ANOMALY-PERIMETER DRILLINGJ. P. Land Associates, Inc.
HOW WELL DO YOU KNOW YOUR DRILLSITE?
(Condensed from Nonseismic methods can provide many views of a drillsite: Oil & Gas Journal. February 26, 1996)
In a world of low petroleum prices it's well to keep in mind advanced reconnaissance techniques that have the ability to quickly focus on promising exploration targets that can then be evaluated and defined by the more expensive methods. This article looks at the predictive accuracy of such methods and the positive effect their input can have on discovery. rates, development, finding costs and reducing the number of dry holes.
Let's say you've developed an exciting 3D seismic picture of a structural anomaly. Now you're preparing to spend an even greater sum of money drilling your wildcat well. Considering the time, energy and money that's already gone into the acquisition and interpretation of data and lease acquisitions, do you have all the information necessary to improve your chances of success? Do you have indications of a strong local presence of hydrocarbons? Your subsurface control is somewhat lacking but it was supported by the seismic picture. Do you have any other data supporting the location? What do you know of the section above your first reliable reflections? Any chance your structural high is really a velocity high? Any evidence of an upper-section density anomaly that should be brought into your velocity determinations? Do near-surface gravity or magnetic responses show the prospect area to be anomalous or normal? Any surface indication of hydrocarbon microseepage? If there is, does it center on you planned location? To improve your odds, these questions need good answers.
There are numerous, proven reconnaissance methods capable of producing viable prospect leads and reducing risk. Micromagnetics can isolate local structural anomalies and, like soil susceptibility, radiometrics, induced polarization and certain geochemical methods such as Delta C and Iodine, it can indicate the products of hydrocarbon microseepage, rock property anomalies. Surface geochemistry provides direct indications that the drillsite is within an active hydrocarbon microseep. Do you have any of these indices agreeing that your drillsite is in the most promising location?
There are a number of non-scientific causes for an exploratory well to end up a dry hole but, considering the relative low cost of non-seismic technology, one reason should not be insufficient information.
Alabama's Haynesville formation has proven to be a worthy but elusive target, according to Durham(1993). Some wells in the North Frisco City field have produced in excess of 3,000 b/d from the formation. But, just four miles to the southwest, Howell Petroleum, drilling its Dogwood prospect, proved that the structure and all but one of the essential elements were there as anticipated. Only the oil was missing. Paramount's drilling of two prospects north-northeast of North Frisco City field was repeat of Dogwood. As to what techniques other than seismic were used to pick the well location, magnetics, gravity and surface geochemistry were considered but not employed. A surface geochemical evaluation of any one of the drillsites probably could have been accomplished for no more than $25,000. Dry holes reportedly cost in the order of $450,000.
The following is meant to focus on the predictive accuracy of some of the non-seismic methods and the importance of their being considered when an exploration program is being planned.
ANOMALY-PERIMETER DRILLING
Jenny (1952), noted numerous occurrences of dry holes being drilled on the perimeter of previously mapped micromagnetic prospect leads he'd previously interpreted as structural anomalies. He suggested that mineralized waters focused by deep-seated structure can bring on the cementation of the shallow sediments changing their specific gravities and seismic wave velocities. Undetected, such anomalous densities could lead to erroneous velocity assumptions and cause drill holes to be located on false structures or on the edge of structures.
A review of airborne micromagnetic data covering 5,000 square miles of the Upper Texas Gulf Coast resulted in the definition of approximately 100 prospect leads, many of them coincident with residual gravity anomalies. When the prospect lead map was laid over a production map, several of the magnetic leads were found to coincide with producing fields discovered during the post-survey period. The majority of the leads had not, as yet, been tested by drilling. A large number of anomalous features had dry holes on or just outside their perimeters.
This same situation appeared in a recent review of the interpretation of a 1985 airborne micromagnetic survey in Choctaw Co., Alabama. A number of magnetic prospect leads coinciding with structural highs, are surrounded by dry holes and are still untested. 10,000-12,000 foot dry holes cost $500,000 - $600,000.
Reconnaissance seismic was used in the 1960's 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 (1981 ). Middle Tertiary rocks overlying the Ekofisk structure reportedly have extremely low seismic velocity due to abnormally high pressures. This rock property evidently distorted the structural interpretation and led to the initial theory that the center of the structure was a graben. The drill test turned out to be an edge location and dry. The graben later proved to be a richly productive low-velocity zone. Surface geochemical survey, that likely would have outlined the microseep and its altered lithologies, was reportedly never conducted by Phillips Petroleum, the operator.
REMOTE SENSING
Landsat analysis has valuable attributes for the initial, very low cost, regional focusing on prospect leads. Sundberg (1990), 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 approximately 15%. Havertz and McCoy (1985) 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. Link (1952) showed their worldwide distribution.
Sixty years of surface geochemistry has produced technology that, when properly applied, allows us to detect hydrocarbon seeps that are invisible to the naked eye, microseeps. We can focus on direct evidence of the local presence of trapped hydrocarbons while setting aside areas which have little or no apparent potential for commercial production.
The Destin Dome in the Gulf of Mexico gave us a memorable lesson in the value of surface geochemistry. Davidson (1985) reported that a group of companies, invited by Exxon to join in the 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, however, decided to join Exxon and contributed well over $100,000,000 to the drilling of what proved to be a dry structure.
In 1973 Horvitz conducted a series of proprietary reconnaissance hydrocarbon surveys covering 5,600 square miles of the Texas and Louisiana offshore. Behrman and Land (1992), reported that:
RADIOMETRICS
Weart & Heimberg (1979) reported on radiometric programs in six different sectors of the U.S. from the Rockies to Florida and from North Dakota to the Gulf of Mexico, survey areas involving 724 wells.
MAGNETIC SUSCEPTIBILITY
Geochemical alteration can result in changes in formation magnetic susceptibilities. Locating susceptibility anomalies can provide inexpensive yet high-priority targets for surface geochemical ground-truthing.
Henry (1989) tells of a 14,400 sq. km. frontier-area test comparing soil susceptibilities and adsorbed hydrocarbons in 1,500 soil samples. Susceptibility anomalies were 3-5 times background and hydrocarbon anomalies were 4-7 times background. Correlation of the two properties were positive for three-fourths of the samples.
INDUCED POLARIZATION
Henry (1989) suggests, for interpretation reasons, that Induced Polarization (IP) be used in tandem with magnetic survey for the discrimination of magnetic and non-magnetic mineralization caused by geochemical alteration. Magnetic minerals now present will vary as the environment and chemical processes vary as well as which type of hydrocarbons, sweet or sour, escaped from the reservoir. The Brown-Bassett field of Terrell Co, west Texas has an IP signature due to the high percentage of pyrite (non-magnetic) present. This is the likely explanation for the magnetic anomaly at Brown-Bassett being so different from the majority of anomalies mapped in a 1,250 square mile airborne micromagnetic survey (Land, 1994).
MICROBIAL SURVEY
The microbial oil survey technique (MOST) measures the population of hydrocarbon-dependent bacteria in the soil as an inexpensive indirect indicator of microseepage. Beghtel, et al (1987) reported on a study in Kansas in which MOST surveys were conducted over 86 projected new field wildcat locations: MOST picked 18 to be producers. Of those, 13 were completed as commercial successes, a 72% success rate.
Of the 86 drilled, 26 were completed as commercial producers, a 30% success rate.
Trost (1993 ) evaluated nine prospects in the Denver Basin of Colorado using the Headspace Gas and MOST geochemical techniques. MOST was 66% accurate in predicting producing wells and 100% accurate in predicting dry holes. The Headspace Gas technique was 50% successful in predicting hydrocarbons and 100% successful in predicting dry holes.
MULTIDISCIPLINE EXPLORATION
Thompson et al., (1994) using an undisclosed variety of geological and geochemical based methods reported 10 stratigraphic fields discovered of 19 drilled (53%) during the 1980-84 period on the Eastern Shelf of the Permian Basin. In another area a discovery rate of 36% was achieved with total exploration and development costs of $0.67 per barrel. Overall, new field discovery rates ranged between 36% and 60% using geochemical related methods and 14% when geochemical related methods were not used.
SUMMARY
When, in spite of 3D seismic's powers to define structure, industry's New Field Wildcat success rate is still in the order of 10% it would seem that something more needs to be added to the interpretation/decision mix. The technology is there that will tell us if we're on firm ground and our probability of success. Disregarding the screening techniques and relying too heavily on too little information is courting failure.
