Henrik I. Petersen

Combustion char morphology related to combustion temperature and coal petrography

Abstract The morphology of chars sampled from various laboratory-scale reactors operating at temperatures from 800 to > 1400°C, together with chars collected directly in the flame zone in a full-scale pulverized fuel combustion experiment, was examined. A coal and coal blend dominated by vitrinite-rich microlithotypes together with four coals dominated by inertinite-rich microlithotypes were used to produce the combustion chars. Char samples produced at temperatures above ∼1300°C have a morphotype composition very similar to the composition of the full-scale char samples, whereas the morphotype compositions of those produced at ∼1150°C or lower are significantly different. Correlation between coal petrography and char morphology and determination of char reactivity should thus be attempted only using chars produced at temperatures comparable with those for the intended use of the coal. A clear distinction between the high-temperature char samples (burnout 50–60 wt% daf) emerges which is related mainly to the parent coal petrography and probably secondarily to the rank. Vitrite, clarite and vitrinertite V may be correlated with the porous tenuisphere and crassisphere morphotypes, whereas inertite, durite, vitrinertite I, duroclarite and clarodurite may be correlated with the crassinetwork-mixed-network-mixed morphotype group.

Morphology, formation and palaeo-environmental implications of naturally formed char particles in coals and carbonaceous mudstones

Abstract Combustion char morphotypes are derived from pulverised fuel combustion of coal. Microscopic examination of polished blocks of Carboniferous, Permian and Jurassic coals and carbonaceous mudstones has, however, also revealed the occurrence of particles with typical char morphology in these deposits. The particles are whiter than the associated huminite/vitrinite and should be considered to belong to the inertinite maceral group, but in both morphology and derivation they do not correspond to any of the established inertinite macerals. Thus, these morphotypes are considered to represent naturally formed char. They are generally of the dense crassinetwork/mixed network/mixed and inertoid char morphotypes. Low temperature combustion of coal, particularly in a muffle furnace, yields similar char morphotypes, hinting that the naturally formed chars were derived by low temperature burning of gelified organic matter. This may have been during surface/ground fires in peat mines. Thus, despite their minor importance volumetrically, the naturally formed chars may have palaeo-environmental implications, particularly if additional evidence is provided by pyroinertinite and/or pyrolytic carbon.

Petroleum potential of Oligocene lacustrine mudstones and coals at Dong Ho, Vietnam — an outcrop analogue to terrestrial source rocks in the greater Song Hong Basin

The outcrop of Oligocene age at Dong Ho, northern Vietnam, may constitute an immature analogue to offshore terrestrial source rocks in the greater Song Hong Basin. The outcrop includes an interval with two source rocks: (1) highly oil-prone carbonaceous mudstones containing kerogen types IIA and IIA/I, and with TOC contents from 6.48 to 16.89 wt%, and HI values from 472 to 690; and (2) oilprone humic coals (kerogen type III) with HI values from 200 to 242. The mudstones were deposited in oxygen-deficient lakes, which on occasion were subject to marine influence, and the coals accumulated in freshwater peat-forming mires. The coals have broad activation energy (Ea) distributions, while the mudstones have Ea distributions characterised by a pronounced principal Ea. During artificial maturation about 16‐17% of the organic carbon in the coals and 45‐50% of the organic carbon in the mudstones participated in petroleum formation. The two source rocks primary generate oil and secondary generate gas, however, the mudstones realised the majority of their potential over a more narrow temperature range than the coals. The excellent generative potential of the terrestrial source rocks at Dong Ho is encouraging for offshore exploration for reservoirs charged by Cenozoic rift-lake successions. q 2001 Elsevier Science Ltd. All rights reserved.

Relative sea-level changes recorded by paralic liptinite-enriched coal facies cycles, Middle Jurassic Muslingebjerg Formation, Hochstetter Forland, Northeast Greenland

Abstract The Middle Jurassic Muslingebjerg Formation of Hochstetter Forland, Northeast Greenland, consists of a succession of shallow marine sandstones encasing four coal seams formed in low-lying coastal mires. The seams are up to 3.45 m thick and contain coal facies cycles with high liptinite contents (up to 70 vol%). The cycles generally start with huminite-rich bright or banded coal and end with dull coal or clayey coal/coaly claystone. The dull coals are characterized by a high proportion of liptinite, commonly resinite, and often mineral matter. Liptodetrinite, resinite and cutinite dominate the liptinite maceral group. Similarity in the relative proportions of the liptinite macerals in the bright and dull coals and the general association of bright coal and dull coal with a high resinite concentration suggest, that the dull coals represent a residue after selective removal of ligno-cellulosic tissues in the precursor peat of primarily the bright lithotype. The clayey coal/coaly claystone lithotypes have high contents of mineral matter and commonly contain alginite and pyrite. Occurrence of pyrite in levels with high diasterane C 27 C 29 ratios and sometimes also alginite in the clayey coal/coaly claystone lithotypes, indicates a marine influence during flooding of the mires. This suggests a causal link between base-level rise in the coastal mires and relative sea-level rise. Accommodation space available for thick peat accumulations was governed by overall relative sea-level rise and the dulling-upward cycles record outpacing of peat accumulation due to accelerated rise in base level/relative sea level. The Muslingebjerg Formation consists of four depositional sequences, beginning with a thick coal seam resting on a sequence boundary. Peat accumulation reflects onset of base-level rise in the coastal area and the coals represent the lower part of the transgressive systems tract. The overlying shoreface sandstones form the upper part of the transgressive systems tract and a progradational highstand systems tract. Alternatively the two lower seams in the succession may form a transgressive-regressive couplet separated by a lagoonal sandstone split. In this case a sequence boundary occurs at the top of the upper regressive seam. The dulling-upward cycles within the coal seams represent parasequences or possibly higher order sequences. Minimum age-estimates suggest that they accumulated over 4000–11000 yr.

Organic Geochemistry in Relation to the Depositional Environments of Middle Jurassic Coal Seams, Danish Central Graben, and Implications for Hydrocarbon Generative Potential

Middle Jurassic strata from the Harald gas field in the Danish Central Graben include five coal seams. The precursor mires were situated in coastal plain environments, and peat formation occurred during water table rise related to relative sea level rise. In a sequence stratigraphic framework, the seams are situated at the transition zone between the lowstand and transgressive systems tracts or in the transgressive systems tract close to a level that can be correlated with the maximum flooding surface. Influence from relative sea level rise on peat accumulation is reflected in the petrographic and organic geochemical composition of the seams. The coals that represent peat accumulation during rapid relative sea level rise are characterized by a higher average hydrogen inde , extractability, hydrocarbon yield, and thermally extracted and generated bitumen content (S1+S2) compared to the coals that represent peat formation during the initial stages of relative sea level rise (slow rate of water table rise). In the latter coals, Pr/Ph ratios are higher, and the content of C29 steranes is higher and the content of C27 is lower. With regard to thermal maturity, the coals are in the oil window and may possess the potential to generate and release liquid hydrocarbons. This is supported by the composition of oil samples derived from sandstones in the Harald field. Multivariate data analysis shows a positive correlation between S1+S2, which is taken to indicate the generative potential of the coals, nd in particular the macerals telinite, telocollinite, and cutinite, the microlithotypes vitrite and clarite, and total organic carbon. Such a petrographic composition is favored in precursor mires characterized by continuously waterlogged, anoxic conditions. These conditions are best met in mires situated on the lower coastal plain during rapid relative sea level rise. The knowledge obtained from multivariate modeling of the data, and sedimentological and sequence stratigraphic interpretations of the coal-bearing strata, may thus make it possible to locate the coals with the highest generative potential.

Composition and organic maturity of Middle Jurassic coals, North-East Greenland : evidence for liptinite-induced suppression of huminite reflectance

Abstract Middle Jurassic coals from Kuhn O, North-East Greenland, may contain up to 85 vol.% liptinite, principally resinite (up to 68.6 vol.%). The coals are thus petrographically comparable to the Middle Jurassic Muslingebjerg Formation coals at Hochstetter Forland approximately 40–50 km to the north, and they are inherently excellent petroleum source rocks. Liptinite-poor (≤6.0 vol.%) coal samples from both Kuhn O and Hochstetter Forland yield mean random huminite reflectance values of 0.49–0.53%Ro, implying a rank of sub-bituminous A. Above a threshold value somewhere between 3 and 24 vol.% resinite and 6 and 35 vol.% total liptinite, significant reflectance suppression is induced, and between 24 and 69 vol.% resinite and 35 and 85 vol.% total liptinite, a nearly linear inverse relationship exists between the content of liptinite/resinite and the mean random reflectance values. At the highest liptinite/resinite contents, reflectance suppression may be up to 0.23%Ro. The reflectance suppression is related to bitumen-adsorption caused by bitumen-expulsion from resinite, and total reflectance distributions of the huminite fractions suggest that the reflectance of all huminite macerals is lowered with increasing liptinite/resinite content.

Synchronous Wildfire Activity Rise and Mire Deforestation at the Triassic–Jurassic Boundary

The end-Triassic mass extinction event (∼201.4 million years ago) caused major faunal and floral turnovers in both the marine and terrestrial realms. The biotic changes have been attributed to extreme greenhouse warming across the Triassic–Jurassic (T–J) boundary caused by massive release of carbon dioxide and/or methane related to extensive volcanism in the Central Atlantic Magmatic Province (CAMP), resulting in a more humid climate with increased storminess and lightning activity. Lightning strikes are considered the primary source of wildfires, producing charcoal, microscopically recognized as inertinite macerals. The presence of polycyclic aromatic hydrocarbons (PAHs) of pyrolytic origin and allochthonous charcoal in siliciclastic T–J boundary strata has suggested widespread wildfire activity at the time. We have investigated largely autochthonous coal and coaly beds across the T–J boundary in Sweden and Denmark. These beds consist of predominantly organic material from the in situ vegetation in the mires, and as the coaly beds represent a substantial period of time they are excellent environmental archives. We document a remarkable increase in inertinite content in the coal and coaly beds across the T–J boundary. We show estimated burning temperatures derived from inertinite reflectance measurements coupled with palynological data and conclude that pre-boundary late Rhaetian mire wildfires included high-temperature crown fires, whereas latest Rhaetian–Sinemurian mire wildfires were more frequent but dominated by lower temperature surface fires. Our results suggest a major change in the mire ecosystems across the T–J boundary from forested, conifer dominated mires to mires with a predominantly herbaceous and shrubby vegetation. Contrary to the overall regional vegetation for which onset of recovery commenced in the early Hettangian, the sensitive mire ecosystem remained affected during the Hettangian and did not start to recover until around the Hettangian–Sinemurian boundary. Decreasing inertinite content through the Lower Jurassic suggests that fire activity gradually resumed to considerable lower levels.

A Middle–Upper Miocene fluvial–lacustrine rift sequence in theSong Ba Rift, Vietnam: an analogue to oil-prone, small-scale continental rift basins

The small Neogene Krong Pa graben is situated within the continental Song Ba Rift, which is bounded by strike-slip faults that were reactivated as extensional faults in Middle Miocene time. The 500 m thick graben-fill shows an overall depositional development reflecting the structural evolution, which is very similar to much larger and longer-lived graben. The basal graben-fill consists of thin fluvial sandstones interbedded with well-oxygenated lacustrine siltstones in the basin centre, while very coarse-grained fluvial sandstones and conglomerates dominate at the basin margins. With increased subsidence rate and possibly a higher influx of water from the axial river systems the general water level in the graben rose and deep lakes formed. High organic preservation in the lakes prompted the formation of two excellent oil-prone lacustrine source-rock units. In the late phase of the graben development sedimentation rate outpaced the formation of accommodation space and fluvial activity increased again. During periods when the general sedimentation rate was in balance with the creation of accommodation space the environment changed frequently between lake deposition and intermittent vigorous fluvial activity. It is likely that the resulting interbedding of fluvial sandstones and lacustrine sediments reflects variations in precipitation. In periods of little precipitation the lakes diminished and lake bottoms became exposed. After heavy precipitation, transverse river systems transported sands from the rift shoulders across the exposed lake bottom and fluvial sands were deposited on lake bottom sediments. Subsequently, lake level rose due to increased water supply from the axial river and the sands were drowned and topped by transgressive lacustrine mudstones. These sandstones may function as carrier beds, whereas the braided fluvial sandstones and conglomerates along the graben margins may form reservoirs. The Krong Pa graben thus contains oil-prone lacustrine source rocks, effective conduits for generated hydrocarbons and reservoir sandstones side-sealed by the graben faults toward the footwall granites. In addition to the structural and climatic signals recorded by the graben-fill, sediment partitioning among the partly isolated basins along the rift axis seems to have been important.

Source rock evaluation of Middle Jurassic coals, northeast Greenland, by artificial maturation: aspects of petroleum generation from coal

Paralic liptinite-enriched coals and carbonaceous mudstones in northeast Greenland constitute potential highly oil-prone source rocks, whereas the humic coals may be marginal source rocks. The liptinite-rich coals are dominated by resinite or fluorescing amorphous organic matter and alginite, resulting in hydrogen index (HI) values generally above 300 and reaching up to 728. During artificial maturation up to 330 degreesC/72 hr, the coals follow the maturation paths of kerogen types I and II on an HI vs. Tmax diagram, and calculations show that upon passage through the oil window, roughly 85% of their generation potential is realized. Activation energy (Ea) distributions with prominent principal Ea values centered around 60-62 kcal/mole and frequency factors from 5.855 x 1015 s-1 to 3.249 x 1016 s-1 strongly influence the generation characteristics from 300 to 330 degreesC/72 hr artificial maturation. Important changes include marked loss of liptinite fluorescence and increase in resinite reflectance; small change in Tmax; significant decrease in HI; pronounced increase in extract yields; increased generation of saturates; and generation of labile bitumen with low Ea values. These observations indicate significant bitumen/petroleum formation from the coals during a relatively narrow temperature range, which, together with the petrographic composition, may facilitate expulsion of a waxy crude oil. The coals demonstrate that under certain depositional conditions, highly prolific coal source rocks can form with the capacity not only to generate but also to expel liquid petroleum. (Begin page 234)

Drowning of a nearshore peat-forming environment, Atane Formation (Cretaceous) at Asuk, West Greenland: sedimentology, organic petrography and geochemistry

Abstract The Cretaceous Atane Formation, Nuussuaq basin, West Greenland, is dominated by non-marine sandstones, shales, coals, and delta-front deposits. Marine incursions are frequent, however, and near Asuk, Disko, a coal seam is encased in shallow marine deposits. Notable changes in both petrography and geochemistry occur through the seam. At the base and top of the seam, the proportions of inertinite and liptinite increase at the expense of the huminite maceral group, and within all maceral groups proportions of detrital macerals increase. Geochemical changes include systematic variations in TOC, TS, n-alkane, acyclic isoprenoid, aromatic hydrocarbon, and di- and triterpenoid biomarkers, which include a number of rearranged hopanes and hopenes, and six isomers of 28,30-bisnorhopane. The variations reflect diagenetic changes related to the availability of clay, as well as changes in depositional environment going from shallow marine conditions, through fresh water mire back to open water conditions and to the eventual return of shallow marine conditions, shown by the occurrence of delta-front deposits containing Ophiomorpha nodosa trace fossils c. 50 cm above the top of the coal seam.