Biomarker Geochemistry of the Miocene Monterey Formation, West San Joaquin Basin, California: Implications for Petroleum Generation

Much of the Miocene Monterey Formation of California is rich in biogenic sediment, especially organic matter and silica. Because of the geologic structure, the Monterey in the subsurface near Lost Hills in the San Joaquin Basin forms a natural laboratory for the study of the diagenetic responses of these materials. Rocks of similar age and lithology are buried to depths ranging between 500 and 3500 m, and are thus exposed to a temperature range of 45–130°C. The diagenetic progression of silica from opal-A to opal-CT to microquartz is well-documented in Monterey burial history studies. However, diagenetic indicators need to be established below the depth of complete conversion of opal-CT to quartz, which may be very shallow (1500 m or less at Lost Hills). Conventional maturity indicators are problematic. The scarcity of vitrinite in many Monterey samples hampers reflectance measurements. In addition, values that are measured may be anomalously low. Maximum pyrolysis temperatures are depressed, due in part to high heavy bitumen content. Biomarker geochemistry provides effective alternative maturity indicators. Of particular interest are the stereochemical variations observed in the assemblages of steranes and triterpanes extracted from oil well core samples. For example, the 20S/20R ratio of 5α(H), 14α(H), 17α(H)-24-ethylcholestane increases from 0.09 to 0.56 over the depth range cited. Samples from petroleum accumulation zones are distinguished by anomalously mature biomarker assemblages considering their depth of burial. This indicates that these bitumens are as mature as the deepest non-production zone sample (3430 m), even though they are from much shallower depths (580–1572 m). Other maturity (and/or migration) indicators confirm this, such as the relative increases in amounts of 14β(H), 17β(H)-steranes, rearranged steranes and tricyclic terpanes. Thus, it appears that any immature indigenous bitumen in production zone samples is overwhelmed by a mature component, which presumably migrated updip, from deeper, warmer strata. Since much oil is produced from shallow, organic-rich fractured Monterey shales, early in situ generation has previously been hypothesized. However, while both source and reservoir rock are lithologically similar and lie within the same formation, biomarker geochemistry indicates that substantial generation occurs only in deeply buried Monterey shales

Effects of Weathering on Aromatic Compounds in Beach Tars from the Deepwater Horizon Disaster, Gulf of Mexico Coast, USA

Operators aboard the Deepwater Horizon drilling platform lost control of the Macondo No. 1 well about 90 km southwest of the Louisiana coast on April 20, 2010, leading to a catastrophic release of ca. 550 Gg of crude oil over the next 86 days [1]. Oil from the spill soon found its way to nearby coastal areas, leaving tarry deposits on beaches and marshes. Oil was reported on the beach at Gulf Shores, Alabama (180 km northeast of the well) on June 5 and the relatively fresh sample discussed herein (GSA) was collected that same day. Oil reached the beach at Grand Isle, Louisiana (180 km east of the well) by May 24, 2010 and the sample (GIL) was collected on Jan. 15, 2011, i.e., about six months after the flow of oil was staunched at the well site. These two tar samples were analyzed directly, without preparation or clean-up, using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) in order to evaluate the usefulness of this technique for rapid forensic characterization of beach tars at spill sites. In this case, the pyrolysis temperature of 610 °C primarily induced vaporization of compounds present with relatively minor formation of pyrolytic artifacts. The analyses revealed a complex suite of polycyclic aromatic compounds, along with acyclic alkanes and both saturate and aromatic biomarkers. The total ion current trace of the fresher sample (GSA) exhibits a series of C16-C32 n-alkanes above a pronounced hump due to an unresolved complex mixture (UCM) of hydrocarbons, whereas in the case of the more weathered sample (GIL), only the UCM is visible, indicating an advanced stage of biodegradation. Distributions of hopanes, tricyclic terpanes, and steranes in both samples show little evidence of degradation and correlate well, indicative of a common origin. The monoaromatic and triaromatic steroid distributions are also very similar in both samples. Differences are evident in the relative proportions of alkylated phenanthrene and dibenzothiophene isomer clusters, as well as in the proportions of individual isomers, particularly among the dimethyl. In contrast, the C0-C3 alkylchrysenes display only minor differences between the two samples. Given that the biomarkers indicate that both samples are of about the same thermal maturity, the observed variations most likely arise due to differences in the severity of degradation. Such alterations must be considered when undertaking forensic evaluations of weathered oil. The Py-GC/MS technique appears to adequately resolve the essential similarities and differences between the two samples. 1) New York Times, 2010. Tracking the oil spill in the Gulf of Mexico. http://www.nytimes.com/interactive/2010/05/01/us/20100501-oil-spill-tracker.ht

Analytical Pyrolysis Principles and Applications to Environmental Science

Over the past half century, analytical pyrolysis has proven itself to be an effective means for the semiquantitative characterization of complex macromolecular organic substances. It has been demonstrated that instruments such as Py-FID, Py-MS, and in particular, Py-GC/MS can provide valuable geochemical insights when applied to a wide variety of problems in environmental science. The more widespread use of analytical pyrolysis methods in the evaluation of environmental pollution is recommended, because of their relatively low cost and information-rich results. Pyrolysis is the heating of organic substances in an inert, oxygen-free atmosphere, thereby avoiding combustion. When performed on a large scale, pyrolysis is involved in industrial processes as diverse as the manufacture of coke from coal and the conversion of biomass into biofuels. In contrast, analytical pyrolysis is a laboratory procedure in which small amounts of organic materials undergo thermal treatment, the products of which are subsequently quantified and/or characterized, for example, by gas chromatography. The pyrolysis may be performed “off-line” or “on-line.” In the off-line case, pyrolysis occurs in stand-alone reactor. The pyrolysis products are then extracted or trapped manually prior to further evaluation by chromatographic or other means. In on-line methods, the pyrolysis reactor is coupled directly to the analytical system, be it the injector of a gas chromatograph or a detector such as a flame ionization device or a mass spectrometer, with the pyrolyzate swept along its course by inert carrier gas. In some cases, a trapping mechanism such as cryofocusing is employed, which can permit the use of multiple detection or analytical systems. On-line methods typically only require milligram or even submilligram quantities of sample. Samples may be analyzed with little pretreatment, thereby minimizing the use of hazardous solvents in the spirit of environmentally conscious “green chemistry.

Fossil Charcoal in Cretaceous-Tertiary Boundary Strata: Evidence for Catastrophic Firestorm and Megawave

Organic matter separated from calcareous sandstone from the upper portion of a deep-water tsunami deposit at Arroyo el Mimbral, Taumalipas (Mexico), which marks the biostratigraphically-defined Cretaceous-Tertiary boundary, consists primarily of fossil charcoal, including semifusinite and pyrofusinite. Analytical pyrolysis-gas chromatography-mass spectrometry revealed the highly aromatic and polyaromatic character of the organic matter assemblage, typical of the products of partial combustion. The organic matter probably originated as terrestrial vegetation that was caught in a firestorm and subsequently transported far offshore in the backwash of a megawave. These data are consistent with the hypothesis of combustion of large masses of vegetation triggered by a giant extraterrestrial impact in the Gulf-Caribbean region (probably forming the Chicxulub crater in Yucatán) at the very end of the Cretaceous Period

Oil Pollution in Water Bodies of Restricted Circulation

Coastal lagoons and embayments near urban centers around the world share many common characteristics and problems. Physical impediments to free water circulation (spits, barrier islands, internal islands, tombolos, submerged sills) often lead to water stagnation and, in the presence of excess nutrients, eutrophication. Urban and industrial activities provoke (usually accidental) spills of hazardous materials into these confined water bodies, such as crude petroleum and refined petroleum products, leading to difficulties for resident biota and potential hazards for human health. The sluggish turnover of these water bodies (or low-energy zones within them) may retard the natural attenuation of the spilled contaminants. The environmental forensics approach uses organic molecular fingerprinting to characterize spills and identify sources, as well as quantitatively monitor the progress of pollutant attenuation. Environmental nuisances and hazards may occasionally be turned to beneficial use, as with the attempt to harvest algal biomass from blooms as a biofuel feedstock and the use of anoxic deep water to sequester polluted sediments. Case studies from locations worldwide including Jamaica Bay (United States), Venice and Orbetello Lagoons (Italy), Lagoa dos Patos and Baía da Guanabara (Brazil), Pearl River Estuary (China), and Grisefjorden (Norway) illustrate these principles

Diagenesis of Miocene Biogenic Sediments in Lost Hills Oil Field, San Joaquin Basin, California

Major portions of the Miocene Monterey formation of California were deposited under low oxygen conditions, with restricted clastic influx, beneath waters with high phytoplankton productivity. The resulting diatomaceous and organic-rich sediments underwent diagenetic modification as they were buried. A suite of core samples was collected from eight wells in the Lost Hills oil field ranging in present depth of burial between 535 and 2285 meters. In one well, the entire Monterey section was sampled. Cores taken from the remaining wells sampled the Reef Ridge and Antelope Members of the Monterey Formation at various burial depths. Silica mineralogy was studied using x-ray diffractometry and optical microscopy. Silica phases exhibit a clear diagenetic progression with depth from the opal-A of the diatom frustules to opal-CT and ultimately to microquartz. The d(101) spacing of opal-CT decreases from 4.088 to 4.044 Å with depth. No opal-CT was detected below 1470 meters. Organic material was studied by bulk pyrolysis (Rock-Eval) and optical methods. Organic carbon is abundant, comprising 1.4-10.2% of the rocks, mainly in the form of Type II kerogen (which reflects a dominantly marine origin) and soluble bitumens. A different organic facies is apparent in rocks sampled higher in the stratigraphic section, evidenced by admixed Type III kerogen. Maturation parameters (maximum pyrolysis temperature, visual T.A.I., and vitrinite reflectance) indicate that none of the samples has yet reached the onset of the main phase of oil generation. Diagenetic transformation of biogenic silica to quartz is therefore complete before the kerogen begins to generate appreciable petroleum. Amounts of detectable free hydrocarbons (S1) and hydrocarbons from kerogen pyrolysis (S2) are consistently high. Heavy bitumens are common and in many cases inflate S2 values. Where this occurs, pre-pyrolysis extraction is necessary to avoid misinterpretation of Rock-Eval data. Oil is produced only in the limited intervals where S1/(S1+S2) is high (~0.4). This suggests that most of the oil migrated and accumulated rather than formed in situ, although some of the heavy bitumens may be indigenous

A Complex Legacy of Contamination in Urban Estuarine Systems

Industrialized urban waterways have typically suffered decades of contamination, varying in source and intensity as manufacturing and transportation practices evolved. The U.S. Environmental Protection Agency designates locales with particularly severe contamination as Superfund sites. Among these, the Gowanus Canal and lower Passaic River in the New York/New Jersey harbor estuary illustrate a complex range of contamination types. The 2.2 km Gowanus Canal, with sluggish circulation driven mostly by tides, accumulated fine-grained sediments (average thickness of 3 m) highly enriched in organic carbon (OC, mean 11 % but up to 49 %) derived from hydrocarbons, sewage, coal, char, and biomass, along with heavy metals (e.g., Pb, mean 736 mg/kg). This contrasts sharply with subjacent sediments deposited prior to mid-19th century canal construction, with mean OC and Pb contents of only 1.9 % and 14 mg/kg respectively. The canal sediments are severely contaminated by polycyclic aromatic hydrocarbons (PAHs), with a mean concentration of 11300 mg/kg (summed EPA PAHs) in the 300 m canal segment adjacent to a former manufactured gas plant (MGP). Although the underlying pre-anthropogenic sediments are low in OC and heavy metals, they are nearly as enriched in PAHs as the canal fill (mean of 6400 mg/kg in the same 300 m stretch), likely the result of subsurface migration of coal tar liquids rather than direct deposition. This occurrence complicates remediation planning. The estuarine lower Passaic River receives freshwater flow from a 1500 km2 watershed, constrained by upstream dams, with episodic storm and snowmelt pulses. A 5 m sediment core taken adjacent to a former MGP reflects further complexity due to a 1.7 m tidal range and mid-20th century navigational dredging. Mean OC and Pb values of 5.4 % and 336 mg/kg, respectively, in the core sediments approach those detected in the Gowanus Canal. Mid-core sediments show the impact of petroleum, polychlorinated biphenyl, and dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) pollution. In the sandy zone encountered at the base of the core, OC and Pb values drop precipitously to 0.2 % and 13 mg/kg, while PAHs increase threefold above the mean to 284 mg/kg. As observed in the Gowanus Canal, these hydrocarbons may have originated at the nearby MGP and been emplaced via subsurface migration

Dam Removal in the USA: Effects on River Water Quality

Dam removal decisions should ideally be made after a thorough cost-benefit analysis. If dam obsolescence, structural safety, harm to fisheries, maintenance costs, and reservoir eutrophication are among the primary concerns, decommissioning would likely be favored. On the other hand, dams provide considerable benefits including water storage for agricultural and urban consumption, renewable electricity generation, support of navigational canal systems, flood control, and lakes for recreation. Because of these competing factors and interests, dam removal decision-making in the United States is often a slow process fraught with controversy, as in the case of the Klamath River. Dams provided mechanical water power essential for mills during the Industrial Revolution, notably on the Passaic River (New Jersey). The 1973 demolition of an old industrial dam at Fort Edward (New York) infamously spread toxic polychlorinated biphenyls (PCBs) downstream in Hudson River sediments, requiring costly remediation and providing a cautionary tale. More recently, carefully planned removals of obsolete dams (on the Cuyahoga, Elwha, and Naugatuck Rivers) were completed without serious environmental impairment, particularly when operators performed a gradual, staged demolition of the dam after the reservoir had been drained. A similarly cautious approach proved successful at the Clark Fork River Superfund site (Montana), even with the serious additional complication of heavily contaminated reservoir sediments requiring removal for off-site disposal. Minor run-of-river dams in urban areas have been removed without significantly affecting sediment contamination levels

Chemical Contaminants as Stratigraphic Markers for the Anthropocene

Thousands and even millions of years from now, widespread anthropogenic contaminants in sediments would likely persist, incorporated into the geological record. They would inadvertently preserve evidence of our present era (informally designated as the Anthropocene Epoch) characterized by large human populations engaged in intensive industrial and agricultural activities. Hypothetical geologists in the distant future would likely find unusually high concentrations of a wide variety of contaminants at stratigraphic levels corresponding to our present time, analogous to the iridium anomaly marking the bolide impact event at the close of the Cretaceous Period. These would include both organic and inorganic substances, such as industrially-derived heavy metals (e.g., Hg, Pb, Cr, Zn) and hydrocarbons, both petrogenic (derived directly from petroleum) and pyrogenic (combustion products). While there are natural sources for these materials, such as volcanic eruptions, wildfires, and oil seeps, their co-occurrence would provide a signature characteristic of human activity. Diagnostic assemblages of organic compounds would carry an anthropogenic imprint. The distribution of polycyclic aromatic hydrocarbons (PAHs) in a sediment sample could distinguish between natural and human sources. Stable isotopic signatures would provide additional evidence. Concentrations of contaminants in the sedimentary record would increase exponentially with increasing proximity to urban source areas, where at present billions of people are collectively consuming vast quantities of fossil fuels and generating large amounts of waste. Aolian and marine transport prior to deposition has been seen at present to globally redistribute detectable amounts of contaminants including Hg and PAHs, even at great distances from principal source areas. For organic contaminants, deposition in an anoxic sedimentary environment could insure their preservation, increasing the likelihood of their inclusion in the long-term stratigraphic record, establishing markers of the Anthropocene Epoch for millions of years to come

Significance of Polycyclic Aromatic Hydrocarbons (PAHs) and Petroleum Biomarker Compounds in Contaminated Passaic River Sediments

The lower Passaic River (northeastern New Jersey) flows through one of the most densely populated regions of the United States. The area’s long history of industrial activity is reflected in the complex and variable hydrocarbon composition of the river sediments. Sediments from river bottom grab samples at Newark and a 30 cm deep core at Kearny were subjected to thermal desorption-gas chromatography/mass spectrometry (TD-GC/MS). This technique offers a practical alternative for rapid, inexpensive analysis, simply employing milligram quantities of dry, disaggregated sediment, avoiding the use of hazardous organic solvents. For each sample, a total of 181 hydrocarbons and organosulfur compounds were quantitated, including normal and isoprenoid alkanes, tricyclic terpanes, hopanes, steranes, sterenes, linear alkylbenzenes, C0-C4 alkylnaphthalenes, C0-C3 alkylphenanthrenes and anthracenes, C0-C2 alkylpyrenes and isomers, C0-C2 alkylchrysenes and isomers, 5 and 6 ring parent PAHs, C0-C2 alkyldibenzothiophenes, and C20 isoprenoid thiophenes. As a guide in the interpretation of the results, principal components analysis (PCA) was employed. The resulting first two principal components accounted for 65% of the variance in the data set. While all samples appear enriched in PAHs and petroleum biomarkers, there are considerable differences in the distributions of these compounds from sample to sample. PCA results delineate three distinct chemostratigraphic zones in the Kearny core, each approximately 10 cm thick. The lower zone is enriched in alkylated three and four ring PAHs and dibenzothiophenes, as well as five ring parent PAHs and isoprenoid thiophenes, relative to rest of the core. The middle zone shows relative enrichment in isoprenoid and normal C14-C24 alkanes, alkylnaphthalenes, and dibenzothiophenes. The upper zone exhibits relative enrichment in C25-C31 n-alkanes, sterenes, linear alkylbenzenes, parent PAHs and isoprenoid thiophenes. The Newark surface grab samples resemble the upper Kearny core samples, although they show relatively higher concentrations of hopanes, steranes, linear alkylbenzenes, and isoprenoid thiophenes. The PCA results indicate distinct differences between the grab samples themselves, but of lesser magnitude than those observed within the core