| Surface geochemical exploration for petroleum is the search for chemically identifiable surface or near-surface occurrences of hydrocarbons, or hydrocarbon-induced changes, as clues to the location of oil and gas accumulations. It extends through a range of observations from clearly visible oil and gas seepage (macroseepage) at one extreme to identification of minute traces of hydrocarbons (microseepage) or hydrocarbon-induced changes at the other. Surface geochemical methods have been used since the 1930s, but the past decade has seen a renewed interest in geochemical exploration. This, together with developments in analytical and interpretive methods, has produced a new body of data and insights about geochemical exploration. Many of these developments are summarized in "Hydrocarbon Migration and Its Near-Surface Expression" (AAPG Memoir 66). Geochemical surveys and research studies document that hydrocarbon microseepage from oil and gas accumulations (1) is common and widespread, (2) is predominantly vertical (with obvious exceptions in some geologic settings), and (3) is dynamic (responds quickly to changes in reservoir conditions). The principal objective of a geochemical exploration survey is to establish the presence and distribution of hydrocarbons in the area and, more importantly, to determine the probable hydrocarbon charge to specific exploration leads and prospects. For reconnaissance surveys, seeps and microseeps provide Editor's Note: The Geologic Column, which appears monthly in TLE, is (1) produced cooperatively by the SEG Interpretation Committee and the AAPG Geophysical Integration Committee and (2) coordinated by M. Ray Thomasson and Lee Lawyer. |
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| to the surface. This has long been an established fact, and the close assocation of surface geochemical anomalies with faults, productive fairways, and specific leads and prospects is well known. It is further assumed, or at least implied, that the anomaly at the surface can be reliably related to a petroleum accumulation at depth. The success with which this can be done is greatest in areas of relatively simple geology but becomes increasingly difficult as the geology becomes more complex. The geochemical anomaly at the surface represents the end of a petroleum migration pathway, a pathway that can range from short-distance vertical migration to long-distance lateral migration. An example of these contrasting seepage styles and migration pathways is illustrated in Figure 1. Seepage activity. Seepage activity refers to the relative rate of hydrocarbon seepage. Active seepage refers to areas where subsurface hydrocarbons seep in large concentrations into shallow sediments and the overlying water column. Active seeps often display acoustic anomalies on conventional and high-resolution seismicprofiles. Such seepage occurs in basins now actively generating hydrocarbons and/or that contain excellent migration pathways. Active seeps are easily detected by most geochemical sampling methods. Examples of active seeps are found in the Gulf of Mexico, offshore California, parts of the North Sea, the southern Caspian Sea, offshore West Africa, and offshore Indonesia. Areas where subsurface hydrocarbons are not actively seeping are said to be characterized by passive seepage. Such seeps usually contain low molecular-weight light hydrocarbons and volatile higher molecular weight hydrocarbons above background concentrations. Acoustic anomalies my be present, but water column anomalies are rare. Anomalous levels of hydrocarbon seepage may be detectable only near major leak points or at greater than normal sampling depths. Passive seepage occurs in basins where hydrocarbon generation is relict or migration is sporadic or inhibited by a major migration barrier. Areas with passive seepage include many intracratonic basins, offshore Alaska, the northwest shelf |
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recent review of more than 850 wildcat wells-all
drilled after completion of surface geochemical surveys-finds that 79% of
wells drilled in positive geochemical anomalies resulted in commercial oil
or gas discoveries; in contrast, 87% of wells drilled in the absence of an
associated geochemical anomaly resulted in dry holes. Data such as these
represent powerful, if empirical, evidence for vertical migration and
microseepage of hydrocarbons. The surface geochemical expression of petroleum seepage can take many forms, including (1) anomalous hydrocarbon concentrations in sediment, soil, water, and even atmostphere; (2) microbiological anomalies and the formation of "paraffin dirt"; (3) anomalous non-hydrocarbon gases such as helium and radon; (4) mineralogical changes such as the formation of calcite, pyrite, uranium, elemental sulfur, and certain magnetic iron oxides and sulfides; (5) clay mineral alterations; (6) radiation anomalies; (7) geothermal and hydrologic anomalies; (8) bleaching of redbeds; (9) geobotanical anomalies; and (10) altered acoustical, electrical, and magnetic properties of soils and sediments. Figure 3 represents a generalized model of hydrocarbon microseepage and their varied geochemical and geophysical effects on soils and sediments. Survey design and interpretation. The importance of proper survey design and sampling density for target recognition cannot be overstated. Hydrocarbon microseepage data are inherently noisy and require adequate sample density to distinguish between anomalous and background responses. The major causes of ambiguity and interpretation failures involving surface geochemical studies are probably undersampling and/or selection of an improper survey method. To optimize the recognition of an anomaly, the sampling pattern and sample number must take into consideration the objectives of the survey, the expected size and shape of the anomaly (or geologic target), the expected natural variation in surface measurements, and the probable signal-to-noise ratio. Defining background values adequately is an essential part of anomaly is an essential part of anomaly recognition and delineation. For prospect evaluation, as many as 70% of the samples collected should be obtained outside the area of immediate interest. |
| For properly designed surveys, and under ideal geologic conditions, the areal extent of surface geochemical anomalies can closely approximate the productive limits of the reservoir at depth. How does one select a method (or methods) for a surface geochemical exploration program? The choice of method(s) depends on the kinds of questions you hope the survey results will answer. In other words, what are the objectives of the survey? Is it to demonstrate the presence of an active petroleum system in a frontier area, or to high-grade previously defined exploration leads and prospects, or to determine the type of petroleum (i.e., oil versus gas) likely to be encountered? What other data are presently available in the area of interest (satelite imagery, aeromagnetics, gravity, seismic, etc.)? What geochemical methods have previously been used successfully in the area of interest, or in a geologic analog? What limitations are imposed by the survey area (onshore or offshore, deep water or shallow, jungle or desert, mature basin or remote area, budget and personnel constraints, etc.)? It is beyond the scope of this article to discuss the advantages and limitations of specific methods or sampling procedures, but such information is readily available in published literature. As a generalization, direct hydrocarbon methods are preferred over indirect methods because they can provide evidence of the very hydrocarbons we hope to find in our traps and reservoirs. Additionally, chemical and isotopic analysis of these hydrocarbons, especially the high molecular weight hydrocarbons, can provide |
insight into the nature and maturity of the source rocks that generated these hydrocarbons. If surface conditions or budgetary constraints preclude the use of direct hydrocarbon
detection methods, the next best choice is one of those indirect methods most closely linked to hydrocarbons and hydrocarbon accumulations. Whenever possible, it is recommended to use more than one geochemical survey method, for example, combining a direct method with an indirect method. The use of multiple methods can reduce interpretation uncertainty because seepage-related anomalies will tend to be reinforced while random highs and lows tend to cancel each other out. Summary. The past decade has seen a renewed interest in surface geochemical exploration which, together with developments in analytical and interpretive methods, have produced a new body of data and insights that establish the validity of many of these exploration methods. Surface exploration methods cannot replace conventional exploration methods, but they can be a powerful complement to them. Geochemical and other surface methods have found their greatest |
utility when used in conjunction with available geologic and geophysical information. The need for such an integrated approach cannot be overemphasized. Seismic data, especially 3-D data, are unsurpassed for mapping trap and reservoir geometry; however, only surface geochemical methods can consistently and reliably map hydrocarbon leakage associated with those traps. Properly acquired and interpreted, the combination of surface geochemical data and subsurface exploration data has the potential to reduce exploration and development risks and costs by improving success rates and shortening development time. Suggestions for further reading. Hydrocarbon Migration and Its NearSurface Expresssion by Schumacher and Abrams (AAPG, 1996). Soil Gas and Related Methods for Natural Resource Exploration by Klusman (Wiley, 1993). Surface Exploration Case Histories by Schumacher and LeSchack (AAPG-SEG Special Publication, in preparation). Corresponding author: GMT geochem@aol.com |
| Deitmar ("Deet") Schumacher is currently a principal in Geo-Microbial Technologies, Ochelata, OK. He was formally a Research Professor with the Earth Sciences and Resources Institute (ESRI) of the University of Utah in Salt Lake City. He received his B.S. and M.S. degrees in geology from the University of Wisconsin and his Ph.D. from the University of Missouri. Deet taught geology at the University of Arizona for 7 years before joining Phillips Petroleum as a research geologist in 1977. He held a variety of positions at Phillips, including Research Supervisor for petroleum geology and Senior Geological Specialist. Deet then joined Pennzoil in 1982 and served as manager of geology/geochemistry before transferring to assignments with Pennzoil International, Pennzoil Offshore, and Pennzoil's Technology Group. In 1994, Deet accepted a position as Research Professor at ESRL. He is presently an Associate Editor of the AAPG Bulletin and a past president of both the Houston Geological Society and the Association of Petroleum Geochemical Explorationists. Deet has had a long-standing interest in exploration and development applications of petroleum geochemistry, particularly surface exploration methods. It is this interest that resulted in his convening (with Michael Abrams) the AAPG Hedberg Research Conference "Near-Surface Expression of Hydrocarbon Migration" and the editing of this volume. |  |
