstones of the Theresa-Potsdam formation by a dark organic rich silt layer is unusual.

Other basement tests in the area have a normal sand section, and although gas shows are common, there are no reported indications of oil staining.

The operators of the Keysor well anticipated gas. However a possible interpretation of the origin of the silty section suggests that the sequence may not be unique but could be the indicator of a series of potential oil reservoirs.

All formations in central New York dip southward into the Appalachian basin.2 The pre-Knox formations dip at approximately 100 ft/mile, and those above the unconformity dip at a lesser rate.

There was loss of section in the Lower Ordovician and Upper Cambrian through both depositional thinning and Middle Ordovician erosion. The pinchout of the preKnox sedimentary section is immediately south of Lake Ontario.

Fisher pointed out that the Potsdam sandstone was laid down transgressively on a faulted and undulating Precambrian surface.3 The Potsdam is a basal sandstone, and the lowermost member tends to reflect the composition of the immediate source, which is the Grenville basement.

The Grenville rocks in central New York display considerable variation. Basement samples in central New York commonly present a range of gneisses, shists, or marbles, and the local bedrock is often apparent in the lowermost few feet of the overlying sandstone.

Except for this basal section, the Potsdam is a clean, white, medium grained sandstone with numerous coarse grains. The sandstone usually displays some porosity, occasionally 1 0% or higher, and there are local zones that have been cemented with a dolomitic cement.

The transgression that deposited the Potsdam onlapped the Precambrian basement from southwest to northeast. Consequently the Potsdam sandstone is time transgressive and ranges in age from Cambrian to Lower Ordovician at its northern most extent.

This Cambrian and Lower Ordovician section thickens southward and is superseded by a dolomitic sandstone, the Theresa, which is in turn overlain by the Little Falls dolomite.

During the Middle Ordovician, Knox uplift and erosion beveled the underlying rocks so that only the pre-Knox sandstones remain across northern New York.4 Consequently there is a broad band across northern New York where the unconformity surface is incised directly into the Theresa-Potsdam sandstones.

This sequence forms the unconformity surface in the area extending from Lake Ontario southward for some 50 miles. It probably appeared as resistant upland crossed by down cutting streams.

The Middle Ordovician Knox uplift resulted in extensive erosion that would have incised rivers deeply into any upland areas. These river valleys would then become filled with post Knox sediments during the later transgression.

In this way a stream valley cut into the pre-Knox sandstones could become filled with Upper Ordovician organic rich silts and clays plus Theresa-Potsdam sands. The stream migration and deposition in the river valley plus wave action as the seas completed the transgression would have a strong sorting action on the valley sediments.

There would be areas of fine material, such as at the Keysor location, in addition to locations where there would be coarse channel sands and point bars.

Erosion and valley fill patterns of this form are common in both ancient and modern buried stream channels and appear to be a reasonable explanation for the Keysor feature.

The top of the silt section in the Keysor well begins at the base of the easily correlated unconformity zone. Overlying the unconformity surface there is a section of dark colored limy shales that represent the first detrital material laid down during the post Knox transgression.

There is considerable organic material in the shales and also in the overlying Black River limestones as well as in the fossiliferous Trenton limestone. The geochemical analysis infers this organic rich section of shaly limestones could qualify as a potential source.

With only a single well intersection, only a very few of the possible stream valley attributes can be estimated. Maximum depth and related fill thickness is likely greater than 145 ft. Valley width must be less than 1 mile at the Keysor location.

Well control to the north and south limit the possible extent and suggest the trend would be mainly east-west.

Magnetometer maps of the area indicate the basement trends are northeast-southwest and joint patterns in the Ordovician rocks dominantly trend north east and northwest. Positive location of this and other channels will require additional information.

Fortunately the section is shallow. Depth to basement is 3,600 ft at the Keysor well and less than 4,500 ft in the most prospective area.

The section should respond to shallow high resolution seismic, which may be the best method for mapping a very interesting feature. Then selective drilling might discover commercial accumulations of Ordovician oil in New York state,

REFERENCES

1. Kreidler, W.L., Petroleum Potential of New York, Selected References for Region 10: Future Petroleum Provinces of the U.S.Their Geology and Potential, AAPG Memoir 15, 1971, pp. 1,258-1,260.

2. Kreidler, W.L.. Van Tyne, A.M., and Jorgenson, K.M., Deep Wells in New York State, N.Y.S. Mus. and Sci. Ser. Bulletin No. 418a, 1972, 335 p.

3. Fisher, D.W., Correlation of the Haydrinian, Cambrian, and Ordovician Rocks in New York State, N.Y.S. Mus. and Sci. Ser. Map and Chart Series, No. 25, 1977, 75 p.

4. Rickard, L.V., Stratigraphy and Structures of the Subsurface Cambrian and Ordovician Carbonates of New York, N.Y.S. Mus. and Sci. Ser. Map and Chart Series, 1973, No. 18.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.