FINDING AFFORDABLE GEOPHYSICS IN TOUGH TIMES

Based upon a workshop co-sponsored by PTTC's Appalachian Region on May 18, 1999, in Morgantown, WV.

BOTTOM LINE

Seismic and non-seismic geophysical methods, used judiciously, are effective in high-grading exploration areas, re-evaluating old data, reducing risk, and increasing production.

PROBLEM ADDRESSED

A computer-based system that re-evaluated old seismic lines particularly when combined with new processing, has helped generate new prospects adjacent to the Ordovician unconformity in the Appalachian Basin. Because the greatest obstacle for small operators in adopting seismic exploration is cost, non-seismic exploration methods—such as aeromagnetics and photo lineament analysis—can help high-grade prospective areas, reduce risk in locating hydrocarbons, and save money.

KEY WORDS:

Aeromagnetic Techniques, Seismic Fracture Detection, PC-based Software, Lineament Analysis

SPEAKERS

Basement Fault Block Pattern Delineation
S. Parker Gay, Jr., Applied Geophysics, Inc.

High Resolution Seismic Imaging for Fracture Detection
Ernie Majer, Lawrence Berkeley National Laboratory

PC-based Seismic Software to Prioritize Rose Run Prospects in the Western Appalachian Basin
Charles Weisenberg, Lauren Geophysical Services

Non-Seismic Exploration Methods
Ray Garton, Mammoth Geophysical, Inc.

TECHNOLOGY OVERVIEW

Both seismic and non-seismic geophysical methods can evaluate archived or newly acquired data.

Seismic Methods
There are several different types of seismic techniques, depending on the computer capabilities available.

High-Resolution Seismic Imaging for Fracture Detection. Past advances in seismic imaging that use surface methods can identify anisotropy, which can be caused by the geologic fabric, as well as fractures. Therefore, an approach is needed to sort out the different effects to identify the fractures controlling fluid flow.

In the “mixing model,” attenuation and velocity depend upon the resonance frequency of gas bubbles in fluid. Beyond the resonance frequency, the velocity of the air-water mixture equals water velocity; at frequencies below resonance, velocity is a combination of water and air velocities. The mixing model explains the large amplitude drop and small velocity change within fractures. Seismic resolution depends upon velocity inside of the fracture. This approach can be used to compute fracture aperture profiles.

PC-Based Seismic Interpretation Software
Compared to examining paper data plots, computerbased seismic analysis software systems allow rapid viewing of seismic lines in a wide variety of versions. For example, line flattening on a particular horizon can be done in less than a minute, polarity can be rapidly reversed, color-variable density plots can be quickly created and colors tuned to highlight features, and a variety of seismic-attribute plots can be quickly displayed.

Old seismic lines from the Rose Run play in Ohio were re-interpreted using PC-based seismic software to determine if this capability could be cost-effective for small operators.

Frequency filtering was useful for over-producing remnants related to the basement-controlled structure. For example, filtering to show only low frequencies (15-30 Hz) often shows a pronounced positive amplitude anomaly at the regional Ordovician Knox unconformity that was not obvious on standard plots. Using middle frequencies (30-50 Hz), the amplitude anomalies are less apparent, but the structure is sometimes more clearly portrayed. Producing remnants often show a low-frequency anomaly at the Knox unconformity level and there is sometimes a mid-frequency anomaly above the remnant. Plots flattened on a particular reflection are often useful to show slight thickening of the upper Knox, thinning above a remnant, or to highlight the structural history.

On the re-examined old lines, numerous undrilled marginal prospects were identified, including small structures. There also was evidence of a rise of the unconformity level and a small amplitude anomaly. Most operators in the Rose Run play have large libraries of old, previously evaluated seismic lines, representing a substantial investment. Re-evaluation using a computer system may produce new prospects from this data, particularly if combined with new processing.

Non-Seismic Methods
Non-seismic exploration methods include earth resistivity, radiometrics, geochemistry, halogens, microbiological, pH/Eh methods, gravity, magnetics, microwave/radar, radio wave, remote sensing/lineament mapping, and others. These techniques should be used to high-grade prospect areas for seismic planning, verify a seismic prospect through increased data and knowledge of the prospect (as stand-alone prospecting tools), and save time and money.

Basement Fault Block Pattern
Complex patterns of basement faulting are present under sedimentary cratonic basins, which include the majority of global hydrocarbon producing basins. A subsequent movement of basement faults, and the blocks between them, localized most of the structure and stratigraphy in the sedimentary sections. The basement fault block pattern largely controls topography of the basement, which affects structure and stratigraphy through gravitational compaction within the overlying sedimentary basin fill.

Correlating the basement fault block pattern—mapped by aeromagnetic techniques—with structural and stratigraphic features of the sedimentary basin fill, was demonstrated in the Appalachian Basin. Potential prospects, which are mappable using residual magnetics, can be generated from fault block maps by integration with known geology. These can be turned into actual prospects by follow-up seismic profiling.

Explorationists who follow this procedure can concentrate on high-priority areas, lower finding costs and increase reserves. A $50,000 expenditure to map the fault blocks with magnetics at 1/2-mile spacing over a 40x40 mile area costs only 5-10% of a typical seismic budget. Additionally, with a basement fault pattern map in hand, it is possible to reduce the seismic budget by 20-80%, compared to a seismic-only program, and end up with the same amount of drill able prospects. The saved money could be invested later in a 3-D seismic grid for definitive prospect evaluation.

Basement fault block maps can also assist interpretation after seismic data has been acquired. For example, it can be used to determine the correct alignments of structural or stratigraphic trends that 2-D seismic lines locate but typically cannot connect between lines.

Horizontal drilling may also benefit from basement fault block mapping. A recent study of the Bakken Play in North Dakota showed that the intersections of basement shear zones provided the best yields for horizontal wells by a factor of two or three.

At this time, magnetics appears to be the only feasible method to inexpensively map basement surfaces below the sedimentary cover. Gravity and seismic imaging are excellent follow-up tools for making cross sections of promising leads developed from basement patterns determined by magnetics.

Photolineament Analysis
In the central Appalachian Basin, productive gas wells and dusters are often located close together with no general trends visible. In these highly heterogeneous reservoirs, hitting a fracture often makes the well productive. A lineament can be a continuous linear/curvilinear feature or it may be an alignment of points or short linear features seen on an image.

Some lineaments may be obvious, while others are more subjective. Significant differences in lineament maps for the same area may be caused by image factors and image types. Image factors include season or time of day, cultural disturbance, scale, and contrast and density. Scale is the most important factor because, as the scale changes, so does the resolution. For example, individual fractures in an aerial photograph may appear as a single lineament.

Widely available image types include Landsat, US Geological Survey mapping photography, topographic maps, Digital Elevation Models, and Side-Looking Airborne Radar. Mapping techniques may be automated or based on visual analysis of images. In either case, the development of an exploration model using photolineament analysis requires finding a relationship between the value of a well and its spatial position with respect to lineaments.

Most previous analyses have used simple linear relationships between well value and distance to lineament, lineament intersection, orientation, or lineament density. Determining the “value” of the well is not straightforward because of problems including the measure of production used, liquid-gas interference, group metering, and management decisions. Initial open flow is the most abundant form of data and may be the most consistent between areas. Cumulative production, where available, may be a better measure, but is affected by management decisions.

Multiple regression analysis is generally used to find a better-than-average probability of a good well. Unfortunately, correlation coefficients typically are not large because lineaments represent only vertical fractures, which have a limited effect on production in many geological settings.

A major conclusion is that lineament analysis should always be used in combination with other techniques for locating well sites. In addition, the proximity of wells to lineaments may affect gas production either positively or negatively. Furthermore, the proximity to lineaments may affect gas well value by about 10 to 20%, but the amount of improvement may be enough to make the well profitable.

FIELD RESULTS

The photolineament analysis results in the central Appalachian Basin are different in each area investigated. This was demonstrated in a paper by Fagan and Copley, “Aeromag Interpretation Technology Helps Chase Cambrian in New York” (1998). It documents an example of using aeromagmetics to delineate deep-seated structural patterns related to Cambrian oil and gas reservoirs. This technique was used to extend the Rose Run play into western New York. This play has been historically important in Ohio and Pennsylvania where 57% of Rose Run wells were productive in 1995.

Aeromagnetics were used to image the basement by identifying faults. Because most sedimentary rocks have very low susceptibilities, the stratigraphic column contributes little to the aeromagnetic response from the basement. Crystalline basement in New York contains a great amount of lithologic variation. Mapping smaller subparallel zones of deformation within the basement on either side of the Central Metasedimentary Belt and the Central Gneiss Belt in western New York is of primary importance in oil and gas exploration in the area.

The study demonstrated a relationship between basement variations and structural features associated with hydrocarbon production. Because of lack of deep-well control, the lower Paleozoic in western New York is still a frontier play. In such a setting, successful exploration programs must high-grade thousands of acres. The results from this project showed that aeromagnetic data can be used as an exploration technique to image basement that underlies and is adjacent to Cambro-Ordovician reservoirs, thus reducing the area to be investigated in detail.

CONNECTIONS:

S. Parker Gay, Jr.
Applied Geophysics, Inc.
675 S 400 East
Salt Lake City, UT 84111
Phone 801-328-8541, Fax 801-363-6243, E-mail benagi@aol.com

Ernie Majer
Lawrence Berkeley National Laboratory
1 Cyclotron Road Mailstop XXX
Berkeley, CA 94720
Phone 510-486-6709, Fax 510-486-5686, E-mail ELMajer@lbl.gov

Charles Weisenberg
Lauren Geophysical Services
12150 E. Briarwood Ave., Suite 230
Englewood, Colorado 80112
Phone 303-799-1637, Fax 303-799-1650 E-mail info@laurengeo.com

Ray Garton
Mammoth Geophysical, Inc.
PO Box 200,
Barrackville, WV 26559
Phone 800-822-6788, Fax 304-366-8019 E-mail garton@mammoth-geo.com

For information on PTTC’s Appalachian Region and its activities contact:
Douglas Patchen, Program Director, Appalachian Oil & Natural Gas Research Consortium
West Virginia University, NRCCE-Evansdale Dr., PO Box 6064
Morgantown, WV 26506-6064
Phone 304-293-2867 x-5443, Fax 304-293-7822, E-mail dpatch@wvunrcce.nrcce.wvu.edu

Disclaimer: No specific application of products or services is endorsed by PTTC. Reasonable steps are taken to ensure the reliability of sources for information that PTTC disseminates; individuals and institutions are solely responsible for the consequences of its use.

The not-for-profit Petroleum Technology Transfer Council is funded primarily by the US Department of Energy’s Office of Fossil Energy, with additional funding from universities, state geological surveys, several state governments, and industry donations.

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