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Organic Petrology
Prasanta K. Mukhopadhyay

Organic petrology is the science of comprehensive visualization of organic matter that exists as microscopic particles, such as solids (coal, coke, graphite, kerogen), semisolids (solid bitumen), liquid components (crude oil and extractable bitumen) and recent anthropogenic residues in any terrestrial or extraterrestrial geological environment. Organic petrologists perform microscopic examinations of macerals (or palynomorphs), which are discrete organic components within rocks and carbonaceous residues similar to minerals in other rocks.

The science is built on the pioneering research of transmitted light petrology of coal in the early 20th century. Incident light microscopy of coal emerged a decade later. The perspectives of coal petrology and palynology applied to the study of coal and petroleum source-rock kerogens have changed significantly with the landmark development of fluorescence microscopy since 1967. Organic petrologists examine organic matter using a complex incident-and-transmitted light microscope with ultraviolet blue and white light sources, mostly in a nonpolarized state (except for the study of coke and carbon fibers). This microscope is sometimes attached to VIS spectrometers, infrared or Raman spectroscopes, and laser fluorescence scanning instruments or is worked in conjunction with a scanning electron microscope with EDX or a transmitted electron microscope to better evaluate the morphology, genesis and molecular characterization of the macerals within various organic matter types.

Historic contributions

Between 1935 and 1975, two books, two atlases, one glossary/handbook (revised several times) and innumerable publications from all over the world established the foundation of present-day organic petrology. Until 1965, coal petrologists mainly concentrated their research on the genesis and rank determination of macerals for paleoenvironmental assessment or on applications of coal-mixturefor the coke oven, power generation, iron and steel industries. Simultaneously, palynologists working in industry, academia and government promoted the development of thin-section petrology of organic matter (palynology) and have made significant contributions to our understanding of the genesis and rank determination of source rocks and coals. American and European research methods in these areas finally merged into the modern science of organic petrology in the early 1970s. Significant advances made with qualitative and quantitative fluorescence microscopy during the mid-to late-1970s unified coal petrological and palynological concepts.

Since then, publications and research notes have directed the maturing science of organic-matter research toward a better understanding and exploration of new earth-science horizons. Today's organic petrologists practice their science by advancing earlier research or developing new concepts.

Building on earlier research

Through existing research protocols, organic petrologists are advancing their science into the future. They have been working to fingerprint lipid components and maturation to determine the genesis of hydrocarbon expulsion and migration in petroleum source and reservoir rocks. Organic petrologists have also been identifying the physical properties of macerals and correlating them with geochemical properties to study coal or coaly shale as a source rock for liquid (crude oil) and gaseous hydrocarbons (natural gas and coalbed methane) and their feasibility for carbon-dioxide sequestration. Additional research has been on ascertaining anoxia, heat flow and hydrocarbons to understand the origin of kerogen, oil and gas in deep- and ultra-deepwater marine environments, and visualizing morphology in a sequence-stratigraphic framework to determine the depositional environment of peat mire and petroleum source rocks

New concepts

Innovative organic-matter research concepts now being pursued will be applied to our own science and other disciplines in the 21st century:

Astrogeology
Fingerprinting biological components, including oil and gas, to assess the morphology of known macerals, other organic components and organometallic complexes in Archean terrestrial and extraterrestrial environments (carbonaceous chondrites) and possibly in future rock samples from Mars
Environmental Geology
o Environmental fingerprinting and remediation — quantitative assessments of anthropogenic and natural components at recent aqueous systems or waste-disposal sites
o Climate change — comparing morphological changes in peat and lignite macerals to changes in temperature and sea level in recent geologic periods
o Carcinogenic metals and polyaromatic hydrocarbons (PAHs) in coal — identifying the mode of toxic-metal occurrence in coal and mass balance of coke/char or the mobility of metals and organic components in combustion by-products
Basin and Petroleum System Modeling - assessing the maturity and organic facies components are the key ingredients for modeling hydrocarbon expulsion and preservation refining quantitative petroleum assessments. Automotive and aviation industries — utilizing the petrology of carbon fibers, graphite, and char (combustion by-products) to design brakes for automobiles, aircraft and space vehicles
Meteoric impacts and their signature on organic matter — illustrating the morphology of fossil charcoal and its geochemical signature to examine catastrophic firestorms and megawaves in geologic periods
Archeology — correlating crude oil and other archeological tools to their source areas
Maceral morphology and its geochemical signatures — establishing the molecular characterization of macerals or hydrocarbon residues using laser scanning, scanning electron or infrared microscopes and microprobes.

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Mukhopadhyay is president of Global Geoenergy Research Ltd., and adjunct professor in the Earth Science Department of Dalhousie University. E-mail: muki@global-geoenergy.com.

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