Surface Geochemical Exploration for Petroleum PDF

Surface Geochemical Exploration for Petroleum C.D. Adenutsi, Ph.D./ W.A. Marfo Department of Petroleum Engineering, KNUST Office: Petroleum Building, PB 318/317 May, 2024  Introduction Geochemical exploration for petroleum is the search for chemically identifiable surface or...

Surface Geochemical Exploration for Petroleum C.D. Adenutsi, Ph.D./ W.A. Marfo Department of Petroleum Engineering, KNUST Office: Petroleum Building, PB 318/317 May, 2024  Introduction Geochemical exploration for petroleum is the search for chemically identifiable surface or near-surface occurrences of hydrocarbons and their alteration products, which serve as clues to the location of undiscovered oil and gas accumulations. All surface geochemical methods assume that hydrocarbons generated and trapped at depth leak in varying but detectable quantities to the surface.  Introduction Geochemical exploration techniques can be direct or indirect. Direct techniques analyze small quantities of hydrocarbons that occur in the pore space of soil, that are adsorbed on the fine-grained portion of soil, or that are incorporated in soil cements. Indirect geochemical methods detect seepage-induced changes to soil, sediment, or vegetation.  Surface Geochemical Exploration Principles Surface geochemical exploration principles include the following: 1. All petroleum basins exhibit some type of near-surface hydrocarbon leakage. 2. Petroleum accumulations are dynamic and their seals are imperfect. 3. Hydrocarbon seepage can be active or passive and is visible (macroseepage) or only detectable analytically (microseepage). 4. Hydrocarbons move vertically through thousands of meters of strata without observable faults or fractures in a relatively short time (weeks to years). 5. Migration is mainly vertical but can also occur over great distances laterally. 6. Relationships between surface anomalies and subsurface accumulations range from simple to very complex  Significance of Anomalies Surface indications of oil and gas seepage have led to the discovery of many important petroleum producing areas. Although the discovery of a surface geochemical anomaly does not guarantee the discovery of commercially significant petroleum, it does establish the presence of hydrocarbons in the area of interest. Hydrocarbon seeps at the surface represent the end of the migration pathway. Traps and structures along such pathways should be considered significantly during prospecting than those not associated with such anomalies  Assumptions Traps Leak 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. Anomalies relate to traps A further assumption is that the anomaly at the surface can be related reliably to a petroleum accumulation at depth. The geochemical or microbial anomaly at the surface represents the end of a petroleum migration pathway  Assumptions Seepage Styles It is assumed that there are different migration pathways that lead to different seepage styles  Limitations of Surface Geochemical Exploration Limitations related to Geology The following are limitations of surface geochemical prospecting related to geology: 1. The geochemical expression of seepage is complex and varied. 2. There is generally no simple one-to-one correlation between a surface anomaly and a subsurface accumulation. 3. Successful integration of surface geochemical data with subsurface geology becomes increasingly difficult as the geology becomes more complex. 4. False seep anomalies can be caused by reworked hydrocarbons and/or reworked source rocks. 5 Reservoirs that are significantly underpressured or contain heavy oil may not be detected by some surface geochemical methods.  Limitations of Surface Geochemical Exploration Limitations related to the method The following are limitations of surface geochemical prospecting related to the method: 1. No single method works everywhere; there are many methods to choose from. 2. A surface anomaly generally cannot be related to a specific source reservoir or depth; however, compositional fingerprinting techniques can sometimes discriminate seepage from different reservoir zones. 3. Undersampling and/or use of improper sampling techniques causes ambiguity that leads to interpretation failures. 4. Discovery of a surface geochemical anomaly does not guarantee discovery of commercially significant volumes of hydrocarbon. 5. Geochemical exploration methods cannot replace existing exploration technology; however, they can add value to existing geological and geophysical exploration  Seepage Activity Seepage activity refers to the relative rate of hydrocarbon seepage. Abrams (1992) defines two distinct end members of seepage activity: active and passive. Active Seepage The term active seepage refers to areas where subsurface hydrocarbons seep in large concentrations into shallow sediments and soils and into the overlying water column. Active seeps often display acoustic anomalies on conventional or high- resolution seismic profiles. Active seepage occurs in basins now actively generating hydrocarbons or that contain excellent migration pathways. These seeps are easily detected by most sampling techniques.  Seepage Activity Passive Seepage Areas where subsurface hydrocarbons are not actively seeping are referred to as characterized by passive seepage. Such seeps usually contain low-molecular- weight hydrocarbons and volatile high- molecular-weight hydrocarbons. Acoustic anomalies may be present, but water column anomalies are rare. Anomalous levels of hydrocarbon seepage may only be detectable near major leak points or below the zone of maximum disturbance  Seepage Activity Active and passive.  Seepage Activity The zone of maximum disturbance is a near-surface zone of variable depth and thickness in which sedimentary and biological processes alter or destroy volatile hydrocarbons.  Seepage Activity Anomalous concentrations of hydrocarbons may not be detectable if samples are not obtained from below the zone of maximum disturbance. The figure illustrates the zone of maximum disturbance in shallow marine sediments. Deeper sampling may be required in areas of passive seepage  Macroseepage versus Microseepage Macroseeps The term macroseepage refers to visible oil and gas seeps. Macroseeps are very localized areas containing large concentrations of light hydrocarbons as well as, if available, high-molecular-weight hydrocarbons. They are localized at the termination of faults, fractures, and outcropping unconformities or carrier beds. These visible seeps have led to the discovery of many of the world’s important oil and gas producing areas  Macroseepage versus Microseepage Microseepage Microseepage is defined as high concentrations of analytically detectable volatile or semivolatile hydrocarbons in soils, sediments, or waters. These invisible seeps are recognized only by the presence of anomalous concentrations of the following: 1.Light hydrocarbons (principally C1–C5) 2. Volatile or semivolatile high-molecular-weight hydrocarbons (such as 2–4 ring aromatics) 3. Hydrocarbon-oxidizing microbes 4. Hydrocarbon-induced alteration products  Macroseepage versus Microseepage Hydrocarbon microseepage is predominantly vertical and is dynamic; Migration rates range from less than 1 meter per day to tens of meters per day Most surface geochemical methods, including both direct and indirect methods, were developed to detect microseepage.  Macroseepage versus Microseepage The existence of microseepage is supported by a large body of empirical evidence. This includes the following: 1. Increased concentration of light hydrocarbons and hydrocarbon- oxidizing microbes in soils and sediments above hydrocarbon reservoirs. 2. Increased 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. Macroseepage versus Microseepage Surface Expressions Surface expression refers to the geochemical evidence of seepage on the surface. Geochemical evidence of seepage The surface geochemical expression of petroleum seepage can take many forms: 1. Anomalous hydrocarbon concentrations in sediment, soil, water, and even the atmosphere 2. Microbiological anomalies 3. Anomalous nonhydrocarbon 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 Geochemical evidence of seepage 5. Clay mineral alterations 6. Radiation anomalies 7. Geothermal and hydrologic anomalies 8. Bleaching of red beds 9. Geobotanical anomalies 10. Altered acoustical, electrical, and magnetic properties of soils and sediments  Surface Expressions Oxidation-Reduction Zones Bacteria and other microbes play a profound role in the oxidation of migrating hydrocarbons. Their activities are directly or indirectly responsible for many of the diverse surface manifestations of petroleum seepage. These activities, coupled with long-term migration of hydrocarbons, lead to the development of near-surface oxidation- reduction zones that favor the formation of this variety of hydrocarbon-induced chemical and mineralogical changes.  Surface Expressions This seep-induced alteration is highly complex, and its varied surface expressions have led to the development of an equally varied number of geochemical exploration techniques. Some detect hydrocarbons directly in surface and seafloor samples, others detect seep-related microbial activity, and still others measure the secondary effects of hydrocarbon-induced alteration. The figure shows a generalized model of hydrocarbon microseepage and hydrocarbon-induced effects on soils and sediments  Hydrocarbon Detection Methods Direct Methods Direct detection methods are geochemical exploration methods designed to detect the presence of hydrocarbons in soils, near-surface sediments, seafloor sediments, and waters. Detection of Light Hydrocarbons The analysis of light hydrocarbons (chiefly methane through pentane) in soils and soil gases represents one of the earliest surface geochemical methods used and is one of the most researched and tested geochemical survey approaches.  Hydrocarbon Detection Methods Light hydrocarbons can reside in soils and shallow sediments in a number of ways: 1. Free gas in the effective porosity 2. Interstitial gas occluded in pore spaces between grains 3. Gas adsorbed onto sedimentary particles or trapped within carbonate cements 4. Gas dissolved in water or present in the atmosphere  Hydrocarbon Detection Methods Detection of Heavier Hydrocarbons Volatile and semivolatile heavier hydrocarbons such as aromatic compounds, gasoline range hydrocarbons, and even normal or biodegraded oils can also be found, particularly where migration occurs along fault and fracture pathways. These different manifestations have led to the development of different techniques for sampling and analyzing hydrocarbons.  Hydrocarbon Detection Methods Direct Indicators of Hydrocarbon Microseepage Direct indicators of hydrocarbon microseepage concentrates on analyzing for the presence of thermogenic hydrocarbons in soils or near-surface sediments. Majority of the direct methods focus on detecting microseepage using the C1–C5 hydrocarbon gases in soils (the soil gas methods), Other methods detect higher molecular weight components by employing an adsorber buried in the sediments (passive collection or adsorber method)  Hydrocarbon Detection Methods 1.Soil Gas Soil gas methods look for C1–C5 hydrocarbon gases in the surface soil profile and near-surface sediments. There are two basic methods for sampling soil gas for analysis. The first uses a gas probe inserted at least a meter into the soil. The probe is connected to a pumping system to pull the gas out of the soil. Often, this method incorporates a portable gas chromatograph for field analysis of the gas. The second method is to dig down at least a meter to sample the soil and quickly place the sample in a sealed can for later laboratory analysis.  Hydrocarbon Detection Methods 2.Adsorbers Microseepage sampling with adsorbers is sometimes referred to as passive soil gas sampling. The use of adsorbers to sample for microseepage has two distinct advantages over soil gas methods. First, adsorbers collect potential microseepage over a period of days or weeks to eliminate the atmospheric and other effects that make it difficult to collect comparable samples with soil gas. Second, they collect a larger molecular weight range that provides more opportunities to recognize thermogenic hydrocarbons.  Hydrocarbon Detection Methods Indirect Methods Indirect methods for detecting hydrocarbon seepage and microseepage are based on what are assumed to be seepage-induced soil and sediment alteration. Indirect detection methods include the following: 1. Microbial 2. Helium 3. Radiometrics 4. Iodine 5. Soil alteration 6. Trace elements 7. Electrical 8. Magnetics 9. Biogeochemical  Hydrocarbon Detection Methods Some indirect detection methods are better understood and more consistently reliable than others. Microbial methods, for example, detect the presence of hydrocarbon- oxidizing microbes in soils and sediments. These microbes would not be expected to be present in significant concentrations if there were no hydrocarbon source present. Helium, by contrast, is not uniquely associated with petroleum. However, it is a common constituent of petroleum accumulations and due to its mobility, chemical inertness, and abiogenic nature forms a very good indirect geochemical marker.  Hydrocarbon Detection Methods The formation of radiation anomalies and other secondary alteration anomalies (soil carbonate, iodine, trace metal, Eh, pH, electrical, magnetic, geobotanical, etc.) is less well understood. The cause of these altered soils and sediments may well be seepage related, but migrating hydrocarbons are an indirect cause at best and not always the most probable cause. Even if due to hydrocarbons, the cause could be shallow biogenic gas and thus unrelated to leakage from deeper oil and gas accumulations.  Hydrocarbon Detection Methods Indirect Indicators of Hydrocarbon Microseepage Not all microseepage exploration methods look directly for low concentrations of thermogenic hydrocarbons in near-surface sediments. Many are focused on searching for evidence of hydrocarbon-induced alteration of sediments that may have taken place, while others look for microbial or botanical changes that could be attributed to hydrocarbon seepage. While these methods may have plausible theoretical bases, there is often more than one possible explanation for the phenomenon they describe.  Hydrocarbon Detection Methods 1. Microbial Microbial surveys entail collecting soil sample and culturing the microbial communities present. The objective is to identify microorganisms capable of metabolizing light hydrocarbons, mainly the ethane through butane-oxidizing microbes. Elevated populations of these organisms are then used as an indicator of the potential presence of elevated light hydrocarbon concentrations due to hydrocarbon seepage  Hydrocarbon Detection Methods 2. Iodine Using soil iodine as an indicator of hydrocarbon seepage is based on the affinity for light hydrocarbons seeping to the surface to interact with iodine to form low-volatility iodorganic compounds in surface soils. Although other chemical characteristics of soils may affect the amount of iodine present, it is thought to be primarily dependent on the concentration of one or more of the light alkanes ethane, propane, and the butanes. These iodorganic compounds are metastable and will decompose in a matter of months if the hydrocarbons source is shut off. Because of this, analysis for iodine must be done as soon as possible after sample collection  Hydrocarbon Detection Methods 3. Induced Polarization and Resistivity Induced polarization (IP) and resistivity are two electrical properties measured in near-surface sediments as indirect hydrocarbon indicators. They are usually measured at the same time by inserting two electrodes into the earth surface and passing a current through them. After the resistivity measurement is made, the current is shut off and the IP is measured. Both the increases in resistivity and IP are thought to be in response to the presence of diagenetic minerals that could be related to hydrocarbon seepage  Hydrocarbon Detection Methods 4. Magnetic Contrasts The presence of seeping hydrocarbon may either induce the formation of some magnetic minerals, such as magnetite and pyrrhotite, or the destruction of others, such as hematite. These contrasts when detected as used as indirect indicators of hydrocarbon seepage Hydrocarbon Detection Methods Contoured ethane concentration map from soil gas  Hydrocarbon Detection Methods Onshore Macroseepage While microseepage may be controversial, macroseepage, especially onshore, is a proven exploration tool that goes back to the beginning of the modern era of petroleum exploration. By recognizing that oil and gas seeps were the product of leakage from petroleum accumulations and that understanding their geology could point back to the location of that accumulation, the petroleum industry got its start. Macroseepage resulting in oil and gas seeps is often closely associated with faults or fracture zones that extend down to the vicinity of the reservoir.  Hydrocarbon Detection Methods These faults and fractures provide a migration pathway for significant volumes of petroleum to move from the reservoir toward the surface. In addition to faults and fracture zones, macroseepage may be the result of a structural readjustment that exposes a carrier bed at the surface, exhumation of a reservoir, migration up the sides of salt structures, and migration along unconformities. This macroseepage can manifest as surface pools (tar pools), gas vents gas bubbling up in springs, streams, ponds, or lakes.  Hydrocarbon Detection Methods Offshore Macroseepage Offshore macroseeps can provide valuable insight into the petroleum systems in a basin prior to any exploration drilling. Seafloor seeps can help identify areas with high potential, provide detailed information about the oil, its source rock, and thermal history. Taken together with the subsurface geology, seafloor seeps can be used to identify and map exploration play fairways ahead of the drill bit.  Hydrocarbon Detection Methods A good example of seep structure is in the model shown in the Fig. Macroseepage from an oil reservoir leaks to the seafloor. A portion of the petroleum remains at or just below the sediment–water interface while some of the hydrocarbons are released to the water column, with gas bubbles and oil droplets heading toward the surface.  Hydrocarbon Detection Methods Some of this oil and gas will go into solution in the seawater. Oil droplets that do reach the sea’s surface will “pancake” out forming a very thin layer of oil as a slick. The slick is acted upon by a variety of processes including evaporation, biodegradation, oxidation, and emulsification.  Hydrocarbon Detection Methods Eventually, some of the oil may congeal into dense masses and sink to the ocean floor. This model indicates that clues to the locating seafloor seep can be found on the sea’s surface, in the water column, and on the seafloor.  Hydrocarbon Detection Methods 1. Sea Surface Slicks Observations using synthetic aperture radar (SAR) satellite imagery, airborne laser fluorescence, and aerial photography are all used to look for sea surface slicks that might indicate seafloor seepage. SAR satellite imagery is the most commonly used method for sea surface slick detection. SAR detects the backscatter of radar energy off waves on the ocean’s surface. A slick appears in the SAR satellite image as dark spots against a lighter background because the slick reflects back less energy to the satellite due to this suppression of capillary waves. Hydrocarbon Detection Methods An example of some SAR images of oil slicks is shown  Hydrocarbon Detection Methods 2. Water Column “Sniffers” In addition to sea surface slicks, there are clues to the location of seafloor seeps that can be found in the water column. It is realized that light hydrocarbon gases can be detected in water column above natural hydrocarbon seeps A shipboard system was developed for “sniffing” hydrocarbon gases in seawater. It consists of a towed “fish” at some depth below the surface with an inlet for a pumping system that would bring water samples to a surface ship. The water is stripped of its dissolved gases which are then analyzed by GC. The position of the ship at the time of sampling is also recorded.  Selecting a Survey Method Principal objectives The principal objectives of a geochemical exploration survey are to 1.establish the presence, distribution, and composition of hydrocarbons in the area of exploration or development interest and 2.determine the probable hydrocarbon charge to specific exploration leads and prospects. Selecting a Survey Method Reconnaissance objectives The objective of a reconnaissance survey is to find seeps and microseeps that provide direct evidence that thermogenic hydrocarbons have been generated, i.e., they document the presence of a working petroleum system. Additionally, the composition of these seeps can indicate whether a basin or play is oil or gas-prone. Hydrocarbons from surface and seafloor seeps can be correlated with known oils and gases to identify the specific petroleum system(s) present. Seepage data allow the explorationist to screen large areas quickly and economically, determining where additional and more costly exploration is warranted  Selecting a Survey Method 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 rock that generated these hydrocarbons.  Selecting a Survey Method Offshore Methods The table below lists the principal geochemical methods used for offshore exploration.  Selecting a Survey Method Onshore Methods The table below lists the principal geochemical methods used for onshore exploration. Designing a Geochemical Survey Survey design and sampling strategy for geochemical surveys should be flexible and must be dictated by the following: Exploration objectives Geologic setting Basin hydrodynamics Anticipated target size and shape of the anomaly(or geologic target) Ability to sample along(and/or between) key seismic lines Logistical considerations Expected natural variation in surface measurements Probable signal-to-noise ratio(Matthews,1996b)  Designing a Geochemical Survey Procedure Use the table below as a guide for designing a surface geochemical survey.  Designing a Geochemical Survey Sample Locations Geochemical exploration often begins with a search for, and analysis of, visible oil and gas seeps. Additional geochemical data may then be acquired along the trace of existing seismic lines or along regional geochemical traverses located to cross features of geologic and structural significance. Depending on survey objectives, sample spacing for geochemical surveys may vary from 500–1,000 m at one extreme to 50–100 m at the other. Sampling along geochemical grids is recommended for small exploration targets and/or 3-D seismic programs; however, grids are not cost effective for large reconnaissance surveys.  Designing a Geochemical Survey Analogs Whenever possible, it is advisable to acquire surface geochemical data over a nearby geologic analog or recent discovery Seeps Oil and gas seeps, if present, are also valuable analogs because they permit direct correlation of seeping hydrocarbons with soil gas and fluorescence data as well as other microbial or geochemical data. Old producing fields may not provide good analogs since production and pressure decline may have reduced or even eliminated their surface geochemical expression  Designing a Geochemical Survey Sample Density Hydrocarbon microseepage data are inherently noisy and require adequate sample density to distinguish between anomalous and background areas. Undersampling is probably the major cause of ambiguity and interpretation failures involving surface geochemical studies  Designing a Geochemical Survey Recognizing Anomalies Defining background values adequately is an essential part of anomaly recognition and delineation; It is suggested that as many as 80% of the samples collected be obtained outside the area of interest. However, for very small targets such as pinnacle reefs or channel sandstones, optimum results are obtained when numerous samples are collected in a closely spaced grid pattern, (100–160-m sample interval or less) over the feature of interest  Designing a Geochemical Survey Example The recognition of surface geochemical anomalies improves by increasing sample number and reducing sample spacing.  Interpretation Guidelines 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 accumulation or leakage along a carrier bed or other migration pathway. Anomalies defined by multiple samples from one or more survey lines may indicate the location of discrete structural or stratigraphic targets within the survey area.  Interpretation Guidelines Anomalies and Vertical Migration If the basin or play is characterized by predominantly vertical migration, then the correlation 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, we assume the trap is not charged with hydrocarbons  Interpretation Guidelines Anomalies and Lateral Migration If the structural or geologic setting of the area suggests that microseepage may be predominantly lateral or pathway selective, such as along dipping stratigraphic surfaces and unconformities, the interpretation will be more difficult since geochemical anomalies may then not be located vertically above a trap. Because relationships between surface geochemical anomalies and subsurface accumulations can be complex, proper interpretation requires integration of surface geochemical data with geologic, geophysical, and hydrologic data.  Interpretation Guidelines Hydrocarbon Composition from Macroseeps Hydrocarbon seep composition can play an important role in evaluating the exploration potential of a basin, play, or prospect. Petroleum in most visible oil and gas seeps (i.e., macroseeps) generally has been altered by processes such as biodegradation, water washing, and evaporative loss of volatile components. Despite these changes, chemical and isotopic analysis of such seeps can enable inferences about the nature of the source rock facies and maturity as well as permit correlation with known source rocks and reservoired petroleum  Interpretation Guidelines Hydrocarbon Composition from Microseeps Obtaining compositional information from the analysis of hydrocarbon microseeps is more difficult because microseeps generally consist of only light hydrocarbons (methane through pentane). Sometimes, however, the heavier gasoline-range and aromatic hydrocarbons are also present. One can infer the composition of the migrating petroleum from these light hydrocarbons from soil gas/hydrocarbon ratios, carbon isotopic composition of soil gases, fluorescence characteristics of soil or sediment extracts, and chromatographic analysis of such extracts.  ASSIGNMENT Please learn the Geological Time Scale. Oxidizing and Reduction Zones END