Oil and Gas Seeps in the Gulf of Mexico
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Oil and gas seeps are common worldwide; occur on land and beneath the ocean; are numerous in the Gulf of Mexico; and are biogenic, thermogenic, or mixed in origin. Seeps occur as gases, liquids, asphalts, and tars. Seeps are estimated to account for about 95 % of oil annually discharged to the Gulf of Mexico waters. Biogenic gas seeps have a microbial metabolic origin. Thermogenic hydrocarbons rise to the surface from more deeply buried source rock horizons or accumulations. The seepage of oil and gas into marine sediments initiates a complex biogeochemical cycle. A unique ecology has evolved in association with oil and gas seeps based on chemosynthesis and symbioses. Consortia of microbial species mediate the geological and biogeochemical processes that are essential for supporting what are commonly referred to as cold-seep communities. At these locations, bacteria oxidize hydrocarbons to carbon dioxide or bicarbonate ions, which favor the formation of hard ground substrate in otherwise mostly muddy environments. Thermogenic oil and gas seeps and biogenic gas seeps are pervasive and intrinsic features of the Gulf of Mexico. Thermogenic seeps will persist as long as oil and gas continue to migrate to the seafloor.
KeywordsOil and gas seeps Seep gasses Seep liquids Biogenic seeps Thermogenic seeps Cold-seep communities
Understanding the location, type, and volume of petroleum seepage is important as indicators of deeper petroleum reservoirs, the presence of faults, and geohazards. Seeps release oil to the sea and greenhouse gases to the atmosphere (Etiope 2009, 2012, 2015; Coleman et al. 2003; Ciais et al. 2013). Conversely, the geographic distributions of oil and gas production and reserves, subsurface geology and sedimentary basins, salt structures, sea-surface slicks, seep-related water column and seafloor features, gas hydrate, and cold-seep communities can be used to infer the presence of seeps.
5.2.1 History of Oil and Gas Seeps Worldwide
The first evidence of humans using petroleum from seeps dates to more than 40,000 years ago, associated with stone tools used by Neanderthals at sites in Syria (Hirst 2009; Etiope 2015). The use of seeping petroleum as a sealant, adhesive, building mortar, incense, and decorative application on pots, buildings, or human skin has been documented worldwide (Krishnan and Rajagopal 2003). More than 5,000 years ago, ancient Sumerians, Assyrians, and Babylonians used asphalt from seeps along the Euphrates for waterproofing (PBS 2004). Ancient Egyptians used liquid oil for medicinal purposes and embalming (Harwell and Lewan 2002; Barakat et al. 2005; Rullkötter and Nissenbaum 1988). In North America, prehistoric Native Americans used tar as a glue to bind stone tools to wooden handles and as a waterproof caulking for baskets and canoes (Harris and Jefferson 1985). In 480 BC, Persian military forces used oil-soaked flaming arrows during the siege of Athens (PBS 2004). The first oil well is believed to have been drilled in 347 AD when the Chinese used bamboo poles to bore as deep as 244 meters (m) (800 feet [ft]) into the subsurface (Kuhn 2004). In the sixteenth century, oil imported from Venezuela was used to treat Holy Roman Emperor Charles V for gout. The word petroleum, Latin for rock oil, was first used by German mineralogist Georg Bauer in 1556 (PBS 2004). In the eighteenth century, Lewis Evans’s “Map of the Middle British Colonies in America” noted the presence of petroleum seeps in Pennsylvania (PBS 2004). During the Revolutionary War, Native Americans taught George Washington’s troops how to treat frostbite using seep oil, and Seneca Oil was advertised as a cure-all tonic. As early as 1815, some streets in Prague were lit with petroleum-fueled lamps (PBS 2004).
The modern history of petroleum exploitation is closely linked to petroleum seeps. Kerosene was produced from seepage oil in 1823. The process of refining kerosene from coal was developed in 1846 (PBS 2004; Kindersley 2007). This process was improved to refine kerosene from seeps in 1852. The first rock oil mine was dug in central Europe in 1853. In 1854, Benjamin Silliman was the first American to fractionate petroleum by distillation. These advances were rapidly adopted around the world (PBS 2004). The first commercial oil well was drilled in Poland in 1853 and the second in nearby Romania in 1857 at seep sites. This was followed by the opening of the world’s first oil refineries (Stoicescu and Ionescu 2014). By the end of the nineteenth century, the Russian Empire led the world in petroleum production. In North America, the first oil well was dug in Oil Springs (named for a nearby seep) in Ontario, Canada, in 1858 (Kolbert 2007). The U.S. petroleum industry began in 1859 on Oil Creek (named for a nearby seep) near Titusville, Pennsylvania (PBS 2004). In the 1860s to the 1900s, sources of oil were discovered in association with petroleum seeps in Peru (1863), the Dutch East Indies (1885), and Persia (1908), as well as in the Americas in Venezuela, Mexico, and the Canadian province of Alberta. By 1910, some of these sites were being developed at an industrial level. In the late nineteenth century and early twentieth century, the demand for petroleum, created by improvements in the internal combustion engine and replacement of horse-drawn carriages, quickly outstripped the supply from seep-related sources. Surface seeps remained a primary indicator of deeper reservoirs of petroleum for many years until the advent of seismic technologies that could visualize the deep subsurface. The first commercial discoveries of oil using seismic methods were in 1924 in Mexico and Texas (Sheriff and Geldart 1995).
During World War I, oil was increasingly viewed as a strategic asset due to the use of oil-powered naval ships, new horseless army vehicles (such as trucks and tanks), and military airplanes (PBS 2004). Oil use during the war increased so rapidly that a severe shortage developed in 1917–1918. By the middle third of the twentieth century, transformative changes occurred in the oil industry. Beginning with Standard Oil’s activities in Saudi Arabia, oil prospecting began a global expansion. The internationalization of oil exploration, production, and distribution played an important role in World War II. Superior access to oil aided the Allied effort. Scientific discoveries and inventions also created a vast market for petroleum products in plastics, synthetic chemicals, and other industries.
5.2.2 History of Oil and Gas Seeps in the Gulf of Mexico
To assess the association of seepage and subsurface accumulations of petroleum, Schumacher (2012) compiled seepage survey results for more than 2,700 exploration wells and compared the results with subsequent drilling outcomes. Locations were in frontier and mature basins, onshore and offshore, and in a wide variety of geologic settings. Subsurface drilling targets were from 300 m (984 ft) to more than 4,900 m (16,076 ft), and there was a full spectrum of trap styles. The presence of seepage was inferred from soil gas, microbial, iodine, radiometric, and/or magnetic surface surveys. Eighty-two percent of wells associated with surface seepage anomalies were considered commercial discoveries, and 11 % of wells drilled without a documented surface seepage anomaly resulted in discoveries. The measure of association was economic viability determined by external factors, and not the presence or absence of petroleum in the subsurface. The sites chosen for analysis in this study were not random; they were based on conventional prospect evaluation methods. This study illustrates that seeps are often only surveyed for in areas suspected of being oil and gas prospects, thus limiting geographic coverage.
Extraction of petroleum
Transport-ation of petroleum
Consumption of petroleum
The immensity of the volume of oil released by seeps in the Gulf of Mexico is indicated by the larger relative contribution of petroleum seepage to oil in North American waters as compared to worldwide estimates (Coleman et al. 2003). The alteration of petroleum, once released to the environment, introduces considerable uncertainty in estimating the volume of petroleum seepage. Gaseous hydrocarbons are particularly susceptible to alteration after seepage and are rarely considered in global seep inventories since little is known about the rates and volumes of seepage, though gas seeps are known to be common (Kvenvolden and Cooper 2003). Most gas seepage is either dissolved in seawater or quickly metabolized by microbes, leaving scant evidence of its presence. Because methane is a greenhouse gas, the contribution of atmospheric methane from natural seepage (mostly biogenic in origin) has been estimated (Etiope 2015). While petroleum seeps have been reported extensively worldwide, global inventories remain incomplete and uncertainties in volume estimates are large.
140,000 tonnes (42 million gal) total natural annual loadings: 70,000 (21 million gal) tonnes in the northeastern Gulf of Mexico and 70,000 tonnes (21 million gal) in the northwestern Gulf of Mexico.
25,400 tonnes (7.62 million gal) total anthropogenic annual loadings: 4,400 tonnes (1.32 million gal) in the northeastern Gulf of Mexico and 21,000 tonnes (6.3 million gal) in the northwestern Gulf of Mexico.
Negligible total natural annual loadings (few known seeps)
17,740 tonnes (5.322 million gal) total anthropogenic annual loadings: 2,660 tonnes (798,000 gal) in the northeastern Gulf of Mexico and 15,080 tonnes (4.524 million gal) in the northwestern Gulf of Mexico.
A tonne equals about 300 gal of oil (Kvenvolden and Cooper 2003). Kvenvolden and Cooper (2003) provide a detailed review of estimates of oil seepage rates as of 1975, 1985, and 2000. In the latest estimates (Coleman et al. 2003), the authors note that the number of regions known to have significant seeps increased mainly due to detection by satellite remote-sensing techniques. The authors note further that seepage rates in the Gulf of Mexico are much higher than first estimated in 1975 and 1985 as the number of known seeps has significantly increased. Based on satellite remote sensing, MacDonald (1998) and MacDonald et al. (1993, 1996) estimated total seepage to be from 4,000 to 73,000 tonnes (1.2–21.9 million gal) per year in the northern Gulf of Mexico (Kvenvolden and Cooper 2003). Assuming a seep rate for the entire Gulf of Mexico is about double the northern Gulf of Mexico estimate, the total Gulf of Mexico seep rate is estimated to be about 140,000 tonnes per year (42 million gal).
Based on these estimates, most petroleum seepage occurs in the northwestern and north-central deepwater region of the Gulf of Mexico coincident with oil and gas production and is negligible in coastal waters. The high estimate for the offshore northeastern Gulf of Mexico region is due to one seep site reported in the far western part of the northeastern sector, but oil seeps are generally absent in the region. Similar estimates are less certain for the southern Gulf of Mexico, but many seeps are known in this region both onshore and offshore.
5.4 Petroleum Geology
Overpressure is caused by the rapid loading of fine-grained sediments, which prevents expulsion of water and equalization of the pressures created by the overburden. Intermittent resealing of breaches can occur and slowing or cessation of burial allows time for excess pressures to dissipate. Fluid expansion, which is a change in volume, is a second cause of overpressure. Overpressure is caused by the thermal expansion of water, clay dehydration, and the thermal cracking of source-rock organic matter to form oil and gas. Depending on the degree of overpressure and the mechanical strength of the encasing rocks, seepage can be widespread and diffuse. A slow seepage rate is commonly referred to as microseepage. In instances when the rocks fracture, focused high-volume seepage is commonly referred to as macroseepage.
5.4.1 Source Rocks and Petroleum Generation
Northern Gulf of Mexico Source Intervals (ages) and Source-Rock Correlations (Hood et al. 2002; AAPG©2002, reprinted by permission of the AAPG whose permission is required for further use)
Rock Oil Type
Lower tertiary (centered on Eocene)a
Tie with high maturity cores of south Louisiana multiple-maturity suites and south-central Louisiana offshore Texas (salt sheath)
Upper cretaceous (centered on Turonian)a
Marine—low sulfur—no tertiary influence
Direct ties with mature source rocks: offshore-eastern Gulf of Mexico, onshore Tuscaloosa trend, and Louisiana and Mississippi Giddings trend, Texas
Direct ties with source rocks: South Florida Basin
Calcareous—unidentified cretaceous—production from fractured lower cretaceous black shale—south Texas
Uppermost Jurassic (centered on Tithonian)a
Inferred tie to postmature, organic-rich calcareous shales of the eastern Gulf of Mexico and oils in lower cretaceous reservoirs on Florida shelf where the Turonian/Eocene section is immature
Marine—moderately high sulfur—Jurassic
Upper Jurassic (Oxfordian)
Tie to postmature, organic-rich carbonates—Mobile Bay
Triassic (Eagle Mills)
Tie to postmature, organic-rich cores—northeast Texas (paleontology and palynology confirm nonmarine source character)
5.4.2 Migration Pathways
As elsewhere in the world, seepage and subsurface petroleum systems in the Gulf of Mexico are closely correlated. Petroleum seepage patterns and analysis in the offshore Gulf of Mexico have been used to extend mapping of hydrocarbon systems and maturity maps beyond subsurface core data (Hood et al. 2002). As described above, the basic requirements for petroleum to reach the surface are common in the deepwater region of the Gulf of Mexico. Multiple prolific source rocks are present that have been deeply buried by sediment deposition over geologic time. Burial results in maturation contributing to overpressuring that, combined with buoyancy, drives upward fluid migration. The same geological processes produce large sandstone bodies. These bodies serve as excellent high porosity reservoirs where some of the generated liquid and gaseous hydrocarbons are trapped. Salt tectonics involving the underlying Jurassic Louann Salt has created deep subsurface faults. These faults provide conduits for not only migration of petroleum into reservoir rocks but also breaching reservoir seals allowing seepage to the surface.
Petroleum seeps in the Gulf of Mexico are a highly variable mixture of chemical compounds reflective of the subsurface source materials and postseepage alteration processes. Seeps exhibit the full spectrum of alterations from pristine (e.g., unaltered) to severely biodegraded. Seeps can be 100 % methane, while in other instances, a complete suite of hydrocarbons typically found in oil is present. The chemical compositions of seeps have been determined by sampling and analysis of air, water, and sediments using seafloor coring devices and remotely operated vehicles and manned submersibles. Sea-surface slicks are collected with adsorbents and screens. Each petroleum seep has its own chemical signature.
Collectively, Gulf of Mexico seeps contain gaseous compounds with 1–5 carbon atoms, volatile compounds with 6–12 carbon atoms, and higher-molecular-weight hydrocarbons with 13 to more than 60 carbon atoms. Seeps can contain alkanes, branched alkanes, cycloalkanes, and aromatic (unsaturated) hydrocarbons. As with petroleum, heteroatomic compounds (containing oxygen, nitrogen and sulfur), resins, asphaltenes, metals and sulfur can be present in oil seeps as well. Complex biochemical-derived compounds that can be linked to known biological precursors, the so-called biological markers or biomarkers, are also commonly present in seeps.
Various compositional ratios of C1–C4 gases have been used to infer origins and maturity. Being derived from fossil carbon, thermogenic gases contain no radiocarbon (Figure 5.25). Thermogenic methane is enriched in 13C relative to microbial-derived methane, with most stable carbon isotopic values ranging from −50 to −35 ‰ (parts per thousand—denoted as ‰—enrichments or depletions relative to a standard of known composition). Microbial gases stable carbon isotopic values vary from −120 to −60 ‰ (Whiticar 1999). Methane hydrogen stable isotopic compositions (1H, 2H [deuterium]) provide additional information about the origins of gases. The hydrogen isotopic composition of methane derived from bacterial carbonate reduction ranges from −250 to −150 ‰, whereas values for methane derived from bacterial methyl-type fermentation range from −375 to −275 ‰. Thermogenic methane deuterium values range from −300 to −100 ‰ (Schoell 1980). Seep gases can be mixtures of multiple sources, and stable isotopic compositions can be altered by microbial oxidation confounding determination of original compositions.
It has been observed that migrated gasoline-range hydrocarbon compositions can vary from those found in reservoir oils (Abrams et al. 2009). The origins of gasoline-range (volatile) hydrocarbons (C5 to C12) in near-surface sediments are difficult to determine due to limited knowledge of inputs from recent organic matter. Seep gasoline-range hydrocarbons are often highly altered by microbes as a readily available source of labile reduced carbon.
The chemical and isotopic analyses of Gulf of Mexico oil seeps have been used to infer origins based on individual hydrocarbon concentrations and ratios; sums of homologue concentrations and ratios (e.g., alkanes and polycyclic aromatic hydrocarbons); stable carbon, hydrogen, and sulfur isotopic ratios; sulfur and metal content (e.g., Ni/V ratios); and biomarker compositions. Seep biomarker compounds provide information that can be used to correlate surface seep to subsurface oils and/or source rocks and indicate source-rock maturity and geologic age. Biomarkers commonly analyzed by gas chromatography/mass spectrometry include, but are not limited to, hopanes, steranes, tricyclic/tetracyclic terpanes, diasteranes, monoaromatic steroids, and triaromatic steroids. Low-intensity seeps can be overprinted by recent organic matter, which can obscure origins (Cole et al. 2001). Not all high-molecular-weight thermogenic hydrocarbons in recent sediments are due to oil seepage. Eroded material from surface exposures of thermally mature source rock can be redeposited in recent sediments.
Evaporation is an important weathering process if seeping petroleum reaches the air/water or air/land interface. In particular, low-molecular-weight hydrocarbons (C1 to C12) are subject to evaporative loss (Coleman et al. 2003). Gases can reach the atmosphere with little or no alteration, depending on the physical setting, and compounds with higher molecular weights may be little altered by evaporative losses. Petroleum seeps can be a mixture of hundreds of compounds that vary from location to location and over time, and evaporative losses can be quite complex, variable, and often difficult to predict.
Emulsification is the process where water mixes with oil changing the properties and characteristics of seepage and susceptibility to biodegradation. Additionally, the volume of the seep increases due to the addition of water (Coleman et al. 2003). Emulsification of seeping oil requires turbulent mixing and is therefore mostly restricted to higher energy marine settings. Emulsions do not spread and tend to form lumps or mats. Tar balls, tar mats and pavements, and asphalt flows have been recovered from Gulf of Mexico shorelines, sea surface, and seafloor. These materials can have differing origins including formation in place (due to emulsification), seepage of oil degraded in the subsurface, eruptions of molten asphalt, and formation at the sea surface due to weathering, which can be followed by sinking to the seafloor once their densities exceed that of seawater (Alcazar et al. 1989; MacDonald et al. 2004). In the Gulf of Mexico, natural and anthropogenic tar balls commonly wash up on shorelines and after storm events. Large tar mats and pieces of tar pavements (or reefs) have been observed on beaches (Van Vleet et al. 1983, 1984).
While generally hydrophobic, hydrocarbons have measurable solubility in water. Gases are the most water-soluble constituents of seeps. In most cases, dissolution accounts for only a small portion of oil seep loss but is important because some of the more soluble components of oil, particularly low-molecular-weight aromatic compounds (e.g., benzene, toluene, alkylated benzenes, and naphthalenes), are more toxic to aquatic species than aliphatic hydrocarbons (Coleman et al. 2003). Dissolution can be extensive in marine settings due to long-term exposure to seawater.
Two oxidative processes, photooxidation and biological oxidation, can alter seeps. Photooxidation includes a wide variety of light-catalyzed reactions. Photooxidation binds oxygen to carbon substrates transforming hydrocarbons into functionalized compounds such as alcohols, ketones, and organic acids that are more water-soluble than the original aliphatic hydrocarbons. If oxygen, light, and time are unlimited, the end products of photooxidation are carbon dioxide and water. Photooxidation is usually unimportant from a mass-balance consideration for seeps but may play an important role in the removal of dissolved hydrocarbons in high-light environments (e.g., on land or in shallow water). Some oxidized by-products are more toxic than precursor compounds (Coleman et al. 2003). The chemistry and extent of photooxidation of hydrocarbons can be quite complex. Its course and importance is dependent on a number of compositional and environmental variables.
Biogeochemical processes are fundamental to, and a critical connection between, commonly expressed phenomena at petroleum seep sites. The primary effects of seeps are the introduction of reduced labile carbon as oil and gas and biological utilization of the labile carbon as an energy source. Other seep effects are those related to the toxicity of some petroleum constituents, and yet other processes involve the by-products (i.e., carbon dioxide and sulfide) and metabolites of hydrocarbon oxidation. Many of these processes are complex, unfold in a stepwise fashion with subsequent processes dependent on the preceding process, have rate-dependent or concentration-threshold limitations and often, these processes are not fully understood. These biogeochemical manifestations of oil and gas seeps have been widely used to recognize the presence of seeps in the absence of direct measurements of hydrocarbons.
A wide range of biota have the capacity to oxidize hydrocarbons, including bacteria, fungi, heterotrophic phytoplankton, and some higher organisms. There are two types of biological oxidation: metabolic detoxification after ingestion and microbial utilization. These two types have markedly differing biochemistries and end products. Metabolic detoxification of hydrocarbons by higher organisms exposed to aromatic hydrocarbons converts them to water-soluble compounds (e.g., alcohols, ketones, phenols, epoxides, and organic acids) that are excreted by the organisms as a protective mechanism. This process is biochemically complex and involves specialized enzymes (e.g., mixed function oxygenases). Not all organisms have the capacity to detoxify hydrocarbons. From a mass-balance perspective, metabolic detoxification of hydrocarbons is unimportant in removing seep hydrocarbons from the environment.
In contrast to metabolic detoxification, microbial oxidation, which occurs commonly, is important in removing hydrocarbons from the environment. Many seeps are highly altered by these processes. Microbial oxidation utilizes hydrocarbons as a carbon source to produce energy from the breaking of carbon bonds and is often referred to as biodegradation. Biodegradation causes two important effects: the effect of the by-products/metabolites of hydrocarbon oxidation and change in the residual oil and/or gas. As with photooxidation, the ultimate end products of biodegradation of hydrocarbons can be carbon dioxide and water, but a range of intermediates, such as organic acids, are also formed. The chemical and stable isotopic compositions of residual hydrocarbons are often altered.
The local chemical environment and the rate of hydrocarbon seepage control carbonate rock formation. Carbonate nodules of up to about 2 centimeters (cm) (0.8 inches [in.]) in diameter scattered throughout sediments are often incorporated into composite aggregates, occur in association with mussel and tubeworm communities, and form deep in sediments most likely in response to slow hydrocarbon flux rates (Figure 5.34). Chimneys as long 50 cm (19.7 in.) can occur as broken pipe-like shapes protruding from muddy sediments and may form due to focused vertical migration of hydrocarbon-rich fluids possibly associated with animal burrows. Slabs can be developed with rough surfaces, sometimes multiple layers and mostly composed of aragonite, suggesting precipitation from sulfate-rich porewaters. Carbonate blocks can be heterogeneous and up to several meters in diameter and length and contain void-lining aragonite-splay cements and brecciated structures of unknown origin (possibly related to abrupt expulsion of hydrocarbons or the decomposition of gas hydrate). Carbonate rocks can be rich in mussel and clam shells and contain relics of burrowing activity and occasionally have iron and manganese coatings (Callender et al. 1990, 1992; Feng et al. 2010).
5.6 Terrestrial Environments
Seeps on land can increase hydrocarbon concentrations in soils and the overlying atmosphere; enhance microbiologic activity; introduce minerals (such as uranium creating radiation anomalies); form calcite, pyrite, elemental sulfur, magnetic iron oxides, and sulfides; bleach red beds; alter clay minerals; affect soil electrochemical properties; and modify biogeochemical and geobotanical processes (Schumacher 2012). Liquid seepage is adsorbed onto soils, while gas seepage can move, mostly unaltered, directly into the atmosphere. Due to the relative lack of water at land sites of seepage compared to marine environments, emulsification is unimportant, but photooxidation can occur on direct exposure to sunlight. The geochemistry of mineral formation mediated by microbiota is similar to that described for marine settings and can encapsulate gases and liquids in mineral interstices. Due to changes in the chemistry of soils and the toxicity of some components of petroleum, land seeps can affect surrounding vegetation health and composition. Soil and air have been analyzed to detect seep-induced surface anomalies on land. Techniques also have been developed to detect these changes using airborne and satellite imagery, spectral reflectance, and other sensors. These surveys typically map suspected seep indicators and variations in vegetation health and types. There are various limitations to these methods and ground-truth is essential to confirm correlations with seepage. While there are many prospect-specific examples, few surveys are in the open literature that would allow assessment of the regional occurrence of land petroleum seeps in the Gulf of Mexico region.
5.7 Marine Environments
5.7.1 Sea-Surface Slicks and Water Column Plumes
Hydrocarbon gas distributions in seawater have been surveyed by ships that deploy equipment, collectively called sniffers, to pump seawater to the surface for analysis. These techniques have found wide use in oil and gas exploration and in the Gulf of Mexico since the 1960s (Dunlap et al. 1960; Lamontagne et al. 1973, 1974; Bernard et al. 1976; Brooks and Sackett 1973; Sackett and Brooks 1973; Sackett 1977). Hydrocarbon sniffers consist of a gas extraction system, adsorbents to concentrate the hydrocarbons, and a gas chromatograph equipped with a flame ionization detector to separate and measure individual hydrocarbon gases. Modern sniffers employ real-time, hydrocarbon detection systems based on various concepts and are deployable on remotely operated and autonomous vehicles. Some sniffers can detect gaseous and liquid hydrocarbons and some have used low-flying airplanes to detect methane in the air overlying the ocean (and land); the use of sniffers in drones has been proposed. Most of the methane detected in the Gulf of Mexico water column is of recent microbiological origin.
5.7.2 Seafloor Sediments
The first reports of retrieval of oil-stained seafloor sediments in the Gulf of Mexico began appearing in the literature in the 1980s. Anderson et al. (1983) noted high concentrations of biodegraded oil, carbonate deposits, and organic sulfur in north-central Gulf of Mexico continental slope sediments recovered by coring. Chemical and stable carbon isotopic compositions indicated that the observed high concentrations of methane to pentane must have been produced thermally at depth beneath the seafloor and had reached the surface through faults and fractures associated with salt diapirs. The authors also noted anomalous seismic reflections that suggested the presence of gas hydrate. Since this first report, the seafloor of the Gulf of Mexico has been extensively sampled by various coring devices from ships, remotely operated vehicles, and manned submersibles.
5.7.3 Gas Hydrate
Until the 1980s, gas-hydrate deposits were believed to occur deep in the subsurface as inferred from seismic records based on bottom simulating reflectors, indicating a phase change from gas to solid hydrate, which crosses subsurface strata (e.g., simulates the seafloor surface). Analyses also indicated that the gas in deep hydrates was solely methane of recent microbiological origin. This view changed when, for the first time, core samples of surface sediments in the Gulf of Mexico recovered thermogenic gas hydrates (Brooks et al. 1984, 1986, 1994) that contained substantial amounts of thermogenic methane and higher-molecular-weight hydrocarbon gases. Following the Gulf of Mexico discoveries, gas hydrates have been recovered from surface sediments cores in the Cascadia continental margin of North America, the Black Sea, the Caspian Sea, the Sea of Okhotsk, the Sea of Japan, and the North and South Atlantic Ocean (Collett et al. 2009).
Seeping hydrocarbon gases can crystallize as gas hydrate in layers, as nodules, and as exposed mounds and vein fillings in sediments (Figure 5.58) (Sassen et al. 2001a, b). Biogenic gas hydrate is white, while thermogenic gas hydrate can be stained with oil or encrusting bacteria giving the hydrate a yellow to orange color (Figure 5.58). Gas-hydrate-derived seepage is largely restricted to occurrences at shallow depths in sediments or outcroppings on the seafloor. In general, gas hydrates tend to accumulate in near-surface sediments, not decompose (Sassen et al. 2001c, 2004). It has been suggested that warmer bottom water temperatures in the ocean can initiate seepage from gas hydrate, which has generated interest in the stability and contribution of gas hydrate to atmospheric greenhouse gases (Sassen et al. 2004). Methane gas hydrate can occur as three different crystalline structures, and all have been observed in nature (Figure 5.59) (Brooks et al. 1984; Sassen and MacDonald 1994; Sassen et al. 2000).
5.7.4 Cold-Seep Communities
In the deep sea, a highly specialized ecology has developed that thrives and depends on petroleum seeps. The discovery of hydrothermal-vent communities in 1977 marked a major change in the understanding of life on Earth and how it might have evolved (Ballard 1977). Until this discovery, it was believed that the primary source of energy available to support life in the oceans was the sun through the process of photosynthesis. Deep-sea hydrothermal-vent communities are supported by alternative sources of energy from reduced chemicals escaping at the deep seafloor. The biological conversion of one or more carbon molecules (usually carbon dioxide or methane) and nutrients into organic matter using the oxidation of inorganic molecules (e.g., hydrogen gas, hydrogen sulfide) or methane as a source of energy is known as chemosynthesis. Shortly after these deep-sea discoveries, similar assemblages of organisms were recovered on the continental slope of the north-central Gulf of Mexico at a petroleum seep and at a brine seep at the base of the escarpment off the shore of western Florida (Kennicutt et al. 1985; Paull et al. 1984, 1985; Brooks et al. 1987a, b; Brooks et al. 1989). These unique biological assemblages have become known as cold-seep communities as contrasted to hydrothermal-vent communities.
Surveys and process studies have established that the critical connection between geological and biogeochemical processes that make cold-seep communities viable is the presence of a wide range of microbes (Fisher et al. 2007). Cold-seep communities in the Gulf of Mexico are unique in that the methane that fuels these bacteria is predominantly thermogenic in origin, whereas at other worldwide sites, microbiologic methane is generally more important. Symbionts, microbial mats, and free-living bacteria are ubiquitous, serving as the primary producers of cold-seep food webs (MacAvoy et al. 2005; Fisher et al. 2007). Consortia of bacteria are capable of critical metabolic conversions such as oxidizing methane and reducing sulfate ions, thereby supporting macrofauna communities and producing critical hard substrate via carbonate precipitation (see Sections 5.5.3 and 5.5.4).
Due to the limitations of most manned submersibles to a water depth of 1,000 m (3,281 ft) and difficulties in sampling the deep sea until recently, few seep sites were discovered in water depths greater than 1,000 m (3,281 ft) in the Gulf of Mexico (Brooks et al. 1990; MacDonald et al. 2003, 2004). Previous surveys and the brine-associated community known offshore Florida suggested that cold-seep communities might be present in water depths greater than 1,000 m (3,281 ft). Oil and gas seeps were known to extend to the abyssal plain, and there was no empirical evidence that water depth limited the occurrence of cold-seep communities. In 2006 and 2007, the presence of cold-seep communities was confirmed at 15 sites on the lower Louisiana slope in water depths greater than 1,000 m (3,281 ft), which significantly expanded the geographic range of sites in the Gulf of Mexico (Fisher et al. 2007; Roberts et al. 2010). These sites contained dense communities of tubeworms and mussels, communities of deep-living soft and hard corals, the largest mussel bed known in the Gulf of Mexico, an actively venting mud volcano, asphalt flows, a brine lake, and a variety of new species, including two in the genera Lamellibrachia and Escarpia. The same species of mussel found at shallower sites, Bathymodiolus childressi, was observed in water depths as great as 2,200 m (7,018 ft). Follow-up studies showed that these deeper living populations were genetically isolated from shallower ones (Cordes et al. 2007b). At water depths greater than 1,000 m (3,281 ft), Bathymodiolus brooksi, a mussel with both methanotrophic and chemoautotrophic symbionts, was also observed (Fisher et al. 1993), and at sites deeper than 2,200 m (7,018 ft), a third mussel species, Bathymodiolus heckeri, with symbionts that utilize reduced sulfur and carbon (methane and perhaps methanol) substrates for energy was the dominant mussel (Roberts et al. 2007, Duperron et al. 2007). At these deeper sites, several other types of biological communities were present including vesicomyid clams in low densities, high-density communities of symbiont containing pogonophoran tubeworms and large aggregations of heart urchins residing in highly reduced sediments (Fisher et al. 2007; Roberts et al. 2010). It is now believed that if the requisite environments are present, cold-seep communities can exist throughout the deep sea regardless of water depth.
If there were no oil and gas seeps in the Gulf of Mexico, many of the phenomena described above would be absent and the mass loading of petroleum to the northern Gulf Mexico would be greatly reduced from current estimates. The distribution of methane seeps would be largely the same since the origin of this methane is predominantly microbial reworking of recent organic matter. Liquid hydrocarbons would be exclusively due to anthropogenic inputs and concentrated in the coastal regions rather than the deep sea in the absence of seeps. It could be reasonably expected that tar balls and mats would be substantially reduced on beaches and elsewhere but still be present due to human activities. Shallow gas-hydrate occurrences would likely be absent as sediment gaseous hydrocarbon concentrations would rarely reach supersaturation. It would be expected that hard sea-bottom substrate occurrences would be reduced on average, but relic, shallow water, and erosion-exposed hard bottom would still be present. From an ecological standpoint, the picture is more complex in regard to an absence of seeps.
The predominant megafauna at cold-seep communities require elevated sulfide concentrations associated with seeps to support endosymbiosis. It is known that these communities have ceased to exist when seepage is no longer present. Since many cold-seep species are endemic (i.e., found only at seeps), Gulf of Mexico biodiversity would be decreased. Studies have shown that cold-seep communities are largely oases of life in an otherwise relatively uniform deep-sea environment. MacAvoy et al. (2005) concluded that some heterotrophic fauna collected in close association with cold-seep communities most likely obtain the bulk of their nutrition from chemosynthetic production through a combination of grazing on free-living bacteria and directly consuming faunal biomass. However, other background deep-sea fauna have been shown to contain little evidence of the utilization of cold-seep primary production, so the broader ecological importance of cold-seep communities to the deep sea remains largely a mystery (Carney 2010). On the other hand, Boetius and Wenzhofer (2013) concluded that, on a global basis, seep sites on continental slopes sustain some of the richest ecosystems in the deep sea and that cold-seep communities utilize about two orders of magnitude more oxygen per unit area than non-seep communities. Other studies have shown that cold-seep ecosystems contribute substantially to the microbial diversity of the deep sea. Hydrocarbon seeps have been described as “…geologically driven hot spots of increased biological activity on the seabed…” (Foucher et al. 2009), and it has become increasingly recognized that biological hot spots are critical to sustaining biodiversity. The differences in the larger Gulf of Mexico ecosystem that might be expected if there were no seeps is difficult to predict given the present state of knowledge but the effects are expected to be limited, as most Gulf of Mexico biomass and diversity occurs in coastal regions beyond the influence of seeps. However, oil and gas seeps are an intrinsic feature of the region and are expected to persist as long as oil and gas remains deep within the basin and finds its way to the surface.
5.7.5 Exemplar Sites
Petroleum seepage is a prevalent, natural worldwide phenomenon that has occurred for millions of years and is especially widespread in the deepwater region of the Gulf of Mexico. As one of the most prolific oil and gas basins in the world, the Gulf of Mexico has abundant deep-seated supplies of oil and gas to migrate to the surface. The deepwater region of the Gulf of Mexico is an archetype for oil and gas seepage, and most of our knowledge of petroleum seeps is based on studies of the region. The essential geological conditions for seepage are met in many areas of the deepwater region of the Gulf of Mexico region including multiple deeply buried mature source rocks and migration pathways to the surface. The northern Gulf of Mexico basin has been a depocenter for massive amounts of sediments over geologic time, and salt tectonics are prevalent, setting boundaries on the geographic patterns of petroleum seepage. Gulf of Mexico seeps are highly variable in composition and volume and include gases, volatiles, liquids, pitch, asphalt, tars, water, brines, and fluidized sediments. Seeps are dynamic over a range of temporal scales and can be ephemeral or persist for many years. In the Gulf of Mexico, seeps annually release vast amounts of oil and gas to the environment. In the Gulf of Mexico region, seeps occur on land; however, most petroleum seepage is in the northwest and north-central offshore regions. Collectively, petroleum seeps in the Gulf of Mexico are sources of highly variable mixtures of hydrocarbons, which are often altered by the weathering processes that occur after seepage. Seeps can be pristine to severely biodegraded. The prevalence, persistence, number, and volumes of petroleum seeps in the Gulf of Mexico display a spectrum of characteristics typical of petroleum seeps. Biogeochemical processes are the critical connections between commonly expressed phenomena at petroleum seep sites, including topographic features and authigenic minerals. The Gulf of Mexico continental slope and abyss are complex topographically with areas of high seafloor reflectivity and acoustic wipe-out zones caused by the active influx of gases and fluids, lithification, physical disruption of sediments, and gas-hydrate formation and decomposition. Gas seeps are widespread in the Gulf of Mexico and most have microbiological origins, but thermogenic gas seeps are also common. Gas hydrate occurs in near-surface sediments at water depths below about 500 m (1,640 ft), which defines their upper stability limit. Surveys and studies have shown that cold-seep chemosynthetic communities are common at macroseeps Gulf-wide, including on the abyssal plain and in the southern Gulf of Mexico. Geological and biological manifestations at petroleum seeps on the seafloor are controlled by the composition of released gases and fluids and the rate and history of seepage. The rates of seepage of oil, gases, brines, and fluidized sediment vary from slow seepage to rapid venting. As these fluxes vary over time, cold-seep community assemblages evolve, and when seepage ceases, seep communities disappear. In the offshore Gulf of Mexico, the geographic distributions of source-rock horizons, salt basins, oil and gas production platforms, satellite and air-borne images of sea-surface oil slicks, regional oil and gas reserves, cold-seep communities, and gas hydrates illustrate the close association of petroleum seepage and these phenomena. Petroleum seepage in the Gulf of Mexico has occurred for millions of years and is widespread and active today.
- Ballard RD (1977) Notes on a major oceanographic find. Oceanus 20:35–44Google Scholar
- Brooks JM, Bernard BB, Sackett WM, Schwarz JP (1979) Natural gas seepage on the South Texas Shelf. In: Offshore Technology Conference, Houston, TX, USA. OTC 3411:471–478Google Scholar
- Brooks JM, Kennicutt MC II, MacDonald IR, Wilkinson DL, Guinasso NL, Bidigare RR (1989) Gulf of Mexico hydrocarbon seep communities: Part IV descriptions of known chemosynthetic communities. In: Offshore Technology Conference, Houston, TX, USA. OTC 5954:6Google Scholar
- Cao B, Bai G, Wang Y (2013) More attention recommended for global deep reservoirs. Oil Gas J 111(9). http://www.ogj.com/articles/print/volume-111/issue-9/exploration-development/more-attention-recommended-for-global-deep-reservoirs.html, accessed December 15, 2015
- Chisholm H (ed) (1911) Petroleum. Encyclopaedia Britannica, 11th edn. Cambridge University Press, Cambridge, UK. http://www.studylight.org/encyclopedias/bri/view.cgi?number=25685. Accessed 14 Sept 2014
- Ciais P, Sabine C, Bala G, Bopp L, Brovkin V, Canadell J, Chhabra A, DeFries R, Galloway J, Heimann M, Jones C, Le Quéré C, Myneni RB, Piao S, Thornton P (2013) Carbon and other biogeochemical cycles. In: Stocker TF et al (eds) Climate change 2013: The physical science basis. Contribution of working group I to the fifth assessment report of IPCC. Cambridge University Press, Cambridge, UKGoogle Scholar
- Cole GA, Yu A, Peel F, Taylor C, Requejo R, DeVay J, Brooks JM, Bernard BB, Zumberge, J, Brown S (2001) 21st Annual GCSSEPM Foundation Research Conference—Petroleum systems of basins: Global and Gulf of Mexico experience. Gulf Coast Section. www.gcssepm.org
- Coleman J, Baker C, Cooper CK, Fingas M, Hunt G, Kvenvolden KA, Michel K, Michel J, McDowell J, Phinney P, Rabalais N, Roesner L, Spies RB (2003) Oil in the sea III: Inputs, fates, and effects. Committee on Oil in the Sea: Inputs and Effects, Ocean Studies Board and Marine Board, Divisions of Earth and Life Studies and Transportation Research Board, National Research Council. National Academies Press, Washington, DC, USAGoogle Scholar
- Collett T, Johnson A, Knapp C, Boswell R (2009) Natural gas hydrates: A review. In: Collett T, Johnson A, Knapp C, Boswell R (eds) Natural gas hydrates: Energy resource potential and associated geologic hazards. AAPG Memoir 89:74 (Chapt. 1)Google Scholar
- De Beukelaer SM (2003) Remote sensing analysis of natural oil and gas seeps on the continental slope of the northern Gulf of Mexico. Master’s Thesis, Texas A&M University, College Station, TX, USAGoogle Scholar
- Etiope G (2009) A global dataset of onshore gas and oil seeps: A new tool for hydrocarbon exploration. Oil and Gas Business. http://www.earth-prints.org/handle/2122/6040
- Feng D, Roberts HH, Di P, Chen D (2010) Characteristics of hydrocarbon seep-related rocks from the Deep Gulf of Mexico. Gulf Coast Assoc Geol Soc Trans 59:271–275Google Scholar
- Fisher CR (1990) Chemoautotrophic and methanotrophic symbioses in marine invertebrates. Rev Aquat Sci 2:399–436Google Scholar
- Fisher C, Roberts HH, Cordes EE, Bernard BB (2007) Cold seeps and associated communities in the Gulf of Mexico. Oceanography 20:118. http://dx.doi.org/10.5670/oceanog.2007.12
- Forrest J, Marcucci E, Scott P (2007) Geothermal gradients and subsurface temperatures in the Northern Gulf of Mexico. Search and discovery article #30048. http://www.searchanddiscovery.com/documents/2007/07013forrest/images/forrest.pdf
- Frank DJ, Sackett WM, Hall R, Fredericks A (1970) Methane, ethane and propane concentrations in Gulf of Mexico. AAPG Bull 54:1933–1938Google Scholar
- Frye M (2008) Preliminary evaluation of in-place gas hydrate resources: Gulf of Mexico Outer Continental Shelf. Minerals Management Service Report 2008-004. http://www.mms.gov/revaldiv/GasHydrateAssessment.htm. Accessed 14 Sept 2014
- Gay A, Takano Y, Gilhooly WP III, Berndt C, Heeschen K, Suzuki N, Saegusa S, Nakagawa F, Tsunogai U, Jiang SY, Lopez M (2011) Geophysical and geochemical evidence of large scale fluid flow within shallow sediments in the eastern Gulf of Mexico, offshore Louisiana. Geofluids 11:34–47CrossRefGoogle Scholar
- Geology In (2015) The Petroleum System. http://www.geologyin.com/2014/08/petroleum-system.html. Accessed June 2015
- Geyer RA (ed) (1980) Marine environmental pollution, 1. Hydrocarbons. Elsevier Oceanography Series. Elsevier Scientific, New York, NY, USA. 591 pGoogle Scholar
- Greinert J, Bohrmann G, Suess E (2001) Gas hydrate-associated carbonates and methane-venting at hydrate ridge: Classification distribution and origin of authigenic lithologies. In: Paull CK, Dillon PW (eds) Natural gas hydrates: Occurrence, distribution, and dynamics. Geophys Monog Series124:99–113Google Scholar
- Harris J, Jefferson G (eds) (1985) Rancho La Brea: Treasures of the Tar Pits. Nat History Museum Los Angeles County Sci Ser 31:1–87Google Scholar
- Hirst K (2009) Bitumen—a smelly but useful material of interest http://archaeology.about.com/od/bcthroughbl/qt/bitumen.htm. Accessed 14 Sept 2014
- Hood KC, Wenger LM, Gross OP, Harrison SC (2002) Hydrocarbon systems analysis of the northern Gulf of Mexico: Delineation of hydrocarbon migration pathways using seeps and seismic imaging, in Surface exploration case histories: Applications of geochemistry, magnetics, and remote sensing. In: Schumacher D, LeSchack LA (eds) AAPG Studies in Geology no. 48 and SEG Geophysical References Series no. 11, pp 25–40Google Scholar
- Hunt J (1996) Petroleum geochemistry and geology. W.H. Freeman, New York, NY, USA. 743 pGoogle Scholar
- Kindersley D Ltd. (2007) Oil and natural gas. Presented by the Society of Petroleum Engineers. (ePub 2013) 978-1-4654-0441-1. http://www.energy4me.org/download/oil_gas_WEB.pdf. Accessed 14 Sept 2014
- Kolbert E (2007) Unconventional crude. The New Yorker Magazine, p 46. http://www.newyorker.com/magazine/2007/11/12/unconventional-crude. Accessed 14 Sept 2014
- Kuhn O (2004) Ancient Chinese drilling. CSEG Record 29(6):39–43Google Scholar
- Levin LA (2005) Ecology of cold seep sediments: Interactions of fauna with flow, chemistry and microbes. In: Gibson RN, Atkinson RJA, Gordon JDM (eds) Oceanography and marine biology: An annual review. Taylor & Francis, Boca Raton, FL, USA, vol 43, pp 1–46Google Scholar
- MacDonald IR (2002) Stability and change in Gulf of Mexico chemosynthetic communities, vol II, Technical report. Gulf of Mexico OCS Region OCS Study MMS 2002-036. U.S. Dept. of the Interior, Minerals Management Service, New Orleans, LA, USA. 456 pGoogle Scholar
- MacDonald IR, Fisher C (1996) Life without light. National Geographic, October:86–97Google Scholar
- MacDonald IR, Reilly J, Best SE, Venkataramaiah R, Sassen R, Guinasso NL, Amos J (1996) Remote sensing inventory of active oil seeps and chemosynthetic communities in the Northern Gulf of Mexico. In: Schumacher D, Abrams MA (eds) Hydrocarbon migration and its near-surface expression. AAPG Memoir 66, pp 27–37Google Scholar
- MacDonald IR, Bohrmann G, Escobar E, Abegg F, Blanchon P, Blinova V, Bruckmann W, Drews M, Eisenhauer A, Han X, Heeschen K, Meier F, Mortera C, Naehr T, Orcutt B, Bernard B, Brooks J, de Farago A (2004) Asphalt volcanism and chemosynthetic life in the Campeche Knolls, Gulf of Mexico. Science 304:999–1002CrossRefGoogle Scholar
- Macelloni L, Caruso S, Lapham L, Lutken CB, Brunner C, Lowrie A (2010) Spatial distribution of seafloor biogeological and geochemical processes as proxy to evaluate fluid-flux regime and time evolution of a complex carbonate/hydrates mound, Northern Gulf of Mexico. Gulf Coast Assoc Geol Soc Trans 60:461–480Google Scholar
- McBride BC, Weimer P, Rowan MG (1999) The effect of allochthonous salt on the petroleum systems of Northern Green Canyon and Ewing Bank (offshore Louisiana), Northern Gulf of Mexico. AAPG Search and Discovery Article #10003. http://www.searchanddiscovery.com/documents/98004/index.htm, accessed December 21, 2016.
- McCartney BS, Bary B (1965) Echo sounding on probable gas bubbles from the bottom of Saanich Inlet, British Columbia. Deep-Sea Res 12:285Google Scholar
- Miles JA (1989) Illustrated glossary of petroleum geochemistry. Clarendon, Oxford, UK. 137 pGoogle Scholar
- Milkov AV, Sassen R, Novikova I, Mikhailov E (2000) Gas hydrates at minimum stability water depth in the Gulf of Mexico: Significance to geohazard assessment. Trans Gulf Coast Assoc Geol Soc 50:217–224Google Scholar
- NASA/Goddard Space Flight Center--EOS Project Science Office (2000) Scientists find that tons of oil seep into the Gulf of Mexico each year. ScienceDaily, January 27. http://www.sciencedaily.com/releases/2000/01/000127082228.htm. Accessed 14 Sept 2014
- Nelson DC, Fisher CR (1995) Chemoautotrophic and methanotrophic endosymbiotic bacteria at vents and seeps. In: Karl DM (ed) Microbiology of deep-sea hydrothermal vent habitats. CRC Press, Boca Raton, FL, USA, pp 125–167Google Scholar
- PBS (Public Broadcasting System) (2004) Extreme oil. http://www.pbs.org/wnet/extremeoil/history/prehistory.html. Accessed 14 Sept 2014
- Peel F, Travis CJ, Hossack JR (1995) Genetic structural provinces and salt tectonics of the Cenozoic offshore US Gulf of Mexico: A preliminary analysis. In: Jackson MPA, Roberts DG, Snelson S (eds) Salt tectonics: A global perspective. AAPG Memoir 65, pp 153–175Google Scholar
- Petty O (2010) Oil exploration. Handbook of Texas online. Uploaded on June 15, 2010. Modified on December 16, 2010. Texas State Historical Association. http://www.tshaonline.org/handbook/online/articles/doo15. Accessed 14 Sept 2014
- Pflaum R, Brooks J, Cox B, Kennicutt M, Sheu DD (1986) Molecular and isotopic analysis of core gases and gas hydrates. Deep Sea Drilling Project Leg 96. In: Reports of the DSDP, Washington, DC, USA. 96 pGoogle Scholar
- Pickwell GV (1967) Gas bubble production by Siphonophores. Report NUWC TP 8. Naval Undersea Warfare Center, San Diego, CA, USAGoogle Scholar
- Railsback LB (2011) Petroleum Geoscience and Subsurface Geology. Prepared for GEOL 4320/6320 Petroleum Geology Course. http://www.gly.uga.edu/railsback/PGSG/PGSGmain.html, accessed December 14, 2015
- Reimer PJ, Baillie MGL, Bard E, Bayliss A, Beck JW, Bertrand CJH, Blackwell PG, Buck CE, Burr GS, Cutler KB, Damon PE, Edwards RL, Fairbanks RG, Friedrich M, Guilderson TP, Hogg AG, Hughen KA, Kromer B, McCormac G, Manning S, Bronk Ramsey C, Reimer RW, Remmele S, Southon JR, Stuiver M, Talamo S, Taylor F, van der Plicht J, Weyhenmeyer CE (2004) IntCal04 terrestrial radiocarbon age calibration. Radiocarbon 46:1029–1058CrossRefGoogle Scholar
- Roberts HH, Cook DJ, Sheeldo MK (1992) Hydrocarbon seeps of the Louisiana Continental Slope: Seismic amplitude signature and seafloor response. Gulf Coast Assoc Geol Soc 42:349–362Google Scholar
- Roberts HH, Feng D, Shedd W, Chen D (2009) Pervasive authigenic carbonate deposition at hydrocarbon seeps of the northern Gulf of Mexico: Geomorphic, petrographic, and geochemical characteristics. Gulf Coast Assoc Geol Soc Trans 59:653–661Google Scholar
- Sassen R, Sweet ST, Milkov AV, DeFreitas DA, Salata GG, McDade EC (1999b) Geology and geochemistry of gas hydrates, central Gulf of Mexico continental slope. Trans Gulf Coast Assoc Geol Socs 49:462–468Google Scholar
- Sassen R, Roberts HH, Carney R, Milkov AV, DeFreitas DA, Lanoil B, Zhang C (2004) Free hydrocarbon gas, gas hydrate, and authigenic minerals in chemosynthetic communities of the northern Gulf of Mexico continental slope: Relation to microbial processes. Chem Geol 205:195–217Google Scholar
- Schumacher D (2012) Pre-drill prediction of hydrocarbon charge: microseepage-based prediction of charge and post-survey drilling results. AAPG Datapages/Search and Discovery Article 90174. In: CSPG©2014 CSPG/CSEG/CWLS GeoConvention 2012, (Vision) May 14-18, 2012, Calgary, AB, CanadaGoogle Scholar
- Seelke CR, Villareal MA, Ratner M, Brown P (2015) Mexico’s oil and gas sector: Background, reform efforts, and implications for the United States. Congressional Research Service 7-5700. ww.crs.gov, R43313. 21 p
- Soley JC (1910) Oil fields of the Gulf of Mexico. Sci Am Suppl 69:1933–1938Google Scholar
- Stoicescu M, Ionescu E (2014) Romanian achievement in the petroleum industry. In: CBU international Conference on Innovation, Technology Transfer and Education February 3-5, 2014, Prague, Czech RepublicGoogle Scholar
- Texas Almanac, Texas Historical Association http://www.texasalmanac.com/topics/business/oil-and-texas-cultural-history; http://www.tshaonline.org/handbook/online/articles/doo15. Accessed 14 Sept 2014
- Whiticar MJ (1999) Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane. Chem Geol 161:291–314Google Scholar
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