SURFACE EXPLORATION METHODS IN MATURE BASINS

Based on a workshop sponsored by PTTC's Midwest Region on December 10, 1999 in Lansing, Michigan

BOTTOM LINE

Surface exploration for hydrocarbons can be optimized by meeting three conditions: (1) more than one well-designed method is used during an interdisciplinary effort, (2) if the results are integrated with subsurface geological and geophysical reality, and (3) if calibration surveys are run for known reservoirs or recent discoveries. By doing so, operators can cost effectively reduce exploration and development risks.

PROBLEM ADDRESSED

Producers are continually looking for cost-effective approaches to reduce exploration and development risks— from pursuing leads and prospects to high-grading leases to assessing reservoir compartmentalization. When combined with subsurface geological and geophysical information, surface exploration methods can reduce risk. Importantly, they can verify the presence of hydrocarbons. Surface expression of hydrocarbon seepage takes many forms, which has led to the development of many surface exploration methods. The most successful methods are based on direct detection of hydrocarbons or hydrocarbon-induced alteration anomalies.

KEY WORDS:

Geochemical anomalies, Hydrocarbon microseepage, Soil hydrocarbons, Surface geochemical exploration

SPEAKERS

Surface Geochemical Technology/Exploration
Dietmar Schumacher, Geo-Microbial Technologies Inc.

Kansas Red Top Program
Lynne Davison, Pangaea Geochemical Technologies, Inc.

TECHNOLOGY OVERVIEW

Since the first development of surface geochemical methods more than 60 years ago, surface prospecting has gone through periods of acceptance and application followed by periods of skepticism. The underlying assumption of all near-surface geochemical exploration techniques is that hydrocarbons are generated and/ or trapped at depth and leak in detectable quantities to the earth's surface. It is further implied that the surface anomaly can be reliably related to a petroleum accumulation at depth. This relationship occurs in places, but not everywhere, hence it is extremely important to integrate surface geochemical methods with traditional subsurface geological and geophysical methods.

Surface expression of hydrocarbon seepages takes many forms, including anomalous hydrocarbon concentrations in sediment, soil, water, and the atmosphere; microbiological anomalies; fluorescence anomalies; mineralogical changes; geothermal and hydrologic anomalies; bleaching of redbeds; geobotanical anomalies; and even altered acoustic, electrical, and magnetic properties of soils/ sediments.

The variety of near-surface expressions has led to development and marketing of many surface exploration methods. Recent studies have established that the most successful methods are those based on direct detection of hydrocarbons or hydrocarbon-induced alteration anomalies. These recent studies also indicate that hydrocarbon accumulation is dynamic and that seals are imperfect; all petroleum basins display some near-surface hydrocarbon leakage; surface expression of leakage is not always detectable by conventional means; hydrocarbon seepage can be active or passive, it can be visible (macroseepage) or only detectable chemically (microseepage). Migration is chiefly vertical and able to migrate through thousands of feet of strata in a short time (weeks to years).

Concerns About Using Surface Exploration Methods. Because the surface expressions of hydrocarbon migration are complex and varied, there is no simple one-to-one correspondence between near-surface anomalies and subsurface hydrocarbon accumulations. And no single method works everywhere. Techniques are generally unable to relate the surface anomaly to a specificsource, depth, or reservoir.

Discovery of a surface geochemical anomaly does not guarantee discovery of commercial hydrocarbons, but it does establish the presence of hydrocarbons in the area of interest. Traps and structures along migration pathways should be more prospective than those not associated with hydrocarbon anomalies. However, because relationships between surface/ subsurface geochemical anomalies and hydrocarbon accumulations are complex, their interpretation requires interdisciplinary integration. Surface exploration methods cannot replace conventional exploration methods. Rather they compliment them. If possible, it is a good idea to employ more than one surface detection method and calibrate surveys over a reservoir or new well.

Assessing Reservoir Compartmentalization. Geochemical and geomicrobiological surveys have documented that hydrocarbon microseepage is common, is predominantly vertical (with obvious exceptions), and responds quickly to changes in reservoir conditions. Because microseepage is nearly vertical, the extent of an anomaly at the surface can approximate the productive limits of the reservoir at depth. Further, the pattern of microseepage over a field can reflect reservoir heterogeneity and distinguish hydrocarbon-charged compartments from drained or uncharged compartments. Microbial Reservoir Characterization (MRC) technology responds to active microseepage; however, such application requires close sample spacing and is most effective when integrated with subsurface data, especially 3-D seismic data. High-resolution microseepage surveys offer a low-cost technology that complements more traditional geological and seismic methods.

Direct Detection Methods for Hydrocarbons. Light hydrocarbons can reside in soils and shallow sediments as free gas in effective porosity, as interstitial gas occluded in pore spaces between grains, or as gas adsorbed onto the sediment particles or within carbonate cements. These differences have led to development of many techniques for sampling of soil hydrocarbons, including: (1) free soil gas probe (1/ 2-in diameter rod driven approximately three feet into the ground), (2) free soil gas augered hole (2-to 4-in diameter hole is augered to a depth of 9-10 ft), (3) canned headspace gas in which a volume of soil/ sediment is collected from bottom of a seismic shothole or augered hole, (4) cuttings gas in which a blender is used to disaggregate sediment to release loosely bound gases, (5) total free gas which is the sum of Headspace and Cuttings gas, and (6) adsorbed gas (acid-extracted gas) techniques.

Indirect Geochemical Methods for Hydrocarbon Detection. Indirect geochemical exploration methods take advantage of hydrocarbon-induced soil or sediment alterations such as microbiological anomalies, mineral changes, clay mineral alteration, or electrochemical changes. Hydrocarbon-induced alteration is highly complex, resulting in varied surface expressions that have led to the development of an equally varied number of surface exploration techniques, including soil carbonate methods, soil halogen and trace metal methods, radioactivity-based methods, and remote sensing methods.

Claims of success for most of these methods rarely with stand rigorous analysis. Of all these indirect geochemical methods, only the microbial method directly correlates with the presence of light hydrocarbons in soil. Other indirect methods could form soil alteration anomalies in response to more than one factor, and thus they could be unrelated to subsurface oil or gas accumulation.

Indirect Geophysical Methods for Hydrocarbon Detection.Potential geophysical evidence of hydrocarbons includes: detection of gas bubbles in the water column; surface mounds, pockmarks and craters; acoustic wipe-out zones, especially beneath surface mud mounds; reflection pulldowns or pull-ups; and reflection offset and terminations. Pyrite mineralization, calcite cementation, clay mineral alteration, brine effects, and formation of a redox potential cell can produce electrical effects.

For example, the induced polarization technique attempts to detect the alteration zone or "pyrite chimney" caused by seepage into iron-rich sediments, whereas the magnetotellurics method measures natural fluctuations in magnetic and electrical fields. Another technique is micromagnetics. Whereas conventional aeromagnetics yields information about basement structure, sediment thickness, and distribution of faults and volcanics, micromagnetics detects the weak signature that may be developed along areas of hydrocarbon seepage due to authigenic mineralization.

Survey Objectives and Design. The objective of the survey (i. e., reconnaissance, prospect evaluation, or field development) plays an important part in optimizing near surface anomaly recognition while minimizing survey costs. Design and density of surface geochemical sampling programs can significantly influence the interpretability of the survey. For regional high grading, a grid with a minimum of two samples across the narrowest expected surface signal appears to be adequate. A higher density of four samples is suggested for prospect high grading. In addition, the sampled area must include sufficient "background" measurements to recognize the existence of an anomaly.

About 80% of the samples should be obtained outside the expected area of interest. The most cost-effective sampling programs are performed in two stages: a low-density regional survey to high-grade the area, followed by a higher density survey within the high-graded area. Insufficient sampling, followed by poor method selection, poor survey design, or failure to properly integrate results of the surface survey with subsurface data, is the major cause of ambiguity and interpretation failure in surface geochemical studies.

Field Results in Kansas with Gas-Sieve Analysis. Pangaea Geochemical Technologies, a Kansas-based geochemical research and service company, reported on the preliminary results of a program, "The Red Top Program," funded in 1999 by the Kansas Technology Enterprise Corporation. The Red Top Program was designed to field test a newly developed surface survey method utilizing gas-sieve technology, crossover technology from the environmental industry for capturing hydrocarbon vapors from a 1000 ml soil vapor sample. Once trapped, hydrocarbon vapors are held tightly (for as long as 30 days without significant loss) so analyses are typically done in the laboratory rather than the field. Since the gas-sieve concentrates the hydrocarbons from the 1000 ml sample, the gas chromatograph used to measure hydrocarbon concentrations can now provide accurate measurements (concentrations can be too low in normal small soil vapor samples).

Thirty Midcontinent field projects were performed in the Red Top Program. Over 600 gas-sieve samples were taken over prospects, with between 20-30 gas-sieve samples per project. Then, over 40 wells were drilled on the survey areas, with additional wells planned. Completed projects, surveyed and drilled, were presented at the workshop. Projects included choosing drilling locations in and at the edges of waterfloods, infill drilling on non-waterflooded locations, trend drilling and wildcats. As the program was incomplete, the final predictive value of the method was not reported with certainty but, preliminary results, with 2/ 3 of the project complete, suggested that the method is over 74% predictive of wells drilled for oil. The predictive value for gas projects was not reported because too few of the gas prospects had been drilled. It was expected that the project would be substantially complete in the first quarter of 2000.

CONNECTIONS:

Dietmar "Deet" Schumacher
Geo-Microbial Technologies Inc.
P. O. Box 132
Ochelata, OK 74051
Phone 918-535-2281, Fax 918-535-2564 Email gmtdeet@aol.com

Lynne Davison
Pangaea Geochemical Technologies, Inc.
1227 N. Covington
Wichita, KS 67212
Phone: 316-721-3737, Fax 316-729-8463 Email pangaea@feist.com

For information on PTTC’s Midwest region and its activities contact:
David G. Morse, Petroleum Geologist,
Oil and Gas Section, Illinois State Geological Survey,
Natural Resources, Bldg., 615 E. Peabody Dr.,
Champaign, IL 61820 ph 217-244-5527, fax 217-333-2830, e-mail morse@geoserv.isgs.uiuc. edu

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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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