Near-surface hydrocarbon seeps as indicators of petroleum charge: The evolution of site selection, sample collection, laboratory analysis, and interpretation
Oil and gas seeps gave the first clues to many of the well-known onshore and offshore petroleum bearing basins historically found. Seep hunting surveys have been based on the observation that migrated petroleum from deep source rocks and reservoirs can be analytically detected or otherwise proxied as thermogenic seepage in near- surface soils and sediments, such that results can be used to help evaluate a prospective petroleum system. The value of survey results has been aided by the evolution of tools and techniques for site selection, sample collection, lab analysis, and interpretation, resulting in our growing ability to determine charge, age, maturity, depositional environment, and even oil quality from the detected seepage. As a part of seep hunting, surface geochemical surveys search for chemically identifiable oil and gas compounds as well as seep-induced physical/geological expressions and biological communities and related features. Petroleum traps are dynamic, theoretically exhibiting some level of leakage, often with a resulting near-surface expression. Seep hunting tools have developed from the early days of onshore geographic grid surveys looking for micro-seepage anomalies related to petroleum traps, to offshore deep-coring, targeted, site-specific micro- and macro-seepage surveys. This talk will summarize the evolution and advancements in the tools and techniques used. Site selection and sample collection: Onshore surveys differ from those offshore. Onshore surveys have tended to focus on specific prospects using a simple grid or transect approach with direct shallow-push-core-sampling above the prospect assuming vertical migration of seepage, whereas marine surveys have developed into more complex deep-core sampling of targeted features related to discrete migration conduits and pathways. Marine seismic and multibeam surveys are used to select coring locations based on various types of surface expression of faults, hardgrounds, mud volcanos, and other seabed features associated with upward migration of oil and gas. Vessels deployed to sample these targets often also collect additional acoustic data with a sub-bottom profiler, looking for subsurface features. Seeping oil and gas can be sampled from these surface expressions depending on the seepage rates and seabed conditions. The depth of sampling can have a significant impact on the results due to an array of near-surface processes (physical, geological, chemical, and biological) that can potentially alter, hinder, or block petroleum seepage. For example, the utility and value of light hydrocarbon gas as a seepage indicator improves significantly with depth over the range retrievable by standard piston coring techniques. Sample extraction and analysis: Direct geochemical measurements by analysis of soil/sediment samples include the determination of a range of low, medium, and high molecular weight hydrocarbon analytes. Measurement of low molecular weight compounds includes the light hydrocarbon gases from methane thru pentanes (C1-C5) and perhaps carbon dioxide and helium. The measurement of medium molecular weight HCs, includes the gasoline-plus range of petroleum hydrocarbons and comprises molecules with 5 to 12 carbons (C5-C12) arranged in linear, branched and cyclic aliphatic structures along with monoaromatic hydrocarbons such as benzene, toluene and xylenes. The high molecular weight hydrocarbon measurements typically include determination of the C12-C35 petroleum related compounds. Light hydrocarbon gases can reside in near-surface sediments simultaneously in a number of different ways: (1) as free gas and gas hydrates in the effective porosity, (2) as interstitial gas which is dissolved and occluded in pore water between grains, and (3) as gas adsorbed onto the sedimentary particles or within carbonate cements. The relative fractions of gas existing in each of these three matrices are different in terrestrial soils from marine sediments. On land, gas may exist in significant proportions in each of these forms in the complicated soil matrix. In the simpler soil matrix of near-surface marine sediments, interstitial gas dominates and may turn into free gas in a retrieved (hydrostatic pressure-released) core. The determination of light hydrocarbons in sediments is useful because gases are the most mobile phase of reservoired hydrocarbons, vary over several orders of magnitude, and can be detected, quantified, and distinguished from local background and biogenic/diagenetic contributions. The bound light hydrocarbon gases are extracted using an acid extraction method pioneered by Horvitz Research Laboratories in the 1970s. The bound gases, also known as adsorbed gas, are believed to be attached to organic and/or mineral surfaces, entrapped in structured water, or entrapped in authigenic carbonate inclusions thus require a more rigorous analytical procedure to extract. The free and interstitial light hydrocarbon gases are extracted by equilibrium partitioning into a clean headspace. The extracted C1-C5 hydrocarbons by either type of extraction are then analyzed by gas chromatography (GC) and their stable carbon isotopes by isotope-ratio mass spectrometry (IRMS). The gasoline-plus range hydrocarbons require a more specialized protocol to collect, analyze, and interpret. The methods currently active include vapor phase extraction by passive samplers such as the Gore Sorber or solid phase micro-extraction (SPME). The extracted C5 to C12 hydrocarbons are then analyzed by GC or GC/MS. For onshore sorber surveys, a pilot hole is developed in the soil to an average depth of perhaps 60-80 cm below grade. A sorber is then tied to a section of cord and inserted into the hole with a rod. The cord is secured at the ground surface by collapsing the hole, and the sorber is left for exposure to volatiles for 15-30 days. Exposed sorbers are then retrieved and analyzed by thermal desorption followed by GC/MS. Marine procedures are the same except that the sorbers are put into selected sections from recovered cores for exposure. After solvent extraction of dried samples, the high molecular weight hydrocarbons are determined using total scanning fluorescence (TSF) and GC. TSF provides information about aromatic hydrocarbons contained in extracts. C15+GC analysis quantifies petroleum-related normal alkanes, isoprenoids, and the unique unresolved complex mixture (UCM) generally found to be associated with extracts that contain at least traces of oil. Selected samples that show promise of seepage from TSF and C15+GC results are further analyzed for saturate and aromatic biological markers by GC/MS or GC/MS/MS. These more detailed and expensive analyses can confirm the presence of migrated petroleum and are also often used to match hydrocarbons in the sediment extract with a specific produced oil and/or source rock, yielding information about age, maturity, depositional environment, and quality. Biological proxies: As seeping hydrocarbons approach the surface (land or marine), they encounter microbial communities that have adapted to make a living in the reduced-carbon-rich (and reduced-sulfur-rich) substrates. The composition of microbial communities associated with petroleum seeps include organisms that are able to metabolize the leaking oil, gas, and H2S along with other organisms up that food chain. The proxy methods include culture based analysis and DNA-based procedures such as serial analysis of ribosomal DNA 16S rRNA (SARD). In addition, chemosynthetic communities that have established a food chain starting with microbes, are proxies for present or past seepage of oil and gas. Indeed, living or long dead communities produce the substrate from which authigenic carbonate hardgrounds are formed, and these features are used to find seep target locations with multibeam backscatter techniques as a part of the seep hunting survey. Interpretation: The end-product of a seep hunting survey is a listing with locations of the acquired cores that display clear evidence of migrated thermogenic hydrocarbons. Such conclusions and interpretive opinions are typically presented as a series of figures and a defining table. Challenges to accurate interpretation of seepage anomalies include (1) distinguishing thermogenic anomalies from background, including a correct understanding of analyte recoveries from the sample matrix, (2) correctly discerning any altered chemistry of the detected compounds, (3) correctly discerning the true proportions of thermogenic, biogenic, and diagenetic hydrocarbons, and (4) correctly discerning whether the tools and techniques used are the right ones for the particular survey. All onshore and offshore sediments contain low-level background concentrations of hydrocarbons related to local microbial generation, diagenesis, and/or reworked petroleum. Properly identifying what is real thermogenic seepage relative to these background signatures is critical. For example, 2-10% mole fractions of the C2+ alkane gases from a producing well are clearly indicative of thermogenic gas, but in marine sediments the background levels of biogenic/diagenetic C2+ gases are often high enough to be misinterpreted as thermogenic. Moreover, microbial activity and hydrate formation can alter the compositions of the hydrocarbon mix. Unlike the simple gases, gasoline-plus range hydrocarbons must be discerned by sophisticated statistical pattern recognition techniques due to the sheer number of individual compounds existing in natural sediments. Likewise, the high molecular weight hydrocarbons are best discerned and screened by plotting the detector response to the bulk aromatics (TSF) against the detector response to the bulk saturates (the UCM), simply because of the overwhelming number of existent compounds. Background issues, mixing with local production, diagenesis, and alterations affect the detected gasoline-plus range and high molecular weight hydrocarbons such that their thermogenic signal is often quite difficult to discern from the background morass. Examples of each of these interpretive challenges are presented.
AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019