Formation of Organic-Rich Sedimentsand Sedimentary Rocks

Ralf Littke and Laura Zieger

Contents1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Depositional Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1 The Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Lakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 Rivers and Peats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Geochemical Transformations in Young Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 Assessment of Organic Matter in Sedimentary Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Research Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

AbstractOrganic matter-rich sediments are deposited in a variety of continental and marinesettings. Their formation strongly depends on bioproductivity and preservation oforganic material, which in turn is affected by sediment composition as well asaerobic and anaerobic microbial activity. Burial, pressure, and temperatureincrease leads to loss of porosity and mineral reactions, ultimately to the forma-tion of sedimentary rocks, and to transformation of primary biomass into insol-uble and soluble sedimentary organic matter, i.e., kerogen and bitumen.

R. Littke (*) · L. ZiegerInstitute of Geology and Geochemistry of Petroleum and Coal, Energy and Mineral ResourcesGroup, RWTH Aachen University, Aachen, Germanye-mail: ralf.littke@emr.rwth-aachen.de; laura.zieger@emr.rwth-aachen.de

© Springer Nature Switzerland AG 2019H. Wilkes (ed.), Hydrocarbons, Oils and Lipids: Diversity, Origin, Chemistry and Fate,Handbook of Hydrocarbon and Lipid Microbiology,https://doi.org/10.1007/978-3-319-54529-5_14-1

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1 Introduction

For the generation of oil and natural gas, the formation of organic matter-richsediments in different environments is of prime importance. The depositional set-tings determine, among others, the amount and quality of organic material, i.e., totalorganic carbon (TOC; Corg) content and kerogen types. The deposition of sedimentsrich in organic matter is taking place in terrestrial, lacustrine, and marine environ-ments in which organic matter is produced faster than it can be destroyed (Tourtelot1979). These sediments are usually fine-grained and either dominated by silicate(clays and quartz) or carbonate minerals; they usually develop under a permanentwater cover with bottom waters being commonly oxygen-depleted. An importantexception is peats, which usually develop in humid climates and in areas with limitedrun-off of surface water leading to a high water level at or above the peat surface.

Organic matter quantity and quality greatly varies even if environments favoringorganic matter accumulation are compared. For example, deltaic and fluvial sedi-ments as well as coals generally do not contain much organic matter derived fromaquatic organisms. However, they often contain tissues of higher land plants in greatquantity. This type of organic material (kerogen type III; Fig. 1) is usually lesshydrogen-rich and less oil-prone than the aquatic type. In contrast, marine andlacustrine sediments with high organic matter contents are commonly characterized

Fig. 1 Kerogen types (left) and microscopy pictures (right) showing respective organic particles(macerals). (a) van Krevelen diagram with the atomic H/C vs. O/C ratios of some organic-rich rocksand kerogens. (Modified after van Krevelen 1961), (b). Botryococcus algae (type I kerogen) underUV-light. (From Rippen et al. 2013), (c) Tasmanales algae (type I kerogen) under UV-light. (FromStock et al. 2017), (d) Carboniferous coal with V vitrinite, C cutinite, S sporinite, and I inertinite

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by a predominance of aquatic organic matter, either of planktonic or benthic origin.This aquatic organic matter is usually rich in hydrogen, contains little oxygen and isclassified as kerogen type I or II.

In the following, the depositional environments are presented, in which organicmatter-rich sediments are deposited as well as the consequences for organic mattercomposition and kerogen quality and quantity. Finally, important methods forkerogen characterization are introduced.

2 Depositional Settings

2.1 The Sea

The deposition of organic matter in modern marine sediments is very complex andstrongly influenced by factors such as bioproductivity, ocean currents, ocean floormorphology, and sediment composition. In general, sediments deposited near con-tinents and sediments from marginal seas are more enriched in organic carbon ascompared to the deep sea. This proximal-distal pattern reflects bioproductivity in theoceans, which is much stronger in continent-near areas and in marginal seas (Fig. 2)due to the higher availability of nutrients there. High bioproductivity exists, forexample, along the northwest and southwest margin of Africa, the west coast ofnorthern South America, and the northwestern coasts of the Indian Ocean (Arabia),where thus bioproductivity is enhanced by upwelling of deep, nutrient-richcold ocean waters. Sediments there are often rich in organic carbon with valuesexceeding 1%.

Fig. 2 Global annual net primary production (gC m�2a�1) from the biosphere. (Modified afterField et al. 1998; see also Huston and Wolverton 2009 for global chlorophyll data)

Formation of Organic-Rich Sediments and Sedimentary Rocks 3

However, although bioproductivity control on organic matter deposition is obvi-ous on a global scale, it can be disturbed significantly on a regional scale due to otherfactors such as sea floor topography, mineralogy, and permeability of the sediments.For the Peru sedimentary margin, for example, the areas of highest primary produc-tivity are not exactly those of highest organic carbon content (Fig. 3). In clay-richsediments or in rapidly deposited sediments, neither oxygen nor other oxidizingagents such as sulfate are transported in great quantity into the sediments thusdecelerating organic matter decay and favoring organic matter preservation. Suchconditions often occur in topographic lows (mini-basins) on the ocean floor.

Decoupling of primary bioproductivity and organic carbon content is even morepronounced in silled marginal seas with limited water exchange with the open ocean.A modern example is the Black Sea, where highest bioproductivity occurs in thenorthwest (Danube Inlet). However, due to water stratification and thus the presenceof anoxic bottom water, the central parts of the Black Sea are much more enriched inorganic carbon, in particular where clay-rich sediments occur and sedimentationrates are high (Stein 1991).

Oxygen concentration in water and at the sediment/water interface greatly influ-ences organic matter preservation and thus the geochemical properties of organicmatter in sediments. Fig. 4 shows oceanic oxygen levels at 300 m depth, i.e., withinthe upper part of the oxygen minimum zone (OMZ) of the oceans. This zone extendsbelow the photic zone in the oceans from about 200 to more than 1000 m depth(Fuenzalida et al. 2009; Paulmier and Ruiz-Pino 2009). Its dimension and intensitydepends on vertical exchange between deep and shallow water masses,bioproductivity, and water temperature. In warm water, less oxygen can be dissolvedthan in cool water. High rates of bioproductivity lead to greater oxygen consumptionby decaying biomass below the photic zone. Thus, both high bioproductivityand high water temperatures are favorable for organic matter accumulation.

Fig. 3 (a) Total primary bioproductivity and (b) sedimentary organic carbon in surface sedimentsin the upwelling area off Peru. (Redrawn from Littke 1993, after Reimers and Suess 1983)

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Therefore, it is not surprising that many “oceanic anoxic events” such as theCenomanian-Turonian Boundary Event coincide with high global temperatures(Sachse et al. 2014).

Organic matter in marine sediments contains not exclusively aquatic (planktonicand benthic) organic matter but also terrestrial organic matter to a variable extent.Terrestrial organic particles (e.g., wood, spores and pollen, charcoal) have to survivea long transport from their site of bioproduction to the site of deposition. Althoughocean currents can be rather fast, e.g., up to 2 m/s for the Gulf Stream, this can takeseveral months or years, because the density contrast between organic matter andwater is rather small and terrestrial particles at some distance from the coast areusually small. According to Stoke’s law, density difference between particle andwater, particle size and viscosity determine the settling of particles. In addition,convective processes partly driven by wind systems can promote rapid settling oforganic particles in the sea (Haake et al. 1993).

Long residence times in the sea will generally lead to a stronger physical andchemical degradation of organic matter; therefore, organic particles in central partsof the oceans are usually very rare, strongly degraded, i.e., hydrogen-poor, andsmall, whereas larger particles occur close to continents and islands (Littke 1993).Along continental slopes, fine dispersed marine organic matter and other sedimentundergo resuspension and redeposition within benthic nepheloid layers above thesea ground, causing a proportion of the organic particles to chemically alter and toage before their final sedimentation (Inthorn et al. 2006; Bao et al. 2016).

Differentiating quantitatively terrestrial-derived and autochthonous marineorganic matter is neither easy for recent nor for ancient sediments/sedimentaryrocks. One possibility is point counting, because terrigenous particles (vitrinite,

Fig. 4 Annual mean of dissolved oxygen in the oceans at 300 m depth. (Modified from NOAA2013)

Formation of Organic-Rich Sediments and Sedimentary Rocks 5

inertinite, sporinite) are usually well-visible in incident light microscopy, especiallyif sections are studied perpendicular to bedding. Other possibilities include lipidgeochemistry (Hopmans et al. 2004) and the usage of carbon/nitrogen ratios whichroughly range from 15 to 35 in higher land plants and 4 to 8 in marine lower plants,zooplankton, and bacteria (Stein 1991).

In summary, different factors govern the formation of organic matter-rich sedi-ments in marine settings including bioproductivity, organic matter type, oceancurrents, sedimentation rate, mineralogy/permeability, and presence of anoxic bot-tom water. High bioproductivity rates, presence of anoxic bottom water, high claycontents/low permeabilities, and high sedimentation rates are favorable for organicmatter deposition.

2.2 Lakes

Lakes are important settings in which organic matter-rich sediments are deposited,often with high percentages of hydrogen-rich lower plants. Deposition in lacustrinesettings follows the same principles as in marine settings but has some particularities.Organic-richness of lake deposits depends on nutrient supply and bioproductivity aswell as water circulation in the lake and stability of water stratification (see Meyersand Ishiwatari 1993). In comparison to marine settings, lakes are usually muchsmaller and thus always near to land. Therefore, deposition is strongly influencedby river inflow, lithology of surrounding rocks, and regional climate. Also, largeterrestrial organic particles can be present in large quantities in lake sediments.

With respect to the geochemical characteristics, organic matter-rich lake sedi-ments are highly diverse but tend to show an excellent preservation of primaryorganic matter as compared to marine organic matter-rich sediments. This is partlydue to the complete absence of oxygen in the bottom waters, whereas many marinesettings are characterized by oxygen-depleted but not completely anoxic conditions,e.g., due to the presence of ocean currents. Furthermore, sulfate is on average presentin higher quantity (by a factor of about 5) in sea water, leading to much more intenseorganic matter degradation by sulfate-reducing microbes. Therefore, on average,organic matter is better preserved in lake than in marine settings, with often highorganic carbon concentrations and high hydrogen over carbon ratios for lake sedi-ments in which organic matter is dominated by phytoplankton (type I kerogen inFig. 1). Another special feature is the high thermal stability of such hydrogen-richkerogen derived from lakes, which is important with respect to petroleum generation(see below).

A recent example for a large stratified lake is Lake Tanganyika, which ischaracterized by high organic carbon contents below a stratified water column(Huc 1988). In such stratified lakes, deposition of organic carbon follows the samepattern observed in the Black Sea: organic matter-rich sediments are deposited in thedeep parts of the lake, where sedimentation rates are high. Due to the fact that manylakes have a stable water stratification and anoxic bottom water, they often containvery well preserved, hydrogen-rich organic matter. Furthermore, sulfate contents are

6 R. Littke and L. Zieger

usually much lower than in marine settings leading to an even better preservation oforganic material (see chapter “▶Lipidomic Analysis of Lower Organisms”). There-fore, kerogen derived from lake sediments is often classified as type I, i.e., very richin hydrogen. Whereas this is favorable for these rocks with respect to organic matterquantity and hydrocarbon generation capacity, lateral extension is often limited. Inlakes, similar to the sea, there is also, aside from aquatic (algal) organic matter, aninput of land plant material. Its quantity is strongly controlled by the vegetationsurrounding the lakes and thus by climate.

A famous fossil example is the Eocene Messel Lake, Germany, studied in muchdetail due to well preserved fossils (early horses, primates, crocodiles, etc.). Organicmatter there consists of a mixture of phytoplankton including green algae, dinofla-gellates and terrestrial material, the latter partly as large fragments (Rullkötter et al.1988). Sediments are to a large extent extremely fine laminated, indicating that noburrowing organisms could exist in the bottom waters due to bottom water anoxia.Thus, aerobic microbial activity was absent and much organic matter was depositedand preserved (Fig. 5). Like most freshwater ecosystems, Lake Messel was sulfate-poor as compared to seawater limiting microbial sulfate reduction (Berner 1984; seechapter “▶Lipidomic Analysis of Lower Organisms”). Accordingly, organic carbonover total sulfur ratios is very high, almost reaching the values of peat and coal(Fig. 5). Under these conditions, excellent preservation of organic matter occurs,leading also to high hydrogen/carbon ratios of the organic matter (Fig. 1). In the caseof Lake Messel, oxygen/carbon ratios are also high due to the admixture of about20% terrestrial higher land plant (woody) material (Rullkötter et al. 1990).

Another type of lake is represented by the Miocene Nördlinger Ries Lake. Morethan 200 m deep and covering 400 km2, it was created by a major meteorite impact15 million years ago. The resultant lake was filled within about two million years,first by lake sediments, later also by terrestrial deposits. Because the lake wassituated between Jurassic limestones, it represents a carbonate lake with a waterchemistry vastly different from most other freshwater lakes. Iron contents, forexample, were very low and sulfate contents high leading to much higher total sulfurover organic carbon ratios as compared to the, in this respect, typical Lake Messel(Fig. 5). Due to the low iron contents, pyrite (FeS2) formation was only possible to alimited extent. Pyrite is common in almost all organic matter-rich sediments (as longas some iron is present). Globally, by far most reduced sulfur in subaquatic sedi-ments is present in pyrite (or in its orthorhombic twin marcasite). Exceptions to thisrule are iron-poor carbonate sediments such as those of the Nördlinger Ries Lake.There, extremely high organic sulfur concentrations can occur (Barakat andRullkötter 1993). There are several other lake sediments with this character, whereasmarine examples for sulfur-rich kerogen include the Miocene, carbonate-siliceousMonterey Formation, California (Baskin and Peters 1992), or the Cenomanian-Turonian carbonates of the Tarfaya Basin, Morocco (Sachse et al. 2011).

Formation of Organic-Rich Sediments and Sedimentary Rocks 7

2.3 Rivers and Peats

A major part of the terrestrial organic matter in sediments is preserved either in theform of dispersed particles in fluviatile siliciclastic rocks or in peats or coal, with thelatter showing the highest concentrations in TOC (�50%) of all organic matter-richsediments. The formation of these sediments is restricted to regions proximate to orwithin areas of intense bioproductivity in humid climates with permanent freshwaterand nutrient supply, where a large proportion of the produced biogenic material canbe preserved under wet, oxygen-depleted conditions.

Such prerequisites are given in peatlands, which are either fed solely by rainwater(ombrotrophic mires) or by a combination of precipitation, flowing water and/orgroundwater (rheotrophic mires). Ombrotrophic mires form raised, or more rarelyblanked bogs, which are characterized by acidic pH values and low siliciclasticinputs, and thus have a relatively lower nutrient supply compared to mires that formunder rheotrophic conditions (Moore 1995). Ombrotrophic peats and the resultantcoals are characterized by low ash/mineral contents. Depending on the height of theirwater table, rheotrophic mires are classified into fens, swamps, and marshes. Resul-tant peats are usually characterized by higher ash/mineral contents including highersulfur contents as compared to ombrotrophic peats. Processes such as peat growth,

Fig. 5 Organic carbon versus total sulfur plot for the marine Posidonia Shale and the lacustrineMessel and Nördlinger Ries Shales. Lines mark the total sulfur/total organic carbon (TS/TOC)ratios of modern sediments in the Black Sea and the oceans. See also Hedges and Keil (1995)

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subsidence, or eustatic sea level rise can affect the hydrological conditions in a peat-mire towards ombrotrophic or rheotrophic, respectively (Moore 1995).

Most modern peatlands are situated within the temperate climate zone and areconcentrated in the northern hemisphere, where the largest areas of peat formationare in Russia, Canada, Fennoscandia, and NW Europe. Bioproductivity is generallyhigher in tropical regions (Fig. 2) leading to thicker peat layers within tropical mires.Although tropical peats only contribute little to the total area of modern wetlands(~10%), they account for 18–25% of the global peat volume (Page et al. 2011).Different from the peatlands of the northern hemisphere, in which the vegetation isdominated by Sphagnum and herbaceous plants, recent mires in tropical regions arecharacterized by a woody, rainforest vegetation, comparable to that of many paleo-mires from the Carboniferous, Jurassic and Miocene that led to the formation of coalseams (Staub and Esterle 1994). Upon burial, peats grade into lignite, subbituminousand bituminous coal, anthracite, and finally graphite.

The type of vegetal material plays an important role in the deposition andpreservation of organic matter in sediments. While the polysaccharides celluloseand hemicellulose have a low resistance to microbial degradation, the wood formingsubstance lignin is relatively stable under anaerobic conditions and is an importantprecursor of vitrinite (Fig. 1a, d), a typical type III kerogen and abundant constituentof coal and the organic matter in most fluvio-deltaic sediments (Hatcher et al. 1982;van Krevelen 1961). Cellulose is, however, still present in lignites (Fabbri et al.2009; Stock et al. 2016). Lignin, on the other hand, can also be degraded substan-tially according to recent investigation (Waggoner et al. 2017). Other importantmacerals of similar resistance are sporinite, derived from spores and pollen ofvascular plants, cutinite derived from waxy protective layers (cuticula) of higherland plants, and inertinite, oxidized, carbon-rich particles resulting from peat fires orfungal reduction (Fig. 1d).

Sites for deposition of dispersed organic matter on continents are, apart fromlakes, lowlands flooded temporarily by rivers, but also backwaters, where conditionssimilar to those in lakes may exist. In humid climate zones, typical sites for thedeposition of organic particles are overbank deposits and crevasse splays. Interest-ingly, organic matter in such fluvial systems is usually more degraded than that inpeat, i.e., the petroleum generation capacity is much lower in fluvial sedimentaryrocks than in the adjacent coals (Jasper et al. 2009).

Coal deposits and related plant fossils reflect very well the terrestrial plantevolution. The terrestrial plant species contributing to the organic matter preservedin sediments evolved and diversified upon geologic times. First land plants appearedduring the Middle Devonian and developed to vascular plants during the earlySilurian (Edwards et al. 1983), delimitating the occurrence of sediments rich interrestrial organic matter to later dates. During the late Devonian, spore producingpteridophyte and early gymnosperm trees populated the continents, leading to anadaptive radiation of land plants and to the formation of extended tropical peat mires.During the Pennsylvanian, these mires covered large areas of present-day North

Formation of Organic-Rich Sediments and Sedimentary Rocks 9

America and Europe. Coal-bearing sequences derived from such tropical, humidenvironments can reach thicknesses of several kilometers with numerous coal seamsas well as dispersed terrigenous organic matter (Scheidt and Littke 1989). Themajority of Permian coals deposited on the former Gondwana continent at highsouthern latitudes in humid cool-temperate climates. The higher inertinite (seeFig. 1) contents in these coals compared to coals that formed during the lateCarboniferous are interpreted as indicators for seasonal changes (Taylor et al.1989). Gymnosperms dominated the peat forming vegetation during the Triassicand Jurassic and angiosperms dominate the terrestrial vegetation since Cretaceoustimes (Fig. 6). The relatively young C4 plants developed during the Oligocene(Vicentini et al. 2008; Christin et al. 2008) and expanded during Late Miocene toPliocene times (Cerling et al. 1997). C4 species like grasses and sedges make up theground cover of modern fens and marshes (Rydin and Jeglum 2013) and have a 25%share in today’s terrestrial net bioproductivity (Still et al. 2003). Because angiosper-mous lignin is more easily degraded than gymnospermous or pteridophytal lignin,(Hedges et al. 1985; Hatcher et al. 1989), vitrinite particles of coals or fluvio-deltaicdeposits that formed from these species tend to be more degraded/detrital comparedto vitrinite from Carboniferous coals.

Fig. 6 Trends in plant evolution. (Modified from Robinson 1990, after Niklas 1986)

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3 Geochemical Transformations in Young Sediments

Organic matter-rich sediments undergo significant transformation upon burialstarting in the very early phases of burial. In young and porous sediments, muchof this transformation is driven by microbial activity (Jørgensen 1982). Organicmatter is oxidized in a sequence of reactions including the reduction of O2, NO3

�,MnO2, Fe2O3, SO4

2� (Froelich et al. 1979), the sequence being determined by theGibbs Free Energy Yield under the respective redox conditions. In organic matter-rich sediments, aerobic activity usually ends several centimeters to meters below thesediment/water interface, whereas the aerobic zone can reach much deeper inorganic-lean sediments (Glud 2008; Fischer et al. 2009). Marine organic matter ismore easily degraded as compared to terrestrial organic matter (Hedges et al. 1988)leading to a diagenetic change of the terrestrial/marine ratio.

In marine water and freshly deposited marine sediments, sulfate is usually presentin high quantities (sea water has an average sulfur content of about 0.1 wt.%) and themain driver of anaerobic degradation. Almost all reduced sulfur is fixed in thesediments during this process of organic matter degradation, mainly as pyrite ororganic sulfur. This leads to a strong decrease of the TOC/TS ratio below thesedimentary surface. An example is shown in Fig. 7a (Lückge et al. 1999), illustrat-ing the change in composition of organic matter-rich sediments. Not only is reducedsulfur fixed in the sediments but also organic matter is also degraded. The degree ofdegradation can be quantified as TOC/TOCor (original TOC) with a ratio of 1 indi-cating no degradation due to sulfate reduction (Lückge et al. 1999). In the course ofthis process, nitrogen and phosphorous are lost from the sediments due to selectivedegradation of nitrogen and phosphorous rich organic matter, and the hydrocarbongeneration potential (HI, see below) as a proxy for organic matter H/C ratio is alsostrongly affected (Fig. 7b–d).

Kerogen, the insoluble organic matter in sedimentary rocks is transformed frombiomacromolecules via selective preservation of nonhydrolyzable macromolecularstructures and also via restructuring of biomacromolecules into thermally morestable moieties (Tegelaar et al. 1989). Chemical changes occurring during diagenesisbefore the onset of thermal hydrocarbon generation, i.e., at temperatures below80–100 �C include incorporation of sulfur into organic matter via natural vulcani-zation (de Leeuw and Largeau 1993), loss of oxygen from the kerogen structure, andloss of much of the organic nitrogen with transformation of a small part of it frompeptide bonds into carbazolic, pyrrolic, or pyridinic structures (Boudou et al. 2008).Changes in molecular structures of specific soluble organic compounds (biomarkers)during the stage of diagenesis have been described in much detail and can be used todecipher temperature history or maximum temperatures reached during burial(Peters et al. 2005).

Sediments rich in organic matter also undergo other changes than structural andgeochemical changes within the kerogen and bitumen. The rocks also experiencesignificant compaction and are transformed from loose sediments into sedimentaryrocks. Peats, for example, have water contents greater than 75%. When transformedinto lignites and finally hard coals, almost all the water gets lost. Mudstones also

Formation of Organic-Rich Sediments and Sedimentary Rocks 11

loose most of their porosity during burial in the first 2 km of the sedimentary column,when they are transformed into solid shales. These changes have great effects ontransport properties, which in turn are extremely important for petroleum migrationduring the stage of catagenesis, i.e., the thermal oil generation stage at temperaturesof about 100–180 �C.

4 Assessment of Organic Matter in Sedimentary Rocks

There is a variety of techniques available to study sedimentary organic matter.Elemental analysis (C, H, N, S, O) is very time consuming due to the necessity todissolve minerals with hydrochloric and hydrofluoric acid, before analyzing theresidual mixture of organic matter and sulfides. Therefore, alternative techniqueshave been developed and applied in the past.

Fig. 7 (a) Depth distribution of TOC/TS ratios in sediments deposited within the OMZ offPakistan. Ratios of TOC/TOCor versus (b) HI, (c) total nitrogen (TN), and (d) total phosphorus(TP). (Redrawn after Lückge et al. 1999)

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Optical microscopy is one of the most common techniques to characterizeorganic matter in sedimentary rocks. In organic petrography, polished sections,preferentially cut and polished perpendicular to the bedding plane, are studied athigh magnification. Organic particles are grouped into numerous macerals, but onlythree maceral groups: inertinite, vitrinite, and liptinite (see Fig. 1). Inertinite is brightunder the microscope, rich in carbon, poor in hydrogen, and derived from, e.g.,charcoal and fungi. Vitrinite is gray under the microscope, rich in oxygen, moder-ately rich in hydrogen, and derived from higher land plants, e.g., wood, bark, roots,parts of leaves. Liptinite, dark grey in reflected light, is derived from a variety ofwaxy, hydrogen-rich plants, or plant constituents such as algae, spores, pollen,cuticular layers, or resins. In most sedimentary rocks, the bulk of the organic matteris visible as particles under the microscope, but there are also sediments dominatedby submicroscopic organic matter. Reflectance of organic particles changes system-atically with burial temperature; this is in particular true for vitrinite reflectance.Therefore, this parameter has been widely applied to reconstruct burial and temper-ature histories of sedimentary rocks and to calibrate numerical models on organicmatter maturation and petroleum generation. There is in addition a large number ofother optical maturity parameters such as solid bitumen reflectance, graptolitereflectance, spore color, and conodont color (Hartkopf-Fröder et al. 2015). Inpalynological studies, rocks are pulverized and treated by hydrochloric andhydrofluoric acids; the residues are studied in transmitted light. Due to the prepara-tion process, much of the fine liptinitic material is here grouped as AOM (amorphousorganic matter), whereas terrigenous (“woody”) material can be well identified.

Pyrolysis techniques have long been used to characterize sedimentary organicmatter including coal, giving access to both the soluble (bitumen) and insoluble(kerogen) parts. As an industry standard, Rock-Eval pyrolysis has been established.In an open system, powdered rocks are heated in inert atmosphere rapidly to about300 �C to vaporize volatile “free” hydrocarbons and then at 25 �C/min to a finaltemperature of about 550 �C in order to pyrolyze kerogen. Also the generated CO2 isquantified. Important parameters include the Hydrogen Index (HI, mg hydrocarbons/g TOC), the Oxygen Index (OI, mg CO2/g TOC), and the temperature of maximumpyrolysis yield (Tmax) which reflects maximum burial temperatures, similar tovitrinite reflectance. Kerogen is commonly classified in a “pseudo van Krevelen”plot (compare Fig. 1) of HI versus OI (Fig. 8a). Rock-Eval parameters do, however,also depend on mineral matter. Presence of low quantities of pyrolyzable organicmatter can lead to retention inside the oven and thus too low HI and too high Tmax

values (Peters 1986). Rocks having sufficiently high quantities of pyrolyzableorganic matter show clear trends of increasing Tmax and decreasing HI values withmaturation (Fig. 8b, c). It should be noted that there is a great number of variouspyrolysis techniques available, e.g., open system, closed system with and withoutwater, Curie-Point pyrolysis, laser-induced micropyrolysis, pyrolysis under con-trolled pressures. In particular, pyrolysis coupled to gas chromatography can providemuch structural and chemical information on organic matter. Further information canbe derived from spectroscopic techniques such as IR, UV, and NMR.

Formation of Organic-Rich Sediments and Sedimentary Rocks 13

Bitumen, the part of sedimentary organic matter which is soluble in organicsolvents, can best be studied using chromatographic techniques, in particular gaschromatography-mass spectrometry (GC-MS). This allows very detailed character-ization on a molecular level, revealing much information on biological precursors ofsedimentary organic matter (biomarkers). The concept of this approach and appli-cations are described in detail in Peters et al. (2005).

5 Research Needs

Both kerogen and bitumen are complex mixtures of organic compounds derivedfrom organisms. Numerous studies have investigated the transformations of organicmatter in very young sediments, in which biological precursors can still be identified,

Fig. 8 (a) Pseudo van Krevelen diagram showing HI and OI values of Posidonia Shale samples(Stock et al. 2017) and sediment from within the Ruhr Basin (Jasper et al. 2009) and theirconcentrated kerogens, (b) development of HI values with increasing maturity (VRr) for PosidoniaShale and Ruhr Basin samples, (c) HI/VRr plot of coal, organic-rich sediment and clastic rocks fromthe Ruhr Basin

14 R. Littke and L. Zieger

such as amino acids, cellulose, etc. However, a complete and quantitative under-standing of transformations occurring at greater depth is still missing, e.g., withrespect to the carbon, nitrogen, sulfur, hydrogen, and oxygen cycles. In particular,microbial gas generation and its impact on kerogen quality and quantity is poorlyunderstood, although much effort has been put into studies on selected aspects.Laboratory experiments can give important insight, but their applicability to geo-logical systems is debatable.

The same holds true for quantification of oil and gas generation from kerogen as afunction of temperature. Diverse pyrolysis experiments in open or closed system,with or without water, with or without controlled pressure have been performed,each one of which giving important hints. From these experiments, kinetic param-eters on petroleum generation have been developed and are widely applied in thepetroleum industry. However, if kinetic parameters on petroleum generation arecalculated from pyrolysis experiments, vastly different results are obtained – provingthat our quantitative understanding is still poor and that extrapolation of laboratoryresults to geological long-time, low-temperature reactors is not possible. Therefore,comparison of pyrolysis experiments with natural maturation series, which are theproducts of the geological reactions, are necessary. Whereas this seems to be simple,it is complex in reality, because rocks of different thermal maturity rarely have theexact original facies, i.e., the same original organic matter quantity and quality aswell as mineralogy.

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