Recent advances in organic petrology and organic geochemistry: an overview with some reference to ‘oil from coal’

Abstract

A B S T R A C T : Although organic petrology and organic geochemistry differ greatly in age, it is during the last 25 years that there has been an almost exponential expansion of research and literature in both fields. Much of the support for the developments that have taken place has come from industrial sources. The steel industry underpinned applied coal petrology in the 1960s to improve carbonization practice related to the production of metallurgical cokes. The petroleum industry has generally supported both organic petrology and organic geochemistry, with a principal aim of improving methods to assess rank (level of maturity) and type (source input) of both coals and kerogens. As precise a determination as possible of both these parameters is as essential in hydrocarbon-prospect appraisal as it is in the raising of coke quality.In organic petrology the construction of fully automated microscope fluorometric systems has been a major advance that has helped to define more precisely the low maturity levels that exist prior to and through most of the 'oil window'. The same optical systems have not been so successful in producing an acceptable single petrographic typing method for kerogens. Used in conjunction with chemical methods, however, microscope fluorometry has certainly contributed substantially towards more precise kerogen typing, allowing more satisfactory discrimination of the liptinite macerals, several of which are known 'oil formers'. Another group of potentially economic rocks, the oil shales, has also been successfully classified, principally through fluorescence methods.Within organic geochemistry, the growth and application of analytical pyrolysis methods have been of relevance and importance. While not capable of high chemical resolution, bulkflow pyrolysis systems (cf 'Rock-Eval') are now widely used to gain a preliminary estimation of maturity level, source input and generating potential of source rocks by the screening of large numbers of samples. The most important single advance in the pyrolysis field, however, has probably been the evolution of a typing method, which, although it utilizes information from both petrography and bulk-flow pyrolysis has, as its essential core, quantitative highresolution pyrolysis data. Kerogens can now be subdivided into eight different end-members and referred to specific depositional environments.When these fluorometric and pyrolysis data are combined with information from biological-marker studies, which have expanded rapidly as a result of the advances in computerized gas chromatography-mass spectrometry over the past decade, it is not surprising that hydrocarbon exploration efforts are better rewarded. There is now less doubt that terrestrial sources have generated economic accumulations of oil. The principal areas where oil has been generated from land-plant material are the Australian and the Canadian Frontier sedimentary basins, and the Chinese intermonbane basins, but other similar environments have been identified and no doubt other basins with te...

Hydrogen Isotopic (D/H) Composition of Organic Matter During Diagenesis and Thermal Maturation

“…CAI: Conodont alteration index, a relative scale of maturity based on the color of preserved skeletal remains from specific marine fauna (conodonts). Compiled and modified after Tissot & Welte 1984;Murchison 1987;Teichmüller 1987;Boreham & Powell 1993;Taylor et al 1998, p. 127;Petersen 2002;Petersen et al 2000Petersen et al , 2004Sykes & Snowdon 2002;and Faiz 2004. deposition (Smith et al 1983) and for constraining paleoclimatic changes across the Cretaceous/Tertiary boundary (Schimmelmann et al 1984). Miocene type-IIS kerogens from California's Monterey Formation contain D/H variability that is correlated to changes in paleoceanographic conditions, although kerogens from the more mature Lion's Head section are enriched in D relative to less mature Naples Beach kerogens (Schimmelmann et al 2001b).…”

Geochemical evaluation of the hydrocarbon prospects of sedimentary basins in Northern Nigeria

“…Coal has long been recognized as a source of gas, primarily methane and carbon dioxide but its importance as a source of economic accumulations of oil has been difficult to prove as coals are often interbedded with shales which are always assumed to be the source beds. Increasing evidence, however, suggests that coals and associated type III kerogens can yield not only gas or condensate (e.g., Tissot and Welte, 1984), but also significant quantities of oil (Murchison, 1987;Hunt, 1991;Hendrix et al, 1995). The traditional view that coals are largely gas-prone may be the result of historical bias in the study of North American and European Paleozoic coals, prior to the study of Mesozoic-Cenozoic coals containing contributions from resinous conifers and angiosperms (Obaje and Hamza, 2000).…”

Coal-Bearing Depositional Systems

“…Over the past 30 years vitrinite reflectance has become the most widely used parameter in assessing coal rank (Murchison 1958(Murchison , 1987Teichmiiller and 224 Coal Facies and Depositional Environment Teichmiiller 1966a;Davis 1978;Neave11981;Bustin et al 1985;Teichmiiller (1987) including maturation levels in dispersed organic matter (DOM), also referred to as kerogen (Bostik 1973(Bostik , 1979Murchison et al 1985;Robert 1981Robert , 1988. Its usefulness in this field is due to its precision, excellent repeatability, satisfactory reproducibility, and the possibility to make numerous low cost assessments on very small sample sizes and quantities.…”

“…At this low rank interval, characterised by a low degree of condensation of the aromatic molecular fabric, reflectance depends on changes in refractive index, which proceed slowly. This changes in the more advanced stages of physico-chemical coalification, when increased condensation and cross-linking of the aromatic clusters results in increased molecular ordering such that absorption becomes the dominant fundamental optical property causing a more rapid increase in reflectance (Murchison 1987).…”

“…Although bacterial sources of some isoprenoids have been found (Volkman and Maxwell 1986), the quantitative contribution of chlorophyll-based phytol to coal probably outweighs any bacterial input. Depending on either transient oxidising or reducing depositional condition (Welte and Waples 1973), phytol is thought to be transformed into either pristane (C 19 ) by phytol oxidation and decarboxylation or into phytane (C 20 ) by dehydration and hydrogenation (Brooks et al 1969;Powell and McKirdy 1973;Murchison 1987). Both products constitute the most frequent isoprenoid hydrocarbons found in coal (White et al 1977).…”