EXPLORATION Nonseismic methods can provide many views of a drillsite

J.P. Land
J.P. Land & Associates
Houston

• Predictive Accuracy Summary [13119 bytes]

In a world of low oil and gas prices it is well to keep in mind other advanced exploration techniques that have the ability to quickly focus on promising targets that can then be defined in detail by the ever improving but more expensive seismic method.

This article looks at the predictive accuracy of various nonseismic methods to suggest the positive effect their use can have on discovery rates, development programs, finding costs, and reducing the number of dry holes.

Enough information

Imagine that you have developed an exciting 3D seismic picture of a structural anomaly and are preparing to spend an even greater sum drilling a wildcat.

Considering the time, energy, and money already spent on acquisition and interpretation of data and leasing, several questions possibly remain:

• Do you have all the information necessary to give you an edge?

• What indications do you have that hydrocarbons are present in commercial quantity?

• Your subsurface control could have been better, but it was supported by the seismic picture. Do you have nonseismic data that support the location?

• What do you know about the section above your first reliable reflections?

• Is there any chance that your structural high is a velocity high?

• Do you have any evidence of an upper-section density anomaly that should be brought into your velocity determinations?

• Do near-surface gravity and/or magnetic responses show the prospect area to be anomalous or normal?

• Is there any surface geochemical indication of hydrocarbon microseepage and the associated alteration products that change lithologies and influence velocities?

• If a geochemical chimney is indicated, does it center on your location?

  Numerous exploration methods capable of prospect targeting and reducing risk are underused by most of the industry.

Micromagnetics can isolate local structural anomalies and, like soil susceptibility, radiometrics, induced polarization, and certain geochemical methods such as delta C and iodine, is a good indirect indicator of hydrocarbon microseepage.

Surface geochemistry provides direct indicators of microseepage. Do you have any of these indices agreeing that your drillsite is in the most promising location?

For industry to optimize drillsite locations and thus improve new field discovery rates and reduce finding costs, it must take advantage of the full range of technology at its disposal. There are a number of nonscientific causes for an exploratory well to end up as a dry hole, but considering the relative low cost of supportive nonseismic technology, one reason should not be insufficient information.

When decision-impacting information that costs a fraction of the cost of a dry hole is neglected or disregarded, needless failure is too often the result.

Field examples
Alabama Haynesville

The Jurassic Haynesville formation in Alabama has proven to be a worthy but elusive target.1

Some wells in North Frisco City field produced in excess of 3,000 b/d from the formation. However, just 4 miles southwest, Howell Petroleum Corp., drilling its Dogwood prospect, proved the structure and all but one of the essential elements were as anticipated.

Only the oil was missing.

Paramount Petroleum Corp.'s drilling of its two prospects north-northeast of North Frisco City was a repeat of Dogwood. The only missing element was the oil.

David Burkett, Howell's manager of geophysics at the time of the Dogwood test, said magnetics, gravity, and surface geochemistry were considered but not employed when selecting the well location.

A surface geochemical evaluation of any one of the three drillsites probably could have been accomplished for no more than $25,000. A dry hole in the area reportedly costs about $450,000.

The following is presented in the interest of helping the reader avoid certain exploration pitfalls and to bring attention to the predictive accuracy of some of the nonseismic methods and the importance of their being considered when any modern exploration program is being planned.

Drilling

Jenny,2 noting numerous occurrences of dry holes on the perimeter of previously reported micromagnetic prospects interpreted as structural anomalies, suggested that in the hundreds or sometimes thousands of feet of section above the first good seismic reflector, mineralized waters focused by deep-seated structure may bring about the cementation of the shallow sediments and thus change their specific gravities, magnetic susceptibilities, and seismic wave velocities.

He then offered several ways the resulting changes in density can affect seismic velocity assumptions and interpretations and cause drill holes to be located on the edge of structures rather than over their centers.

In 1984 the author of this article was contacted to review 12,000 line miles of airborne micromagnetics covering a 5,000 sq mile sector of the Upper Texas Gulf Coast and flown in the late 1950s.

The interpretation of the data resulted in approximately 100 prospect leads being defined. Many were coincident with residual gravity anomalies. In the 25 years of post-survey drilling, several of the features had become producing fields, though most had not as yet been tested by properly located drill holes.

One striking observation was the large number of anomalous features having dry holes on or just outside their perimeter. A current example of anomaly-perimeter dry hole drilling (Figs. 1a-d [94652 bytes]) stems from the review of a 1985 airborne micromagnetic survey in Choctaw County, Ala.

Subsurface geology shows that a number of magnetic prospect leads coincident with structural highs are still untested and prospective. Dry hole costs are $100,000-150,000 for 4,000-6,000 ft tests and $500,000-600,000 for 10,000-12,000 ft tests.

Jenny suggested that in such instances, rather than selecting the next drillsite using the same type of information, better to locate the next test in the center of a prospect lead developed by a different method.

Reconnaissance seismic was used in the 1960s to discover the six Greater Ekofisk fields in the North Sea. The incorporation of information from any other survey methods is not mentioned by Van den Bark and Thomas.3

Middle Tertiary rocks overlying the Ekofisk structure reportedly have ex- tremely low seismic velocity due to abnormally high pressures. This rock property distorted the structural interpretation and led to the early theory that the center of the structure was collapsed, a graben.

The initial drill test turned out to be an edge location and was dry. The "collapsed zone" later proved to be a richly productive "collapsed" velocity zone. Surface geochemical survey, which likely would have outlined the microseep and chimney with its associated altered lithologies, was reportedly never conducted by Phillips Petroleum Co., the operator.

Tompkins4 speaks of direct location technologies (DLTs), as those methods that indirectly relate to hydrocarbon microseepage through the alteration products of microseepage and, the direct location methods such as microbial surveys and soil gas surveys, adsorbed or interstitial.

He reports improved success rates and lower finding costs when DLTs are part of the exploration process:

• The average company with its 13% success rate spends $16-22/bbl.

• If DLTs are also used, the acreage leased is smaller, seismic expenditures are lessened, and apparently small reservoirs disregarded, resulting in $8-10/bbl finding costs and a 35% success rate.

  He mentions one company that improved its success rate to 29% from 8% and finding costs to $3.59/bbl.

Remote sensing

Landsat analysis has valuable attributes for the initial, very low cost focus on valid prospect leads and the initial regional highgrading that is of basic importance to the reconnaissance process.

In an unpublished 1986 article,5 Paul Oman refers to a report by Everett & Petzel on an Anadarko basin study in which 76 Landsat anomalies were identified; 59 of them correlated with oil and gas fields and 11 correlated with nonproductive structures.

Saunders6 reported that in 20 years of drilling Landsat geomorphic anomalies in the Rocky Mountain region, a total of 2,164 geomorphic anomalies were delineated. As of 1979, 1,177 of them had been tested, of which 637 or 54% were producers.

Sundberg,7 discussing the use of Landsat imagery in eastern Colorado, said, "Even the most promising wells from an alteration standpoint are sometimes dry, but a final success rate of up to 25% is obtained" if higher priority signatures are properly considered. The industry average for the region is about 15%.

Havertz and McCoy8-9 cite Vixo and Bryan's mapping of Landsat lineament density anomalies in the Illinois basin in which 67% of the lineament density highs coincided with oil fields.

Surface geochemistry

Most known oil reserves are associated with hydrocarbon seeps, macroseeps visible to the naked eye. Link10 showed their worldwide distribution.

Sixty years of surface geochemistry has produced technology that, when properly applied, allows detection of hydrocarbon seeps that are invisible to the naked eye- microseeps. We can focus on direct evidence of trapped hydrocarbons while setting aside areas that have little or no apparent potential for commercial production.

The exploration of the Destin dome in the eastern Gulf of Mexico gave a lesson worth remembering in the value of surface geochemistry.

Davidson11 reported that a group of companies, invited by Exxon to join exploration of the giant structure, hired Horvitz Laboratories to conduct a surface geochemical survey of the area. Because that survey's results were negative with no indication of hydrocarbon accumulations, the group declined participation.

Champlin Petroleum Co., however, decided to join Exxon and contributed well over $100 million12 to the drilling of what proved to be a dry structure.

An exploration vice-president who used the geochemical findings to keep his company out of the failure felt, "The best way to use geochemistry is to determine where not to drill a well."

Horvitz Laboratories in 1973 conducted a series of reconnaissance hydrocarbon surveys covering 5,000 sq miles of the Texas and Louisiana offshore. Behrman and Land,13 reporting on the results of subsequent drilling, showed that:

• 89 prospective features had been defined by the geochemical surveys.

• 43 of the 89 had been tested by drilling.

• Of the 43 tested, 36 were proven productive, a predictive accuracy of 84%.

• 160 (expensive) dry holes had been drilled on the edges of or outside the prospective features.

Davidson's personal experience14 while with Crown Central Petroleum Corp. was that between 1940 and 1957 the company in using delta carbonate geochemistry found "4.7 million bbl of oil at 1/6 the cost of another 11.5 million bbl found using methods other than geochemistry." The company used delta carbonate geochemistry to purchase leases on the eastern shelf of the Permian basin, and all subsequent production was from stratigraphic traps. During the same period, the company spent 8% of its lease money drilling behind geochemistry, and by 1960 was taking about 65% of its production from those leases.

Davidson also noted a report by Kuzmin of a 58% new field wildcat success rate in six Russian basins when surface geochemistry was used (26 productive pros- pects of 45 tested). Also, the Academy of Sciences and Ministry of Geology of the Former Soviet Union reported success rates credited to geochemical surveys of 90% in the Komi region and 70-80% in the middle Volga region.

Severne et al.15 wrote that in Australia surface geochemistry was being used mainly to rank seismic prospects. They also said surface geochemistry's potential for defining leads to stratigraphic oil is certainly attractive. "...Over 30 wells were drilled in areas condemned by the soil analysis method (adsorbed gas). Twenty nine of the negative predictions proved to be correct..."

Rice16-17 presented examples of the valuable input surface geochemical profiling brings to a seismic section as to if and where a seismic anomaly appears to be actively seeping hydrocarbons.

Radiometrics

Weart & Heimberg18 re- ported on radiometric programs in six sectors of the U.S. from the Rockies to Florida and from North Dakota to the Gulf of Mexico, survey areas involving 724 wells.

• 85.7% of the producers drilled were in radiometrically favorable locations.

• 68.7% of the dry holes were drilled in radiometrically unfavorable areas.

• Overall the authors felt that radiometrics may be correct three out of four times.

  Curry,19 reporting on the results of the drilling of 282 wells in a 4,465 sq mile area of the Powder River basin of Wyoming, gave these figures:

• Of 55 wells drilled in the middle of radiometric anomalies, 36 were productive (65% accuracy).

• Of 38 wells drilled inside the edges of radiometric anomalies, 21 were productive (55% accuracy).

• Of the 93 wells drilled within anomalies, 57 were productive (61% accuracy).

Magnetic susceptibility

The microseepage-related geochemical alteration of formations in the chimney above a reservoir changes formation magnetic susceptibilities. Lo- cating such susceptibility anomalies can inexpensively provide high priority targets to be ground-truthed by geochemical survey.

Henry20 wrote of a 14,400 sq km frontier area test program in which susceptibility measurements were compared with the adsorbed hydrocarbon analysis of 1,500 soil samples collected at the bottom of 10 m shot holes. Susceptibility anomalies were 3-5 times background, and light hydrocarbon anomalies were 4-7 times background. Results showed a positive correlation of susceptibilities and hydrocarbons for three-fourths of the samples measured.

Induced polarization

As Henry20 pointed out, induced polarization, a mainstay method of the mining industry in the exploration for sulfides, has application in petroleum exploration.

Magnetic susceptibilities will vary as the minerals present in a geochemical chimney vary with different geochemmical environments and processes that depend, for one thing, on which hydrocarbons, sweet or sour, leak from the reservoir. Under strongly reducing, moderately sulfidic conditions, certain magnetic minerals may be altered to the nonmagnetic sulfide, pyrite, or magnetite reduced to nonmagnetic siderite. Magnetic minerals such as magnetite and maghemite form through the oxidation of other iron minerals and greigite and pyrrhotite as metastable precursors to pyrite. Induced polarization should thus be useful in combination with micromagnetics in the discrimination of magnetic and nonmagnetic mineralization and the resultant susceptibilities.

Brown-Bassett gas field, Terrell County, Tex., has an IP signature due to the high percentage of pyrite present. This is probably why, in a 1,250 sq mile multidiscipline study,21 micromagnetics showed a linear structured zone coincident with the main fault north of production but no recognized anomaly over the reservoir such as were developed from the subsurface and photogeologic datasets-in other words, no magnetic expression resembling those associated with other producing structures in the survey area.

IP data undoubtedly could have provided clues for interpreting micromagnetic data, another case for multidiscipline exploration.

Microbial survey

The microbial oil survey technique (MOST) is far from new, having been discussed as early as 1937 by Mogilevsky.

Beghtel et al.22 presented results of a 1980 study in Kansas. MOST surveys were conducted over the projected locations of 86 randomly selected new field wildcats, and the MOST data were interpreted to predict the drilling outcomes: