Aeromag interpretation technology helps chase Cambrian in New York

Joseph P. Fagan Jr.
Pearson, deRidder & Johnson Inc.
Lakewood, Colo.

David L. Copley
Ardent Resources Inc.
Buffalo, N.Y.

Building a geologic model from the bottom up, Ardent Resources Inc. conducted a high-sensitivity, high-resolution aeromagnetic survey in western New York. The aeromagnetic survey was flown to delineate deep-seated structural patterns which affected emplacement of Cambrian oil and gas reservoirs.

In an effort to extend the Rose Run play into western New York, Ardent employed Pearson, deRidder & Johnson Inc. (PRJ) to acquire, process, and interpret an aeromagnetic survey covering about 170,000 acres. An exploratory well drilled upon the interpretation results of the survey is an indicated gas discovery in the Rose Run equivalent upper Cambrian Theresa formation (OGJ, Jan. 27, 1997, p. 98).

Rose Run production has long been established in Ohio and Pennsylvania (Fig. 1 [73,393 bytes]). In 1995, over half of all permits issued in Ohio were granted for sub-Knox unconformity wells, and of these over 200 were drilled to the Rose Run sandstone. It should also be noted that 57% of these Rose Run wells were productive (OGJ, Apr. 15, 1996, p. 77).

Extension of the subcrop play into New York has only just begun. It is hoped that the Rose Run can do for New York what it has done for Ohio.

Geologic rationale

The Rose Run play is comprised of heterolithic dolostones and sandstones in a subcrop belt beneath the Knox unconformity. Reservoir heterogeneity is related to basement paleotopography, faulting, and fracturing (OGJ, Oct. 14, 1996, p. 88). Because of its proximity, the Precambrian is intimately involved in the geology of the reservoir rock.

It was hoped that an aeromagnetic survey could be used to image the basement by identifying faults and lithologic contacts. Because most sedimentary rocks have low or very low susceptibilities, the stratigraphic column contributes little to the aeromagnetic response obtained from the basement that underlies western New York.

The crystalline basement of New York is not an isotropic, homogeneous material. A great amount of lithologic variation exists in the material that underlies the Appalachian basin.¹ The basement is part of the Grenville Province, which can be subdivided into several sub-belts (Fig. 2 [44,757 bytes]).

Western New York is near the boundary between the Central Metasedimentary Belt and the Central Gneiss Belt. A shear zone several kilometers wide forms the boundary. Smaller subparallel zones of deformation are also found within the basement on either side of the boundary. Mapping these variations is of primary importance in oil and gas exploration.

Ardent approached PRJ in late 1994 to inquire about the usefulness of an aeromagnetic survey in the western New York state. If relationships between basement variations and structural features associated with oil and gas production could be established, aeromagnetic data could become an inexpensive and useful exploration tool.

Integrated study

This technology transfer project, partially sponsored by the New York State Energy Research and Development Authority (Nyserda), united local geologic expertise with state-of-the-art 3D seismic and aeromagnetic technologies.

Each geophysical analysis was performed independently. That is, both the aeromagnetic interpretation and the 3D seismic interpretation were performed without the benefit of detailed geologic input. Additionally, neither geophysical method had access to the results obtained from the other.

Ardent integrated the survey results after the contractors delivered the final interpretations. Results of the 2.7 sq mile 3D seismic survey were summarized previously (OGJ, Oct. 14, 1996, p. 88). Fig. 3 [49,471 bytes] is reprinted from that issue and shows the time structure of the Knox unconformity from the 3D seismic survey.

The 3D survey shows that despite the northeasterly trend of Silurian and younger age rocks, sub-Knox unconformity sediments have a distinct northwesterly trend. This condition was hitherto unrecognized.

Fig. 4 [172,212 bytes] is a map of the total magnetic intensity corrected for the contributions of the earth's magnetic field. As can be seen in the magnetics, a deep-seated structural trend is evident in the Precambrian rocks. This type of response is associated with a lithologic contact in the basement.

It was still unknown at this point whether or not a structure associated with the lithologic contact was present. To answer this, a patented interpretive tool was used.

Starmag is the trademark name for a form of artificial intelligence that detects and quantifies the size of structures on the surface of the basement. By working with high-sensitivity profile data, Starmag is able to distinguish between high-amplitude, low frequency lithologic changes and low-amplitude, high-frequency structural anomalies.

A high-quality aeromagnetic data set, a first-order approximation to the configuration of the basement, and an idea of the type of target that is to be expected are the input parameters required to begin an analysis.

Fig. 5 [124,076 bytes] is a residual structure map on top of the basement surface from Starmag. A comparison of the 3D and aeromagnetic interpretation results from the Northwoods Project study area shows an excellent correlation.

Future developments

Because of the indication of a gas discovery from the first well drilled, the Northwoods Project can already be considered a success. Leasing has picked up in western New York on the recognition of the northeasterly extension of the Rose Run subcrop belt (OGJ Jan. 27, 1997, p. 98).

One of the biggest problems in expanding the Rose Run play farther into New York is the lack of deep well control. As such, western New York's lower Paleozoic is still a frontier play. Successful exploration programs must first pare down thousands of acres.

One of the most significant results from the Northwoods Project survey is that aeromagnetic data can be used to image the basement that underlies and that is immediately adjacent to Cambro-Ordovician reservoirs. As play size decreases, economics becomes more of a driving force in exploration.

This high-resolution, high-sensitivity aeromagnetic survey in western New York covers not only the Northwoods Project area but an additional 250 sq miles as well. Acquisition, processing, and interpretation of this 160,000+ acre survey were completed for less than $41,000. Acquisition and processing of the 3D seismic for the Northwoods Project were completed for under $80,000.

These costs compare very favorably with the reported $150,000 cost of a dry hole. As a result, the process of culling out nonprospective areas is within the range of nearly all independents.

With the improvements in acquisition, processing, and interpretation technology, not only can aeromagnetic surveys be used as a high-grading tool for the positioning of 3D seismic shoots, they can be used as a primary exploration tool as well. Used alone or in conjunction with state-of-the-art seismic methodology, aeromagnetic data can be employed as an important-and inexpensive-part of any exploration program.

Information about aeromagnetic prospecting is available on-line at www.prj.com

Reference

1. Easton, R.M., and T.R.Carter, "Extension of Grenville basement beneath southwestern Ontario: rock types and tectonic subdivisions," Ontario Geological Survey Geological Compilation Map 2441, Open File Map 162, 1990.

Bibliography

Copley, David L., and Mohar, Oliver N., Three dimensional seismic demonstration: The Northwoods Project, final report for New York State Energy Research and Development Authority Project 4207-ERTER-ER-96, October 1997.

Copley, David L., and Mohar, Oliver N., Aeromagnetic demonstration survey with basement interpretation and deep structure mapping, final report for Nyserda project 4365-ERTER-ER-96, January 1998.

Loewenstein, Stuart, Geologic assessment of core data from the Stahl #1 and correlation with the Rose Run of Ohio, final report for Nyserda project 4378-ERTER-ER-96, pending.

Bibliography reports accessible at www.nyserda.org

Applying Starmag to aeromagnetic data

STARMAG, A SERVICE MARK OF TEXACO, IS A TOOL for interpreting aeromagnetic data using neural network technology.

It employs computers-directed by geophysical interpreters-to use artificial intelligence to recognize patterns of geologic interest in magnetic data sets.

Two advantages are speed and the computer's ability, if properly "trained," to recognize correlations in patterns among hundreds of individual aeromagnetic profiles.

Another plus is the computer's capability to adapt quickly if an operator changes his exploration paradigm in the midst of interpretation.

An operator who has acquired magnetic data must decide whether to use manual or neural network assisted interpretation.

Interpreters who have some idea of the geological character of basement and the types of anomalies to expect in the area of interest create computer models. They tell the computer that various structural patterns in the area should yield certain magnetic fields.

The computer's first results are usually imprecise. Neural connections begin minimizing the errors until the computer's recognition of the patterns converges to acceptable limits, usually plus or minus 10% accuracy.

Then the real magnetic data are fed to a personal computer for the Starmag run. The training and supervisory period usually takes most of a day, and entire data sets can be run in 10-15 min.

Starmag's output is a residual basement structure map that the geologist can take to his desk.

A survey of more than 160,000 acres, the size of the Northwoods Project in New York, generates more than 200 profiles. In that survey, the correlation with 3D seismic was profound.

Best results are obtained when the aeromag lines are flown at relatively tight line spacing, such as 1/4 by 1/2 mile. This facilitates detection of anomalies that, although small, extend across several lines (strike).

Texaco estimated that Starmag can interpret overnight-and often with better results-a 10,000 line mile magnetic survey that would take an experienced interpreter 3-5 months to analyze.

Neural network technology is also capable of interpreting formation permeability from wire line logs, making it useful in enhanced oil recovery efforts.

Pearson, deRidder & Johnson (PRJ) believes that Starmag, available only the last 4 years, may have reached no more than one-fourth of its audience.

Starmag is a highly advanced interpretation technique in the magnetics toolbag, but many operators do not use magnetics. Seismic surveys command a high percentage of oil and gas geophysical spending.

The Authors

Joseph P. Fagan Jr. has been a geophysical engineer with Pearson, deRidder & Johnson Inc. of Lakewood, Colo., since 1993. His principal work involves merging potential field geophysics with other geophysical and geologic data to provide integrated interpretations for the petroleum and mining industries. He holds a BS degree in geology from the University of Illinois at Urbana-Champaign and an ME degree in geophysical engineering from the Colorado School of Mines.

David L. Copley is a petroleum geologist and president of Ardent Resources Inc., Buffalo, N.Y. He has been a practicing petroleum geologist in the Appalachian basin for over 25 years. He holds a BA degree in geology from Alfred University and an MS degree in geology from the University of Toledo. Ardent Resources drills and operates in the northeastern U.S. and Canada.

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