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Finding Oil and Gas in Oklahoma

If precious metals are not your forte, then try the petroleum industry. Exploration for oil and gas has always depended on surface maps of rock types and structures that point directly to, or at least hint at, subsurface conditions favorable to accumulating oil and gas. Mapping begins with reconnaissance, and if that indicates the presence of hydrocarbons, then detailed mapping begins. Originally, both of these maps required field work. Later, the mapping job became easier by using aerial photos.

After the mapping, much of the more intensive exploration depends on geophysical methods (principally, seismic) that can give 3-D constructions of subsurface structural and stratigraphic traps for the hydrocarbons. Then, they are sampled by drilling and their properties measured.

Landsat, and other space imaging systems, serve as mega-photos that depict large areas, within which clues to subsurface conditions may be evident. In general, most of the obvious structures that have surface expression had been discovered and mapped (to varying extents) over much of the world. Some regions, however, were not adequately mapped even in the 1970s, so that the advent of higher-resolution space imagery proved a boon to energy companies seeking new sources of fossil fuels. Sometimes the imagery proved especially sensitive to subtle indications of interior structures. For instance, fractures around structures in known oil/gas fields may extend further, as seen in the coherent space images, than suspected from ground work. Also, drainage patterns at broader scales may reflect control by underlying rocks involved in suitable traps. And even vegetation distribution may disclose signs of structure. These and other indicators discernible in space imagery appealed to exploration geologists as another means to survey large areas.

We illustrate these ideas by examining and evaluating one of the first case studies using Landsat-1 to demonstrate the feasibility of direct exploration from space. This study, conducted jointly by the Eason Oil Corp. and the Earth Satellite Corp., shed considerable light on effective criteria for recognizing conditions that might relate to buried hydrocarbons. In addition, some of the pitfalls associated with the space approach were also discovered by carefully assessing the results reported by these investigators.

The strategy behind the study was to look at Landsat imagery of a region already established as a petroleum province, giving special attention to telltale surface indications of the presence of known fields. They used standard-processed and computer-enhanced versions. Rather than test capabilities in a region where there is obvious structural control and other clear-cut evidence, they selected producing areas, in which the surface does not give clear indication of subsurface conditions. If they could succeed in detecting hydrocarbons under such difficult circumstances, then Landsat would increase in stature as an oil/gas discriminator .

The Anadarko Basin of south-central Oklahoma fits this requirement well. Located in the eastern Great Plains, with most of the land used for farming and ranching, the Basin is one of the great producers of the mid-continent petroleum province, which also includes much of Texas, as well.


Map of the Anadarko Basin in south-central Oklahoma.

The Basin is a down-sag in the crust that has allowed up to 15,200 m (50,000 ft) of Paleozoic sedimentary rock to accumulate. Structurally, the Basin is an asymmetrical geosyncline (a regional-scale downfold), with the deepest part near the south edge. Oil and gas are present in porous rocks associated with structural (anticlines; fault blocks) and stratigraphic traps. Large gas fields occur mainly along the Basin's western half, whereas oil is more common in the eastern half. Wells as deep as 7,600 m (25,000 ft) have recovered both hydrocarbons, although most pay zones are between 2,750-5,250 m (9,000-15,000 ft).

Generally, surface expression of underlying oil or gas traps in the Basin is meager, because first, there are few structural indicators in the flat-lying sediments atop older folded units and second,there is overprinting of geologic features by vegetation and land use (grasslands; hilly sage-covered terrain; and wheat farmlands). The Eason Oil/Earthsat investigators decided to focus on two search elements: previously undiscovered fractures and subtle chemical alterations of surface rocks by escaping hydrocarbons.

Lineaments analysis was conducted by Eason Oil using Landsat image transparencies backlighted on a light table. The linear features they picked are shown by lightweight black lines on the map below. Superposed as brown and green-black heavier lines are faults that had previously been discovered and mapped. As a geographic reference, note the meander bends (curved segments) of the Canadian River, traced in blue. The majority of the Landsat-mapped linear features are inconspicuous in the imagery. Many of them are suspect, i.e., they could be non-geological or some type of lighting artifacts.


Lineament Analysis map conducted by Eason Oil.

A group of four geologists, including this writer, in Code 923 at Goddard Space Flight Center decided to check on the reproducibility of these map results. Each person used the same transparencies (mostly winter images) as Eason Oil and worked independently of one another to minimize bias. When done, we registered the tracings to a base map, on which the Eason Oil lineaments were also plotted, as seen below. The comparison disclosed rather startling discrepancies in terms of variance between the two groups. We found only about 20% of the total linear features in common. Eason Oil chose approximately 35% of the questionable features, exclusively, while Goddard geologists chose the remaining 45%, which represented those "missed" by Eason Oil. We immediately suspected that this kind of result is partially due to considerable subjectivity in deciding whether a given linear feature a) really exists, b) is geological in nature, and c) means anything.

Comparison of GSFC and Eason Oil Linears Selections diagram (A).

This suspicion was reinforced by comparing the linear features selected by the four Goddard geologists. Here are the results - a mishmash that requires the following interpretation:

Comparison of GSFC and Eason Oil Linears Selections diagram (B).

Of the 785 linear features identified by all four combined, only 4 (0.5%) were noted by every operator. From the remainder, 3 operators mutually selected 37 (4.7%), two operators agreed on 140 (17.8%), and the rest, 604 (77%), each operator found exclusively. This type of result has been reported in similar studies, although the above scores were particularly discouraging. Each geologist had ample experience in photointerpretation and special skills in analyzing Landsat imagery. Their choices were justifiable but overall, our results were questionable.

5-10: In this experiment, and in the technique of picking linear features in space imagery, what do you think was really going on behind the end result of some many linears being found but not consistently by multiple interpreters? ANSWER

The bottom line here is that there often is a strong tendency towards overkill in choosing features that appear to be meaningful lineaments. So many are drawn that it would take a monumental field effort to check them out. If plotted as rose diagrams (see Section 2), they may reveal valid trends for the orientations of regional fractures, because statistically lineaments of non-geological nature should be in the minority. A study of obvious lineaments in the Adirondacks confirmed this result. Of the 200+ prominent ones that were field-checked, geological fractures directly or indirectly controlled most of them, but about 20% related to human factors, such as fence lines, roads, etc. Thus, we conclude that we should combine lineaments analysis with other indicators of mineralization or hydrocarbons. This combining would encourage geologists to field-check particular sites to verify the lineament presence and nature and their possible correlation with these indicators.

The Eason Oil study sought to recognize such indicators. Their interpreters delineated certain geomorphic anomalies, such as circular patterns and unusual drainage. In the course of their image appraisals, they noticed unexpected tonal patterns that looked a bit like light-colored smudges on the images. These they called "hazy" features, as seen here:

We labeled three typical hazy patterns A, B, and C. The one at A, at a bend in the Canadian River, is especially prominent, and occurs over a known oil field.

A standard false color subscene around A shows the hazy to have a bluish-white color similar to soils in barren fields. Note the road pattern and white blotches which are accesses to producing wellheads. The yellowish areas coincide with unaltered Permian (late Paleozoic) red beds.


False color subscene at (A) of the previous image.

When we process this April Multispectral Scanner image into three ratio bands that we then combine into a color image (4/5 = Blue; 5/6 = Green; 6/7 = Red), the hazy feature at A takes on a unique yellow-green, and the red beds become orangish.

MSS color image of subscene at (A).

From the multiseasonal data sets, only those scenes imaged in late winter to early spring show hazies. At other times of the year, vegetation masks the phenomenon. To understand their explanation of the features, we look now at this photograph of two rock types:

Color photograph of two rock types found at White Mountain, Utah.

The rock on the far left is a sample from the red beds (sandstones) of Permian age. Next to it is the same material that has been color bleached to yellow-brown by converting iron oxide cement into hydrated iron oxides (analogous to rust). The gray rock on the far right is a limestone (calcium carbonate). To its left is a gypsum rock (hydrated calcium sulphate). Both interior rocks appear to be altered equivalents of the primary exterior rocks. In the field, comparable altered rocks can occupy many square miles.

To account for these hazy features, the Eason Oil people postulated that chemical reactions affected the iron cement or transformed the carbonates into sulphates, when sulphur-laden gases or fluids leaked out of petroleum traps and rose towards the surface, interacted with susceptible rocks, and brought about compositional changes. About the time of their conclusion, evidence for such changes was reported as the doctoral thesis of Terrence Donovan (later of the U.S.G.S.), in which escaping hydrocarbons drastically altered rocks above the Cement Field, at the southeast edge of the Anadarko Basin. Dr. Donovan found a pronounced set of anomalous values of the ratio of C13 to C12 in samples collected over both producing zones in the field, shown as contoured areas below:

Diagram of Dr. Donovan's theory relating C-13 and C-12 in the Cement Field at the southeast edge of Anadarko Basin.

These values represent some of the highest departures from normal ratios known anywhere in the world. He attributed them to the effects of chemical action by carbon-rich fluids on the rocks which, as a consequence, appear bleached. Oddly, while evident on the ground, this alteration is not detectable in the Landsat imagery, possibly because it is not strongly expressed in derivative soils.

Accepting this alteration hypothesis, the Eason Oil group looked for at least partial coincidence between these hazies and the surface projections of subsurface oil or gas fields. Of the 57 anomalies they mapped in a control segment of the imagery, they claimed an association with 42 producing fields. Another six occurred above or near non-producing structures, and only 9 showed no coincidence. If this observation remained true, then detecting hazies, sometimes correlative with lineament concentrations, could promise a powerful new way to hunt for oil and gas using space imagery.

This author , being skeptical, decided to challenge these findings. He traced the outlines of the Eason Oil hazy features (in a hachured pattern) on a transparency and then overlaid and registered it to the oil (pinks) and gas (blues) map of Oklahoma. The resulting combination is shown here:


Map combining the Eason Oil

Visually, the coincidence between hazies and fields does not appear strong. This was supported by a spatial correlation analysis, which demonstrated there is no statistical significance to the pattern distribution, i.e., the coincidence is random rather than associative. In practical terms, there would be at least as much chance of striking oil by drilling into points selected by throwing darts at the map, as there would be in drilling into the centers of hazies. Based on a quick field trip to the A hazy, the author believes hazy features are areas where wind has blown away much of the soil fines, leaving reflective quartz grains behind.

The Goddard geologists didn't perform these studies to discredit the Eason Oil study, which provided some valuable insights into the discerning power of space imagery for petroleum exploration and the potential shortcomings of the apparent results. We did them to independently evaluate this approach and to inject caution into any beliefs that this technique might become a panacea for finding petroleum. Suffice to close with the remark that since the launch of ERTS-1, the petroleum industry has found new oil and gas fields with the aid of space data and has developed criteria from the images that continue to prove worthwhile in planning and conducting exploration programs, which are leading to payoffs.

5-11: Imagine you are a geologist. Describe an approach to exploration for oil/gas using space imagery that could prove workable and successful. ANSWER

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Primary Author: Nicholas M. Short, Sr. email:

Collaborators: Code 935 NASA GSFC, GST, USAF Academy
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