Vertical Migration Article





Vertical Migration Article





Vertical Migration Article





Vertical Migration Article





Vertical Migration Article





Vertical Migration Article





Imaging spectroscopy





Imaging spectroscopy


Chimney Cube Unravels Subsurface



Chimney Cube Unravels Subsurface



Chimney Cube Unravels Subsurface






Pixler - Formation Evaluation by Analysis of Hydrocarbon Ratios


Surface geochemical exploration for oil and gas: New life for an old technology

DIETMAR "DEET" SCHUMACHER, Geo-Microbial Technologies, Ochelata, Oklahoma, U.S.


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.
Figure 1. Spectrum of contrasting seepage styles and migration pathways from the Gulf of Mexico and the North Sea (modified from "Understanding geology as the key to using seepage in exploration: the spectrum of seepage style" by Thrasher et al., AAPG Memoir 66, 1996).
direct evidence that thermogenic hydrocarbons have been generated; that is, they document the presence of a working petroleum system and identify the portions of the basin that are most prospective. Additionally, the composition of these seeps can indicate whether a basin or play is oil-prone or gas-prone. If the objective is to evaluate individual exploration leads and prospects, the results of geochemical surveys can lead to better risk assessment by identifying those associated with strong hydrocarbon anomalies, thereby highgrading prospects on the basis of their probable hydrocarbon charge. For development projects, detailed microseepage surveys can (1) help evaluate infill or stepout drilling locations, (2) delineate productive limits of undeveloped fields, (3) identify bypassed pay or undrained reservoir compartments, and (4) monitor hydrocarbon drainage through use of repeat geochemical surveys. Hydrocarbon microseepage surveys have potential to add value to 2-D and 3-D seismic data by identifying those features or reservoir compartments that are hydrocarbon-charged.
    The underlying assumption of all near-surface geochemical exploration techniques is that hydrocarbons are generated and/or trapped at depth and leak in varying but detectable quantities


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


Figure 2. Seismic cross-section of Ekofisk Field, North Sea, illustrating a well-developed gas chimney caused by low-velocity conditions due to gas-charged sediments (from "Ekofisk: First of the giant oil fields in western Europe" by Van den Bark and Thomas, AAPG Memoir 30, 1990). Hovland and Sommerville (1985) estimated gas-seepage at 1000 liters per hour. Extrapolating this estimate to the toltal area of gas seepage, approximately 100 000 mē containing 140 seeps, gives a net flux of 890 liters per square meter per year.



Figure 3. Generalized model of hydrocarbon microseepage and hydrocarbon-induced effects on soils and sediments (from "hydrocarbon-induced alteration of soils and sediments" by Schumacher, AAPG Memoir 66).
of Australia, central Sumatra, and parts of the North Sea.
    As indicated above, there
is a seepage continuum from the lowest detectable levels at one extreme to visible oil and gas seeps at the other.

Figure 4. Geochemical expression of a stratigraphic trap at about 5600 ft (1.5 s)in the Cretaceous Escondido Sandstone, La Salle County, Texas (from "Exploration enhancement by integrating near-surface geochemical and seismic methods" by Rice, Oil and Gas Journal, 1989). A soil gas hydrocarbon survey was conducted to look for evidence fo hydrocarbon microseepage from a seismically defined trap at CDP 1070 (left). Propane soil gas anomalies were detected at CDPs 1070 and 1096. A wildcat drilled at CDP 1070 resulted in a new field discovery. The geochemical lead at CDP 1096 was reevaluated seismically and, after additional processing, a revised interpretation (right) predicted porosity development coincident with the surface geochemical anomaly. A productive well was subsequently drilled.
Macroseepage refers to the visible oil and gas seeps. Microseepage is defined as elevated concentrations of analytically detectable volatile or semivolatile hydrocarbons, or hydrocarbon-induced changes, in soils and sediments. The existence of microseepage is supported by a large body of empirical evidence in cluding (1) increased concentration of light hydrocarbons and hydrocarbon-oxidizing microbes in soils and sediments above petroleum reservoirs, (2) an increase, in key light hydrocarbon ratios in soil gas over oil and gas reservoirs, (3) sharp lateral changes in these concentrations and ratios at the edges of the surface projections of these reservoirs, (4) similarity with stable carbon isotropic ratios for methane and other light hydrocarbons in soil gases to those found in underlying reservoirs, and (5)the disappearance and reappearance of soil gas and microbial anomalies in response to reservoir depletion and repressuring.
    Microseepage rates and surface hydrocarbon concentrations can vary significantly with time. Surface hydrocarbon seeps and soil geochemical anomalies appear and disappear in relatively short

times-weeks to months to years. Results from studies of natural seeps and underground storage reservoirs, as well as repeat surveys of fields, indicate that the rate of hydrocarbon migration and microseepage varies from less than 1m per day to tens of meters per day. Empirical observations and computer simulations suggest that the mechanism for microseepage is a buoyancy-driven, continuous-phase gas flow through water-wet pores and fractures.
    Nearly all surface exploration methods rely on the assumption that hydrocarbons migrate in a predominantly vertical direction from source rocks and reservoirs to the surface. Evidence for vertical leakage of hydrocarbons is commonly seen on conventional seismic and high-resolution seismic sections. Figure 2 illustrates an example of such a gas-leakage chimney over Ekofisk Field in the North Sea. There are numerous published articles showing apical (or direct) geochemical anomalies over oil and gas fields, as well as over petroleum storage reservoirs. A

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.
    The presence of hydrocarbon macroseeps or microseeps in the area of a geochemical survey is direct evidence that petroleum has been generated. Hydrocarbon seepage at the surface represents the end of a petroleum migration pathway. These hydrocarbons may represent hydrocarbon leakage from an accumulatin, or merely leakage along a carrier bed or other migration pathway. Anomalies defined by multiple samples fromone or more survey lines may indicate the location of discrete structural or stratigraphic targets within the survey area. If the basin or play is characterized by predominantly vertical migration, then the corelation of a strong geochemical anomaly at the surface with a possible trap at depth suggests that the trap is charged with hydrocarbons;conversely, if the trap is not associated with a positive geochemical anomaly, the assumption is that the trap is not charged with hydrocarbons. Becauase relationships between surface geochemical anomalies and subsurface accumulationscan be complex, proper interpretation requires integration of surface geochemical data with geologic, geophysical, and hydrologic data (Figure 4). 

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


  About the Author
    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.
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