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Kerogen Origin, Evolution and Structure

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Kerogen Origin, Evolution and Structure Organic Geochemistry Organic Geochemistry 38 (2007) 719–833 www.elsevier.com/locate/orggeochem Review Kerogen origin, evolution and structure M. Vandenbroucke a,*, C. Largeau b,* a IFP, Geology-Geochemistry-Geophysics Direction, 1-4 Avenue de Bois Preau, 92852 Rueil-Malmaison Cedex, France b UMR CNRS 7618 BIOEMCO, ENSCP, 11 Rue P. et M. Curie, 75231 Paris Cedex 05, France Received 28 April 2006; received in revised form 27 November 2006; accepted 9 January 2007 Available online 16 January 2007 Abstract Kerogen, commonly defined as the insoluble macromolecular organic matter (OM) dispersed in sedimentary rocks, is by far the most abundant form of OM on Earth. This fossil material is of prime importance as the source of oil and natural gas; moreover, kerogen can provide essential information on major topics such as past environments, climates and biota. This review reports the main advances in kerogen studies since the comprehensive synthesis edited by Durand [Durand, B. (Ed.), Kerogen, Insoluble Organic Matter from Sedimentary Rocks. Editions Technip, Paris, 1980.]. It is organized into eight sections. The first two are concerned with the successive definitions of kerogen and the definition used here, the dif- ferent techniques used for kerogen isolation without loss or degradation and basic kerogen analysis. The third and fourth focus on sedimentary OM sources and preservation processes in relation to depositional environment, including sedimen- tation conditions favourable for kerogen accumulation, and extrapolation to past geological time. Great strides have been made in the latter topics over the last 25 years, owing to a combination of classical studies in organic geochemistry and studies in other domains such as biogeochemistry, oceanography, hydrology and soil science, along with the development of powerful analytical tools. The next two sections deal with the different kerogen classifications by type and kerogen evo- lution and maturation upon burial in sediments. Structural modelling of coal and kerogen, based on physical and/or chem- ical structural analysis, is described in the following section. Although, only statistical, the models thus derived provide a synthetic view of the main structural resemblances and differences among various samples in relation to source, maturity or physicochemical properties. Finally, the last section explores some of the advances in kerogen understanding expected for the near future. The review includes a list containing about 500 references. Ó 2007 Elsevier Ltd. All rights reserved. Contents 1. Introduction...................................................................... 721 2. Definitionofkerogen................................................................ 722 2.1. History of successive definitions .................................................. 722 2.1.1. Pioneeringwork........................................................ 722 2.1.2. Presentdaydefinitions................................................... 722 2.1.3. Dependenceofdefinitionuponexperimentalprocedure............................ 723 * Corresponding authors. E-mail addresses: [email protected] (M. Vandenbroucke), [email protected] (C. Largeau). 0146-6380/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2007.01.001 720 M. Vandenbroucke, C. Largeau / Organic Geochemistry 38 (2007) 719–833 2.1.4. Occurrenceofotherinsolublenon-kerogensedimentaryOM........................ 724 2.1.5. Consequences of extended definition on distinction between kerogen formation andevolution.......................................................... 724 2.2. Summary . ................................................................. 725 3. Kerogenisolationandbulkcharacterization................................................ 725 3.1. Isolation. ................................................................. 725 3.1.1. Kerogenconcentrationviaphysicalmethods.................................... 725 3.1.2. Kerogenconcentrationviachemicalmethods................................... 725 3.1.3. Kerogenstorage........................................................ 729 3.2. Bulk and atomic analyses. .................................................... 730 3.2.1. Specificgravity......................................................... 730 3.2.2. Elementalanalysis...................................................... 730 3.2.3. Rock-Evalanalysis...................................................... 731 3.3. Summary . ................................................................. 732 4. Biologicalsourcesanddepositionalenvironment,presentandpast................................ 733 4.1. Biological sources ............................................................ 733 4.1.1. Primaryproducersandthecarboncycle....................................... 733 4.1.2. MicroscopicfeaturesofsedimentaryOMfrommillimetretonanometrescale............ 735 4.2. Changes associated with sedimentation processes . ................................... 739 4.2.1. Depositionalenvironment................................................. 739 4.2.2. Relationshipbetweenorganicandmineralsedimentation........................... 742 4.3. Present and past OM accumulation in sediments . ................................... 743 4.3.1. Quantitativedistributionofkerogeninsedimentaryrocks.......................... 743 4.3.2. EvolutionofOMproductionandpreservationovergeologicaltime................... 746 4.4. Summary . ................................................................. 748 5. Preservationprocesses............................................................... 748 5.1. Degradation–recondensation mechanisms ........................................... 749 5.1.1. Basicmechanismsconsideredforhumification.................................. 749 5.1.1.1. Maillard reactions. 749 5.1.1.2. Oxidative condensation of phenols . 749 5.1.1.3. Oxidative crosslinking of polyunsaturated fatty acids . 750 5.1.1.4. Esterification between fatty acids and phenols . 753 5.1.2. ImplicationofthesecondensationreactionsforsedimentaryOM..................... 753 5.1.3. Therelationshipbetweenkerogenandhumicsubstances........................... 755 5.2. Selective preservation pathway . ................................................ 757 5.2.1. Selectivepreservationofmaterialsfromalgalcellwalls............................ 757 5.2.2. Selectivepreservationofmaterialsinprotectivelayersofhigherplants................. 760 5.2.3. Chemicalstructureofalgaenan,cutanandsuberan............................... 760 5.2.4. Problemsrelatedtoalgaenan,cutanandsuberanisolation.......................... 762 5.2.5. Estimateofalgaenan,cutanandsuberancontributiontokerogen..................... 762 5.2.5.1. Algaenan. 762 5.2.5.2. Cutan and suberan . 766 5.2.6. Respective contributions of selective preservation and degradation–recondensation pathways toalgalkerogenconstituents............................................... 766 5.2.7. Selectivepreservationofothertypesofbiomacromolecule.......................... 767 5.2.7.1. Sporopollenin . 767 5.2.7.2. Lignin . 768 5.2.7.3. Proteinaceous material. 770 5.3. Summary . ................................................................. 771 6. Classificationofkerogenbytype........................................................ 772 6.1. Widely accepted classification.................................................... 772 6.1.1. Referencetypesofkerogen................................................ 772 6.1.2. Influenceofkerogentypeonpetroleummigration................................ 776 6.2. Further modifications the initial classification of kerogen . .............................. 777 6.2.1. Difficultiesduetoextensionofclassificationtootherseries......................... 777 6.2.2. Classificationbasedonbiologicalcommunitiesindepositionalenvironments............. 779 M. Vandenbroucke, C. Largeau / Organic Geochemistry 38 (2007) 719–833 721 6.3. Summary . ............................................................... 780 7. Kerogenevolutionuponsedimentburial.................................................. 781 7.1. Early diagenesis . .......................................................... 781 7.1.1. Nitrogencontent:atomicN/Cdecrease....................................... 782 7.1.2. Sulfurincorporationintoorganicandmineralsedimentfractions..................... 783 7.1.2.1. The S cycle: organic vs. mineral transformations and implication of bacteria . 784 7.1.2.2. Conditions for incorporation of mineral and organic S into sediments as shown by studies of recent sediments ................................................. 785 7.1.2.3. Sulfurization modes and related chemical structures . 788 7.1.3. Oxygenincorporation.................................................... 790 7.2. Diagenesis . ............................................................... 790 7.3. Catagenesis. ............................................................... 791 7.4. Metagenesis . ............................................................... 793 7.5. Pyrolysis as a tool for modelling geological maturation of kerogen . ........................ 794 7.6. Summary . ............................................................... 796 8. Structuremodelling:whyandhow...................................................... 796 8.1. Significance of chemical models .................................................. 796 8.2. Coal modelling .............................................................. 797 8.2.1. Modelsbasedmainlyoncoalliquefactionproducts............................... 797
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