News & Recent Publications
Sunday, June 15, 2014
Outside Publication: Organic Geochemistry
Samples of three high volatile bituminous coals were subjected to parallel sets of extractions involving solvents dichloromethane (DCM), carbon disulfide (CS2), and supercritical carbon dioxide (CO2) (40 °C, 100 bar) to study processes affecting coal–solvent interactions. Recoveries of perdeuterated surrogate compounds, n-hexadecane-d34 and four polycyclic aromatic hydrocarbons (PAHs), added as a spike prior to extraction, provided further insight into these processes.
Monday, January 13, 2014
Outside Publication: Organic Geochemistry
The gas released from coals under hydrous pyrolysis conditions represents a quantifiable mixture of ancient (270 Ma) methane (likely microbial) that was generated in situ and trapped within the rock during the rapid heating by the dike, and modern (laboratory) thermogenic methane that was generated from the indigenous organic matter due to thermal maturation induced by hydrous pyrolysis conditions. These findings provide...
Thursday, January 09, 2014
Susitna basin, south-central Alaska
USGS Publication: Open-File Report 2013–1307
We used Rock-Eval pyrolysis and vitrinite reflectance to examine the petroleum source potential of rock samples from the Sheep Creek 1 well in the Susitna basin of south-central Alaska. The results show that Miocene nonmarine coal, carbonaceous shale, and mudstone are potential sources of hydrocarbons and are thermally immature with respect to the oil window.
Thursday, September 12, 2013
Lithofacies, age, depositional setting, and geochemistry of the Otuk Formation in the Red Dog District, northwestern Alaska
USGS Publication: Professional Paper 1795–B
Complete penetration of the Otuk Formation in a continuous drill core (diamond-drill hole, DDH 927) from the Red Dog District illuminates the facies, age, depositional environment, source rock potential, and isotope stratigraphy of this unit in northwestern Alaska. The section, in the Wolverine Creek plate of the Endicott Mountains Allochthon (EMA), is ~82 meters (m) thick and appears structurally uncomplicated. Note: This is part of the collection of papers that follow continues the series of USGS investigative reports in Alaska under the broad umbrella of the geologic sciences.
Friday, May 31, 2013
USGS Publication: Open-File Report 2013–1118
A variety of sediment and rock samples were collected in and around Tuba City Open Dump in 2008. Whole rock geochemistry has already been reported for sediment/rock/dune samples in Johnson and others (2008). Many of these samples were sieved in order to get grain-size distributions. The resulting grain-size distributions along with new whole rock geochemistry associated with each grain-size category are provided in this follow-up report. In addition, whole rock geochemistry from cores collected at sites WP-03 and WP-14 and a few X-ray diffraction samples are provided.
Friday, May 31, 2013
Natural gas is an important component of the energy mix in the U.S, producing greater energy yield per unit weight and less pollution compared to coal and oil. Most of the world's natural gas resource is thermogenic, produced in the geologic environment over time by high temperature and pressure within deposits of oil, coal, and shale. About 20 percent of the natural gas resource, however, is produced by microorganisms (microbes). Microbes potentially could be used to generate economic quantities...
Saturday, April 06, 2013
Outside Publication: International Journal of Coal Geology
Coal formation dewatering at a site in the Powder River Basin was associated with enhanced potential for secondary biogenic methane determined by using a bioassay. We hypothesized that dewatering can stimulate microbial activity and increase the bioavailability of coal. We analyzed one dewatered and two water-saturated coals to examine possible ways in which dewatering influences coal bed natural gas biogenesis by looking at differences with respect to the native coal microbial community, coal-methane...
Tuesday, January 29, 2013
Shale is an increasingly important source of natural gas in the United States. The gas is held in fine pores that need to be accessed by horizontal drilling and hydrofracturing techniques. Understanding the nature of the pores may provide clues to making gas extraction more efficient. We have investigated two Mississippian Barnett Shale samples, combining small-angle neutron scattering (SANS) and ultrasmall-angle neutron scattering (USANS) to determine the pore size distribution of the shale over the size range 10 nm to 10 μm.
Thursday, December 13, 2012
Comparison of Concentration, Element Ratio, and Isometric Log-ratio
Outside Publication: Mathematical GeosciencesMathematicians and geochemists have long realized that compositional data intrinsically exhibit a structure prone to spurious and induced correlations. This paper demonstrates, using the Na–Cl–Br system, that these mathematical problems are exacerbated in the study of sedimentary basin brines by such processes as the evaporation or dissolution of salts owing to their high salinities.
Geochemistry research has been conducted under the Petroleum Processes Research Project since 1995. The project objectives are to apply and provide cutting-edge research on processes related to petroleum (oil, gas, and asphalt) generation, migration, and entrapment, and the consequences of these processes on petroleum type, quantity, quality, and location. The research results strengthen the scientific credibility of realizing and assessing undiscovered conventional and unconventional petroleum resources at a national and global level and are critical to formulating a better understanding and strategic use of petroleum resources.
Major research topics include origins of petroleum, genetic petroleum correlations, natural gas studies, shale gas, sedimentary basin modeling, and microbial natural gas.
Figure 1. An illustration of a hydrous pyrolysis experimental method used to simulate natural petroleum generation in the laboratory.
Photo: A close-up photographic view of a hydrous pyrolysis experimental method used to
simulate natural petroleum generation in the laboratory.
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The petroleum industry now ranks basin modeling as the top leadership technology that increases exploration success and lowers finding costs. Over the last decade, three-dimensional (3D) imaging and modeling of the subsurface through time have co-evolved and emerged as a major research focus of the petroleum industry. Virtually all major oil companies have independently recognized the need for... [+]
- Organize data, allowing deficiencies or inconsistencies to be identified
- Archive data (data loss due to personnel attrition and reorganization is a major cost factor)
- Facilitate visualization of geologic processes and communication with stakeholders
- Add value by converting static data into dynamic processed data and interpretations and
- Require many disciplines to work toward a single goal.
Major application of 3D petroleum systems models can serve as evolving databases that provide surface and subsurface geologic information for various practical research needs. They allow users to figuratively look within the earth to examine data, appraise the reliability of geological concepts, models, or geochemical input, and extract needed information. Major applications of 3D geologic models include developing predictive exploration and reservoir models, integrating sequence stratigraphy and assessment units, predicting the extent and timing of petroleum generation in source rocks, structural deformation that disturbs basin architecture, migration pathways, and locations of potential traps and accumulations, and analysis of risk based on different geologic, geochemical, or fluid-flow assumptions. Thus, 3D petroleum system models can provide a basic geoscience framework to conduct and record a wide variety of applied and basic research.
As petroleum becomes more difficult to find and reserves become more difficult to replace, 3D petroleum systems modeling has grown because it better quantifies the generation, migration, and entrapment of the remaining resource. It also facilitates interpretation of the stratigraphic and sedimentologic processes that are important to develop a predictive sequence stratigraphic framework. Because of financial and time constraints, large oil companies typically conduct 3D modeling studies only at scales to suit their immediate needs. These studies commonly cover only the acreage held by each company and seldom cover the full extent of each petroleum system. This knowledge gap represents an opportunity for the USGS because many domestic and international companies are willing to supply data and expertise. Companies will benefit from USGS studies that extend 3D interpretations beyond immediate concession areas. For example, many oil companies exhaustively study “postage-stamp” areas within basins, but 3D modeling may not have been conducted over the entire area (e.g., Gulf of Mexico, San Joaquin, Permian, Los Angeles basins, North Caspian Basin).
3D petroleum systems modeling is rapidly growing as a tool to better understand subsurface migration, accumulation, and preservation. The approach is strong as a tool to predict the pod of active source rock, thermal maturity of the source rock, migration pathways, and the timing of petroleum generation. 3D modeling is currently less successful as a means to predict volumes of trapped petroleum, their detailed compositions, or the effects of secondary processes. However, solutions to these questions could have major impact on the domestic and world economy. 3D modeling is a tool that will continue to attract new users because of the potential for high-impact solutions to these problems with respect to exploration, development, and assessment.
Figure 1. Input geometry for a 3D North Slope, Alaska model. The study area polygon shows the position and surface shape of the 3D model (top). The 2D cross section along the key E-W wells (white vertical lines; bottom) was cut from the 3D input model. The 2D cross section was used to reconstruct the shape, current thickness, and eroded thickness of the prograding foresets in the Brookian section. The pre-erosional thickness (maximum burial depth) was determined using a linear extrapolation of measured vitrinite reflectance (Ro) on semilog scale for each well. Reconstructed lost overburden reaches up to ~4000 m in the western part of the section. This lost section controls the burial depth and thermal maturity of the source rock and geometry of the migration paths.
Figure 2. Simplified perspective view of a 3D North Slope model (note north arrow) is based on layered surfaces derived from well and seismic data. Rather than using a stratigraphic subdivision of the Brookian clinoform sequences, timelines of eastward prograding foresets were mapped to allow for a time-transgressive change of geometry. LCU = Lower Cretaceous unconformity.
Figure 3. Three-dimensional numerical models allow predictions of petroleum migration pathways and accumulations through time. Example shows predicted migration pathways and accumulations (green and red for liquid and vapor, respectively) on the North Slope above the Triassic Shublik Formation source rock (hatched gray) 14 million years ago based on a calibrated petroleum systems model created using PetroMod® software.
Determining genetic petroleum families and their correlation with source rocks are critical to defining petroleum systems and interpreting the processes that are controlling the generation, migration and accumulation of petroleum within sedimentary basins. Geochemical parameters based predominantly on biomarker technology have been established to genetically correlate crude oils and their sources... [+]
and aid in defining petroleum systems (see Peters et al., 2005). However, crude oil is not the only petroleum product exploited in sedimentary basin; condensates and gases are also significant. In order to more thoroughly and completely understand the dynamic petroleum processes operating within sedimentary basins, reliable and scientifically sound geochemical parameters are needed to perform (1) condensate-source rock and condensate-oil correlations, (2) gas-source correlations, and (3) identify and quantify mixing of genetically unrelated crude oils, natural gas and condensates. The absence of biomarkers in most condensates precludes the interpretation of the origin of condensates, correlation with sources and the identification of mixtures using established methods. Thus, new tools that integrate molecular and isotopic data to interpret source facies and mixing processes are required. Previous studies have attempted to decipher mixed petroleum systems (Peters and others, 1989; Jiang and Li, 2002; Chen and others, 2003) with some success. However, challenges remain due to the effects of maturity, source rock facies variation, secondary alteration, and migration fractionation. Currently, no established methodology exists to unequivocally differentiate these mixtures. The issue of differentiating crude oils that represent genetic end-members and varying mixtures is particularly important in determining total petroleum systems and their assessment units.
Gas-rich petroleum systems are receiving more attention, as natural gas becomes a more dominant component of our energy mix. Currently, characterizing natural gases is limited to molecular concentrations and stable isotopes of C1 to C5 hydrocarbons. Condensates commonly coexist with natural gases in many accumulations, but correlation of this type of petroleum with natural gases and crude oils to define petroleum systems and assessment units that are dominated by natural gas and condensate is in the early stages of development (Gurgey et al., 2005). Figure 1 (Gurgey et al. 2005) demonstrates how isotopic data can be used to correlate gases and condensates with oils. Integration of genetic molecular interpretations with this type of isotopic data will enhance petroleum system and process interpretation capabilities.
Figure 1: Correlation of condensates and gases with oils based on isotopic data (From Gurgey et al., 2005).
Migration distance/pathways, thermal maturity, and reservoir alteration can affect the geochemical parameters used to correlate natural gas, condensate, and crude oil to their source rock. A robust suite of geochemical parameter parameters that are not affected by these geological factors remains to be established to unequivocally determine the sources of genetically related natural gases, condensates, and crude oils of a petroleum system.
Determining genetic petroleum families and their correlation with source rocks are critical to defining petroleum systems and interpreting the processes that are controlling the generation, migration and accumulation of petroleum within sedimentary basins. Although a variety of geochemical parameters are currently available for correlating oils to oils and their source rocks, there is a definite need to establish geochemical parameters to correlate gas and condensate to one another and to their related oils and source rocks. Results from this research are essential in defining petroleum systems that are gas or oil dominated.
Microbial Natural Gas
Natural gas generated from microbial activity in natural organic deposits (coal, black shale, petroleum) represents an increasingly important natural resource. In the past, producers have tended to ignore microbial-derived natural gas deposits because they were considered too small to be economic. However, the increasing demand for natural gas has encouraged producers to begin targeting these smaller... [+]
smaller microbial natural gas deposits. It is estimated that natural gas from microbial activity (methanogenesis) accounts for about 20% of the world's natural gas resource. Since this gas is biologically produced, it also represents a possible renewable resource. Examples of microbial-produced natural gas deposits include: the organic-rich Antrim shale deposits in northern Michigan, and the shallow eastern edge of the Powder River Basin coal in Wyoming. Significant coal-bed gas resources may also exist in subsurface Wilcox Group and younger (Paleocene-Eocene) coal beds found across much of the Gulf Coastal Plain, and preliminary geochemical and isotopic work by USGS and others has shown that Wilcox coal gas in north Louisiana originates from microbial methanogenesis.
Although a considerable body of research exists on the biology of methanogenesis, there is much less known about the microbial-mediated conversion of geopolymers such as coal, black shale, and petroleum to methane. Methanogenesis involves a large consortium of microorganisms in order to convert the geopolymers in fossil fuels to methane (Figure 1). Methanogenic archaea are the end producers of methane, but the consortia also includes fermenting bacteria that biodegrade geopolymers in the organic deposit to simpler molecules utilized by methanogenic archaea. The nature of the microorganisms, enzyme systems, and decomposition pathways involved in the production of microbial natural gas from organic deposits is actually poorly understood. This task will examine the process involved in microbial production of methane from organic deposits using both field studies and laboratory experiments.
Figure 1. Conceptual model of microbial conversion of geopolymeric carbon to methane.
Organic Origins of Petroleum
Organic Origins of Petroleum
Petroleum is a naturally occurring substance consisting of organic compounds in the form of gas, liquid, or semisolid. Organic compounds are carbon molecules that are bound to hydrogen (e.g., hydrocarbons) and to a lesser extent sulfur, oxygen, or nitrogen.
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As natural gas demand increases in the United States, gas exploration across North America is increasingly focused on “unconventional” reservoirs, including shales (a broad term herein meant to include mudstone, claystone, and other fine-grained rocks). Although shales have been studied extensively in their role as source rocks (the rocks from which oil and gas generate they have only recently become... [+]
important reservoir rocks in North America, so research on them as it bears on their reservoir characteristics is extremely limited. Because North America is a mature petroleum province, shale gas (gas produced from shale reservoirs) is rapidly becoming an important domestic exploration target and an increasingly significant source of natural gas produced in the US and Canada. A systematic study of shale gas reservoirs is thus necessary in order to more fully appreciate the volume of contained petroleum resources and the mechanisms acting to create and preserve these reservoirs through geologic time. Such a study was recently launched by the USGS because a cursory look at some shale gas reservoirs in North America suggests that there are fundamental differences between them, making it difficult to generalize about these important reservoirs. Furthermore, there is little known about the role, if any, of clay minerals (Fig. 1A), the dominant mineral component of shale, in localizing organic material (the source of oil and/or gas) and their role in influencing the concentration of specific types of organic matter (Fig. 1B).
Figure 1A. Scanning electron photo showing
clay minerals (example shown by arrow), the
dominant mineral in shales.
Figure 1B. Scanning electron backscatter photo
showing the complex mixture within shales of
constituents including organic material, clays
(I/S), fossil fragments, pyrite framboids, and
detrital quartz (Q). Other inorganic
constituents include carbonate (C) minerals.
Thus, scientists at the USGS are studying shales to address several fundamental questions concerning these important gas reservoirs such as, 1) what is the possible role of clay minerals in localizing and concentrating organic material in shale reservoirs? 2) what constitutes an effective shale gas source rock? and 3) can we define shale resources within a petroleum system framework? Addressing these questions will provide a greater understanding of the fundamental mechanisms responsible for shale gas accumulations and as such, construct a framework for future assessments of the undiscovered resources within these still enigmatic reservoirs.
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Page Last Modified: Friday, January 31, 2014
Geochemistry Research Topics
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