The stratigraphic distribution of inertinite

The paper presents the results of an investigation into the stratigraphic distribution of inertinite in humic coals, ranging from the Silurian to the Neogene periods. This range equals the time span during which both land plants and atmospheric oxygen evolved. The survey is based on a literature study of inertinite in coals covering a variety of depositional environments and climates in all continents. The inertinite percentages listed in this paper have been arranged in stratigraphic order. They are the mean values based on mineral-free, whole coal compilations. Maceral compositions listed in the literature for individual subsections have been averaged to represent complete coal seams/beds, although this might not have been always possible due to some ambiguity in the published record. Moreover, not all of the published and unpublished information could be used because of poor sample description, obscure geographic origin, or insufficient stratigraphic information. The results show that there have been times in the geological past, e.g. during the Late Permian, in which coals were particularly rich in inertinite compared with other times, e.g. during much of the Late Tertiary, in which many coals contained very little inertinite. Occasionally, such primary, global trends were modulated by a diffuse distribution of inertinite percentages that correspond to differences in climate, depositional settings, e.g. active orogenic margins versus tectonically inactive shelf margins, and other local or regional variations. Although various mechanisms for the generation of inertinite have been proposed in the past, the overall pattern in its stratigraphic distribution supports the notion of incomplete combustion as the main source of inertinite in coal. This points to secular variations in the atmospheric oxygen content as an explanation for the global changes in the inertinite distribution, while differential subsidence and climatically influenced reaction rates governed regional and local variations in inertinite percentages

Utility of coal petrology for sequence-stratigraphic analysis

Sequence stratigraphy seeks to predict the depositional architecture of sediments based on the principle that their superposition and lateral arrangement are largely determined by the rate at which accommodation is made below depositional base level relative to the supply rate of sediments. Depositional base level is generally correlated with sea level which in paralic peat/coal deposits controls the position of the groundwater table above which oxidation, ablation and erosion would occur. The current Late Quaternary sequence is used to discuss some of the constraints on coal formation that stem from the interaction of short- and long-period modulations on sequence-forming eustatic cycles. The results show that the mid-transgressive systems tract is unlikely to produce significant coal deposits, which is supported by a survey of the relevant literature from eight different countries. It reveals a bimodal distribution of coal deposits with the strongest mode being situated in the early transgressive systems tract, while a broader, secondary mode occurs in the early and mid-highstand position. This concentration contrasts with the complete absence of any significant coal deposits in the early and mid-lowstand systems tract, and only a low occupancy of the late highstand systems tracts. The reasons for this distribution are briefly discussed, and attention is drawn to the possible over-stating of the coal-forming potential of the late lowstand systems tract. Following a brief discussion of the sequence-stratigraphic interpretation of coal measures, emphasis is put on the application of sequence stratigraphy to the study of coal seams and their composition at various levels of petrographic detail. While in low-rank coals, botanical affinities contribute much to the lithotype-based analysis, a genetic assessment of bituminous coal seams relies primarily on physical and chemical palaeo-environmental signatures. Simple properties, such as maximum coal thickness, low detrital mineral content and maximum preservation of vitrinite, particularly of telovitrinite, define a good balance between the former rates of accommodation versus peat accumulation. Iso-metamorphic variations in telovitrinite reflectance and its dispersion measured by the coefficient of variation have also contributed to the palaeo-environmental interpretation. This paper confirms the identification within coal seams of most of the sequence-stratigraphically significant surfaces, e.g. sequence and parasequence boundaries, transgressive, flooding, and maximum flooding surfaces. It also gives examples of some new ones, referred to as accommodation reversal, non-marine flooding, give-up transgressive, terrestrialisation, and paludification surfaces. Some of these are specific to coal measures, but the give-up transgressive and accommodation reversal surfaces may have wider significance

Accommodation-based coal cycles and significant surface correlation of low-accommodation lower cretaceous coal seams, Lloydminster heavy oil field, Alberta, Canada: implications for coal quality distribution

Sequence stratigraphy and coal cycles based on accommodation trends were investigated in the coal-bearing Lower Cretaceous Mannville Group in the Lloydminster heavy oil field, eastern Alberta. The study area is in a low accommodation setting on the cratonic margin of the Western Canada sedimentary basin. Geophysical log correlation of coal seams, shoreface facies, and the identification of incised valleys has produced a sequence-stratigraphic framework for petrographic data from 3 cored and 115 geophysical-logged wells. Maceral analysis, telovitrinite reflectance, and fluorescence measurements were taken from a total of 206 samples. Three terrestrial depositional environments were interpreted from the petrographic data: ombrotrophic mire coal, limnotelmatic mire coal, and carbonaceous shale horizons. Accommodation-based coal (wetting- and drying-upward) cycles represent trends in depositional environment shifts, and these cycles were used to investigate the development and preservation of the coal seams across the study area. The low-accommodation strata are characterized by a high-frequency occurrence of significant surfaces, coal seam splitting, paleosol, and incised-valley development. Three sequence boundary unconformities are identified in only 20 m (66 ft) of strata. Coal cycle correlations illustrate that each coal seam in this study area was not produced by a single peat-accumulation episode but as an amalgamation of a series of depositional events. Complex relations between the Cummings and Lloydminster coal seams are caused by the lateral fragmentation of strata resulting from the removal of sediment by subaerial erosion or periods of nondeposition. Syndepositional faulting of the underlying basement rock changed local accommodation space and increased the complexity of the coal cycle development. This study represents a low-accommodation example from a spectrum of stratigraphic studies that have been used to establish a terrestrial sequence-stratigraphic model. The frequency of changes in coal seam quality is an important control on methane distribution within coalbed methane reservoirs and resource calculations in coal mining. A depositional model based on the coal cycle correlations, as shown by this study, can provide coal quality prediction for coalbed methane exploration, reservoir completions, and coal mining

Comparison of two petrographic methods for determining the degree of anomalous vitrinite reflectance

Three Australian Permian bituminous coals from well-documented geological settings chosen to cover a range of rank, and orthohydrous, subhydrous and perhydrous vitrinite compositions, were used to compare and contrast two petrographic methods for correcting anomalies in vitrinite reflectance (VR) due to compositional variation. One of the methods (fluorescence alteration of multiple macerals (FAMM™)) is based on the fluorescence alteration properties of coal macerals, and the other on paired measurements of reflectance and fluorescence intensity. The methods are only applicable to fresh (unoxidised) coals or dispersed organic matter (DOM). Both methods use a reference 'normal' orthohydrous telovitrinite line on which results from perhydrous or subhydrous samples are projected to obtain the equivalent reflectance of orthohydrous telovitrinite. The results of the two methods on the test samples are similar but they differ in detail