David F. Bensley

FOSSIL CHARCOAL IN CRETACEOUS-TERTIARY BOUNDARY STRATA : EVIDENCE FOR CATASTROPHIC FIRESTORM AND MEGAWAVE

Abstract 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 Yucatan) at the very end of the Cretaceous Period.

Organic geochemistry of a lower jurassic synrift lacustrine sequence, Hartford Basin, Connecticut, U.S.A.

Abstract Synrift terrestrial strata of the Lower Jurassic East Berlin Formation (Hanford basin, Connecticut, U.S.A.) record cyclical expansion and contraction of major lakes, six of which were deep enough to develop anoxic bottom waters. We have studied one representative lacustrine sequence in detail, sampling a new roadcut near the village of East Berlin. The section examined is 4 m thick, with a gray silstone at the base, deposited in shallow water, overlain by an organic-rich black shale (deep water), succeeded in turn by another gray siltstone, deposited as the lake waters gradually receded. The upper gray siltstone is chemically distinct from the lower siltstone, as it contains small amounts of corrensite, analcime and gypsum, reflecting the increasing salinity and alkalinity of the contracting lake. The samples in the center of the black shale unit contain laminae of thermally-altered, yellowish orange-fluorescing, mottled telalginite. The fluorescence properties indicate a peak oil generation maturity level, confirmed by a vitrinite reflectance of 1.13% and a Methylphenanthrene Index of 1.08. The other samples have less organic matter, becoming increasingly lean towards the top and bottom of the sequence. Samples in the middle of the black shale unit are distinguished by the presence of an homologous series of tricyclic terpanes extending from C20 to at least C41, and by the near absence of hopanes and steranes. Moving upsection into the gray siltstone, the samples contain markedly lessextractable organic material (EOM) and the concentration of tricyclic terpanes relative to hopanes steadily decreases. In the uppermost sample, hopanes are the predominant terpanes. Moving downsection from the black shale into the lower gray siltstone, EOM and the ratio of tricyclic terpanes also decrease, except in the lowermost samples, which contain terpane distributions like those of the middle part of the black shale. This likely is migrated material, because bitumen fills microfractures and extensive megafractures. The lack of hopanes and steranes in the black shales cannot simply be a maturation effect, as these biomarkers appear in the adjacent beds. Instead, the manual terpane distributions may indicate a changing depositional environment, documenting the geochemical evolution of the lake. Or, more likely, they may result from fractionation during expulsion of petroleum from these mature source rocks.

Correlations between the microscopic and chemical changes in wood during peatification and early coalification: a canonical variant study

Abstract The microscopic and chemical evolution of wood from peats and lignites were investigated in order to achieve a better understanding of the processes associated with the origin of cell-wall-derived huminite macerals in low-rank coals and the relationships between maceral appearance and maceral chemistry. Correlations between the observed microscopic and chemical trends were determined using canomical correlation analyses. These analyses showed that: (1) the birefringence intensity displayed by huminite macerals and pre-macerals generally decreases during peatification and early coalification as their carbohydrate fractions become progressively depolymerized and hydrolyzed (primarily through the action of microbial and fungal enzymes); (2) the intrinsic color of cells walls becomes progressively during peatification and early coalification as the phenolic components of lignins become increasingly dominant and oxidized; and (3) the primary fluorescence intensity of huminite macerals and pre-macerals decreases during peatification and early coalification due to changes in their electronic configurations arising from the partial depolymerization and side-chain modification of the lignin-derived polymers that comprise them. Similarly, changes in the general spectral fluorescence distribution of huminites and pre-huminites (for example the peak wavelength and Q ratio), are also broadly correlated with the aforementioned lignin modifications.

Classification of torbanite and cannel coal i. insights from petrographic analysis of density fractions

Torbanite and cannel coal are considered to be coals because of their low mineral content and overall physical morphology. However, the texture and composition of the organic matter in torbanite and cannel coal are similar to the kerogen occurring in oil shales and lacustrine source rocks. Therefore, understanding the nature and origin of organic components in torbanite and cannel coal is of significance in the study of kerogen and petroleum formation. In this research, a set of torbanites and cannel coals from different locations throughout the world were petrographically characterized and processed using a density gradient centrifugation (DGC) technique. Microscopically, the torbanite and cannel coal are composed of coarser maceral particles set in a fine-grained to amorphous groundmass. The groundmass is a mixture of more than one type of substance and accounts for 10 to 80% (by volume) of the torbanites and cannel coals. Botryococcus-related alginite is the most characteristic component of the torbanite. While sporinite typically is the main phytoclast in the cannel coals, in most cases the groundmass is volumetrically the dominant component, determining the overall character of the sample. This observation calls into question the traditional practice of classifying such coals using the alginite to sporinite ratio. Variations in composition, texture and fluorescence permits the recognition of three different types of groundmass: lamalginitic, bituminitic and vitrinitic. High purity alginite, sporinite, vitrinite and varieties of groundmass were separated using the DGC technique. The distribution of density fractions closely reflects the petrographic composition of the various torbanites and cannel coals. Distinct peaks on the density profiles represent the major organic components and peak magnitudes are functions of the percentage of the components, demonstrating that the density gradient profiles can be used to distinguish the different types of torbanite and cannel coal. The separation data also indicate a gradual shift towards higher density from lamalginitic to bituminitic to vitrinitic groundmass.

Rotational reflectance properties of Arkoma Basin dispersed vitrinite: insights for understanding reflectance populations in high thermal maturity regions

Abstract Analysis and interpretation of dispersed vitrinite reflectance data in regions of high thermal maturity (> 2% vitrinite reflectance) have been equivocal partly because of an increase in width and complexity of reflectance histograms with increasing mean reflectance. Such complexity is illustrated by random reflectance (Rran) data from the Arkoma Basin that display a linear increase in standard deviation of Rran with an increase in mean Rran from 1 to 5%. Evaluating how much of the dispersion in these data is the result of vitrinite anisotropy and how much is the result of mixing of kerogen populations by sedimentary processes and/or sampling procedures has been problematic. Automated collection of reflectance data during polarizer rotation provides preliminary data for solution of this problem. Rotational reflectance data collected from a subset of Arkoma Basin samples reveal positive, linear relationships among maximum (R′max), random (Rran), rotational (Rrot), and minimum (R′min) reflectance, as well as a systematic increase in bireflectance (R′max–R′min) with increasing reflectance. R′max and Rrot display lower standard deviations and narrower, more nearly unimodal histograms than Rran and R′min, suggesting that R′max and Rrot are superior (less ambiguous) indices of thermal maturity. These data patterns are inferred to be mostly an indication of increasing vitrinite anisotropy with increasing thermal maturity, suggesting that the linear covariance observed between mean Rran and standard deviation in dispersed organic data sets from regions of high thermal maturity may be explained mostly as the result of increasing vitrinite anisotropy with increasing thermal maturity.

Fluorescence spectral analysis of sporinite in the Lower Kittanning coal bed

Abstract Sporinite (microspores) from the Lower Kittanning coal bed (western Pennsylvania and Ohio) can be separated into two or more distinct groups, within a single sample at a given rank, on the basis of fluorescence spectral parameters. Differences in sporinite fluorescence color are visually discernible and can be quantified. The wavelength of maximum intensity ( λ max ) between sporinite groups within a sample can differ by as much as 115 nm, with differences of 20–70 nm being common. Spronite groups also exhibit distinctly different spectral quotients ( Q ) and mean spectral areas. The existence of these sporinite groups is due probably to different spore and pollen species in the Lower Kittanning coal. Fluorescence parameters of the dominant sporinite group in each sample vary systematically with rank. Samples in this study range from 0.56 to 1.01% R max ; within this rank range, chromaticity data show that sporinite color shifts from yellow to yellowish orange, and λ max varies from a minimum of 541 nm to a maximum of 667 nm. Comparable changes are observed for Q and mean spectra area. A discontinuity in fluorescence parameters is observed at a reflectance of apporximately 0.9%, and another one may be present at 0.7%. Spectral fluorescence data presented here are in general agreement with those reported by previous workers.

The inherent heterogeneity within the vitrinite maceral group

Abstract Research has been directed towards the characterization of ultra-fine coal. Equipment and procedures have been developed which permit full reflectance characterization on vitrinite particles as small as 1.5 μm diameter using rotational polarization techniques. The application of this technology towards the evaluation of vitrinite macerals separated using density gradient centrifugation has provided insights into the inherent heterogeneity of the vitrinite maceral group. It is concluded that much of the scatter in reflectance values obtained for a given coal is due to the presence of remnant cell structure. This structure cannot only be observed through etching, but can be directly measured using reflectance. Density gradient centrifugation can be used to separate and concentrate these remanent structures in high volatile bituminous vitrinite, provided the sample has been crushed to 5 μm or less. Within this rank range there is a strong trend towards increased reflectance with increasing vitrinite density.

Artificial maturation of alginite and organic groundmass separated from torbanites

Abstract The two principal organic constituents— Botryococcus -related alginite and organic groundmass—were isolated by density separation from two torbanite samples (from the Stellarton Formation, Nova Scotia, Canada and the King Cannel, Utah, U.S.A.). The groundmass consisted of degraded algal, bacterial and terrestrial plant debris. Aliquots of alginite and groundmass were separately heated in gold tubes for 24 h with 70 MPa confining pressure, at fixed temperatures ranging between 250 and 375°C. The 250, 300 and 325°C experiments run on the alginite produced very low yields of CHCl 3 -extractable organic matter (EOM), indicating that very little of the generation potential had been tapped. The alginite reached the onset of generation at 350°C and peaked at 375°C. The groundmass exhibited a distinctly different response to heating. Its 300, 325 and 350°C experiments showed a progressive increase in EOM yield with increasing temperature, producing more EOM than the corresponding alginite runs, in spite of the lower initial generation potential of the groundmass. However, EOM yields were lower at 375°C, indicating that its peak generation had occurred at 350°C. After heating, the CHCl 3 -extracted residues were analyzed by Rock-Eval and flash pyrolysis-GC-MS to determine the remaining petroleum potential and monitor the alterations in the macromolecular structure. In nature, petroleum generated from a torbanite would be a mixture of the liquids generated by each of its components, in a blend that would change as thermal alteration progressed, as the various constituents each reached their peak of generation. Such a multi-component model of torbanite composition can serve to improve predictions of oil generation from torbanites and related source rocks in sedimentary basins.

Low intensity spectral analysis (LISA) of coal macerals and the assessment of DGC fractions

Abstract Low intensity spectral analysis (LISA) represents an extension of fluorescence analysis that enables the study of all maceral groups. This is made possible by replacing the standard double grating monochromator and photomultiplier assembly, used in conventional fluorescence, with a fixed grating, image intensifier and diode array. This detection equipment can be used to measure extremely low intensity signals extending below the visual threshold. The increased sensitivity is achieved by using the capability of the diode array to accumulate a signal over an extended period of time. Techniques have been developed that exploit the signal gathering ability of this equipment enabling fluorescence spectral acquisition from 500 to 850 nm onall coal macerals including fusinite. The sensitivity of this analysis system permits high resolution spectral analysis into the near i.r. The capability of measuring “non-fluorescing” macerals permits the evaluation of maceral fractions obtained from density gradient centrifugation runs. The density interval over which single maceral concentrations are obtained can be rapidly determined using low intensity fluorescence analysis.

In-Situ Microspectrophotometry of Coal Macerals

Fluorescence microspectrophotometry involves the study of coal at high magnification under ultraviolet radiation. For practical reasons we are generally restricted to analysis of that portion of the electromagnetic spectrum detectable with human vision. Although analyses are carried out over a relatively narrow spectral range, the organic petrologist has the distinct advantage of being able to perceive the materials response to excitation in a qualitative manner. The quantitative capabilities of fluorescence spectroscopy are therefore used to supplement an initial qualitative assessment. The major problem encountered in the study of coal under fluorescence excitation is that most of the constituents of coal do not fluorescence, or at least do not fluorescence with enough intensity to be analyzed quantitatively, although a rough qualitative assessment can usually be made. For this reason considerable developmental effort has been directed over the past thirty years toward increasing the range and sensitivity of quantitative fluorescence analysis.