Lavern D. Stasiuk

Effect of maturity and petroleum expulsion on pyrrolic nitrogen compound yields and distributions in Duvernay Formation petroleum source rocks in central Alberta, Canada

Abstract A quantitative study of pyrrolic nitrogen compounds was conducted on a series of marine carbonate petroleum source rock extracts taken from the Upper Devonian Duvernay Formation in the central Alberta portion of the Western Canada Sedimentary Basin. With increasing thermal maturity, concentrations of various pyrrolic nitrogen compounds in the rock extracts increase drastically, together with significant compositional variations related to alkyl substitution position. The study provides circumstantial evidence for adsorptive interactions operating between organic nitrogen compounds and solid organic/mineral phases in subsurface sedimentary rocks during petroleum generation and expulsion. The absolute concentrations and relative distributions of pyrrolic nitrogen compounds in the extracts of different maturity ranges provide background information for the calibration of such data in migrated petroleums, as an independent measurement of secondary oil migration range of Duvernaysourced oils in the basin

The origin of pyrobitumens in upper Devonian Leduc formation gas reservoirs, Alberta, Canada: an optical and EDS study of oil to gas transformation

Pyrobitumens are common within deep basin, Devonian carbonate gas reservoirs of the Western Canada Sedimentary Basin. Incident light microscopy and SEM-EDS have been used to study the origin of pyrobitumens from Upper Devonian isolated reefs of the Leduc Formation, west central Alberta. Reflectance properties (%Romax and %Romin), coke microtextures and relative S/C ratios are used to classify the pyrobitumens. In portions of reservoir units where NSO-enriched, low aromaticity crudes may have been dominant, oil to gas transformations produced mainly low-reflecting isotropic to fine-grained mosaic coke microtextures within the reservoir pyrobitumens (Type A). In contrast, where ‘normal’ crudes were most likely dominant in the reservoir units prior to thermal cracking, pyrobitumens with high bireflectance and very anisotropic coarse flow to domain cokes dominate (Type B). Oil to gas cracking within some of the reservoir units resulted in overpressuring and is represented by highly deformed pyrobitumens with fibrous coke textures and high bireflectance. Relative concentrations of Type A and B pyrobitumens vary vertically and temporally in some of the reefal buildups. The variations in the dominant pyrobitumen types are attributed to crude oil alteration which preceded crude oil to gas transformation (e.g. tar mats) in some zones whereas other zones may have been altered contemporaneously with crude oil cracking. The later may have resulted from the oxidation and sulphur enrichment of the crude oil during thermochemical sulphate reduction. This reaction is prevalent within Devonian evaporite-carbonate strata of the Western Canada Sedimentary Basin, generating copious volumes of H2S. The interaction between crude oil and sulphur-enriched, highly alkaline fluids is suggested by the presence of high S/C, very fine-grained, late stage calcined needle cokes within brecciated pyrobitumens.

Geochemical constraints on models for secondary petroleum migration along the Upper Devonian Rimbey-Meadowbrook reef trend in central Alberta, Canada

Abstract The Upper Devonian Rimbey-Meadowbrook reef trend of central Alberta, Canada, is one of the classical examples that was originally used to support Gussows model of differential hydrocarbon entrapment. However, the clear discrepancies existing between the ideal hydrocarbon distribution predicted by this model and the one actually observed led to several alternative geological models, most of which have not been constrained by organic geochemical data. The present study uses a wide range of bulk and molecular parameters to determine the oil source characteristics and thermal maturity along the reef trend. The concentrations and isomeric distributions of alkylcarbazoles and non-alkylated benzocarbazoles in oils are employed to study petroleum migration pathways and to constrain the existing secondary petroleum migration models. The data indicates that Gussows principle is generally applicable, as the oils in the up-dip direction generally have longer implied migration distances although this is not always the case. Other factors not recognized previously, such as the presence of two subfamilies of the Duvernay Formation sourced oils, may also have contributed significantly to the discrepancies in the oil and gas distributions between the model predictions and the actual observational data.

Geochemical characterisation of Middle Devonian oils in NW Alberta, Canada: possible source and maturity effect on pyrrolic nitrogen compounds

Abstract Molecular geochemical compositions of a suite of Middle Devonian sourced and reservoired oils from the Rainbow–Zama–Shekilie sub-basins in N.W. Alberta, Canada, reveal the presence of at least two oil families in the study area. The distribution of each oil family is geographically restricted to a single sub-basin, consistent with the oils being sourced locally within a series of closed generation/migration/trapping systems. The diversity in the biomarker distributions of the oils indicates the wide range of depositional environments and source materials existing in each sub-basin, rather than mixing of end member oils across different sub-basins. Clear maturity differences are observed between the oils from the Rainbow and Zama subbasins. Pre-Cretaceous thermal anomalies along the reactivated regional Precambrian basement faults are proposed as one of the major causes for the relatively high maturity levels for the Middle Devonian source rocks in the study area. We infer that depositional environment and thermal maturity have had a strong impact on the geochemical characteristics of the saturated and aromatic hydrocarbons in the Rainbow–Shekilie–Zama oils, but may not influence the pyrrolic nitrogen compounds to a significant extent. However, recognition of possible source and maturity effects on pyrrolic nitrogen compounds in other studies suggests that these factors should be considered before the pyrrolic nitrogen compounds are used to characterize petroleum migration.

Organic Facies in Devonian and Mississippian Strata of Western Canada Sedimentary Basin: Relation to Kerogen Type, Paleoenvironment, and Paleogeography

Abstract Petrographic analyses of dispersed organic matter (including macerals and palynomorphs), siliceous and calcareous microfossil assemblages and microtextures (e.g. stromatolitic) have been used to define and interpret five organic facies and regionally map their distribution for the following informal groupings of potential hydrocarbon source rocks in the Western Canada Sedimentary Basin: Upper Devonian Woodbend group, Upper Devonian Winterburn group and Upper Devonian to Lower Mississippian black shales of the Exshaw and Bakken formations. Five petrographic organic facies (A-E) are defined for the potential source rocks based on assemblages of alginites, acritarchs, sporinites, siliceous microfossils and algal mat microtextures. Organic facies A, B (prasinophyte alginites and acritarchs) and C (coccoidal alginite), represent accumulation in relatively deep (basin), intermediate (shelf-platform), and shallow water depths (bank-reef margin to lagoonal). Organic facies D is defined by siliceous microfossils (e.g. Radiolaria) and accumulated in deep basinal to outer shelf settings immediately east of an ancient Pacific Ocean, or south of an ancient Arctic Ocean. This facies may reflect regions of upwelling which extended into intracratonic and epicontinental settings. Organic facies E, characterized by stromatolitic microtextures with or without coccoidal alginite, only occur within Upper Devonian Winterburn Group shallow water, restricted shelf to lagoonal dolostones associated with evaporites. As a whole, the regional distribution of organic facies is related to paleogeography, paleobathymetry or paleostructure in the source rocks. Surprisingly, petrographic organic facies do not show strong positive correlation with kerogen type as defined by Hydrogen-Oxygen indices or TOC-S2 plots.

Revised models for hydrocarbon generation, migration and accumulation in Jurassic coal measures of the Turpan basin, NW China

Abstract Whether or not the Lower-Middle Jurassic coal measures in the Turpan basin of NW China have generated commercial quantities of liquid petroleums is a problem of considerable importance that remains contentious as it has not yet been resolved unequivocally. This study provides evidence against the Jurassic humic coals as the only major source for the oils discovered in the Taibei depression of this basin and suggests additional significant contributions from the Upper Permian and Middle–Lower Jurassic lacustrine source rocks. The Carboniferous–Permian marine source rocks may have been important also in limited locations along the major basement faults. Molecular and petrographic data indicate that the majority of the Middle Jurassic strata are currently immature or marginally mature with respect to hydrocarbon generation. Within the major depocenters, the Middle–Lower Jurassic coal-bearing strata of the Baodaowan and Xishanyao formations has reached the conventional oil window (i.e. with vitrinite reflectance >0.7% Ro). Pre-Jurassic (Upper Permian in particular) derived hydrocarbons appear to be widespread in extracts of fractured Jurassic coal and fine-grained rocks. Large differences have been observed in the absolute concentrations of biomarker compounds in rock extracts of various source intervals. Thus, “coaly” biomarker signatures of the oils most likely resulted from mixing and migration contamination when hydrocarbons derived from mature source rocks migrated up through highly fractured coal seams along deep-seated faults. In addition to conventional exploration targets, revised petroleum generation and accumulation models predict that the focus in the Turpan basin should also include deep structures within the Carboniferous–Permian strata and subtle, low magnitude anticlines and stratigraphic traps within the Triassic–Jurassic sections. Crown copyright

Geochemical characterisation of organic matter in Keg River Formation (Elk point group, Middle Devonian), La Crete Basin, Western Canada

Carbonates from the Keg River Formation, La Crete Basin, Alberta, western Canada were examined in order to define: (a) oil source rock potential; (b) bulk maceral composition; (c) extract yield; and finally (d) facies variations using PY-GC-MS. Thirty samples from 6 different wells were examined from the lower Keg River member and 4 from the upper Keg River member using conventional geochemical methods. As maturity differences are absent within the sample set, variations in TOC, Tmax, hydrogen index, organic petrography and extract yields are caused by variability in organic matter input, which is revealed by molecular characterisation using PY-GC-MS. Lower Keg River member bituminous wackestones are excellent potential source rocks containing Types I–II and Type II organic matter. Types I–II organic matter contains large well preserved (up to 200μm in diameter) thick-walled Tasmanites (10–15% of sample) and akinete algal cells indicative of algal blooms within an amorphous bituminite. Type II organic matter contains a higher proportion of degraded alginites/bituminite relative to well-preserved alginites. Extract yields (mg/g TOC) were seen to increase from Types I–II to Type II organic matter. PY-GC-MS revealed that 1,2,3,4-tetramethylbenzene was a major peak in most samples. This is a pyrolysis product arising from β-cleavage of C40 diaromatic carotenoids incorporated within the kerogen during diagenesis. The source of this compound is thought to be from an unknown diaromatic compound with a 2,3,6-/3,4,5-trimethyl substitution pattern and isorenieratene, which is specific to photosynthetic green sulphur bacteria (Chlorobiaceae) suggesting that the photic zone was at least partially anoxic during deposition of these samples. The relative abundance of this compound/n-C11-alkene and organic sulphur (calculated from the thiophene ratio) both increase from Types I–II to Type II organic matter. This trend was grossly similar to the trend seen in the variability of extract yield with hydrogen index. A similar trend for HI and Tmax indicates samples containing a higher proportion of degraded alginites/bituminite relative to well-preserved alginite are more labile than Type I–II organic matter. Upper Keg River member marls contain Type II organic matter, which is characterised by heavily degraded algal material within a bituminous groundmass. Pyrolysates of two of the marl samples contain only low amounts of 1,2,3,4-tetramethylbenzene, in contrast to the bituminous wackestones, indicating that the depositional environment/source input was different during deposition of the marl samples. Although both marls contain similar organic matter, their pyrolysates were significantly different. One marl (1141.9 m) was highly paraffinic containing dominantly short-chain alkene/alkane doublets, while the other marl (1137.6 m) contained a bimodal n-alkane/alkene distribution and high amounts of alkylphenols, which may be derived from preservation of resistant algal polyphenolic molecules or suggest a terrestrial input.

Confocal laser scanning fluorescence microscopy of Botryococcus alginite from boghead oil shale, Boltysk, Ukraine : selective preservation of various micro-algal components

Abstract Confocal laser scanning fluorescence microscopy (LSM) has exceptional potential for resolving micrometer scale morphological details within fluorescing macerals (e.g. alginite) of hydrocarbon source rocks and oil shales. This investigation of well preserved Botryococcus alginites from a Paleogene boghead oil shale from central Ukraine clearly illustrates that LSM can effectively resolve microalgal cellular organization. A dominance of highly resistant, outermost cell walls in Botryococcus confirms that selective preservation was an effective process during kerogen formation. Three dimensional serial section compilations of images taken through Botryococcus alginites reveals a number of cellular features including: (i) micro-layering within resistant outer cell walls; (ii) preservation of resistant, very thin, outer walls of apical cells and; (iii) stacks of successive thimble-shaped layers and funnel-shaped cups which comprise the resistant stalk framework of fossilized compound colonies. LSM also discloses preservation of pairs of ‘reproducing’ Botryococcus cells consisting of outer resistant walls enclosing mainly unstructured, granular organic matter in the ‘cell contents region’. Some serial images from these areas do, however, show evidence for cellular organization and possible selective preservation of possible resistant biomacromolecules derived from aplanosphores or zoospores.

Reflected light microscopy and micro-FTIR of upper Ordovician Gloeocapsomorpha prisca alginite in relation to paleoenvironment and petroleum generation, Saskatchewan, Canada

Abstract Gloeocapsomorpha prisca alginites from Upper Ordovician Type I kerogen in hydrocarbon source rocks (kukersites), Yeoman Formation, Saskatchewan, Canada, have been studied using incident light microscopy and transmission micro-Fourier transform infrared spectroscopy. Gloeocapsomorpha prisca coccoidal alginite are classified as three maceral varieties: (i) small agglomerations of thin-walled disseminated A, (ii) larger agglomerations of thick-walled disseminated B; and (iii) a stromatolitic variety. Each alginite maceral variety is interpreted as a stage in the life cycle of the algal precursor resulting from progressive normal growth controlled by chemical and physical changes in the paleowater column. Thickened cell walls of disseminated B and stromatolitic G. prisca are characterized by strong i.r. absorbances in the OH region and a higher aromatic C—H absorption compared to the early growth stage of the disseminated A alginite at the same level of thermal maturity. This characteristic is attributed to the presence of highly aliphatic and resistant outer cell wall biopolymers formed by the algal precursors in response to a variation in either oxygen supply or fluctuating salinity levels within the paleoenvironment and is outlined in a proposed paleoenvironmental model. The rate of reflectance increase in oil and the degree of fluorescence red shift (lambda max) with increasing depth of burial and thermal maturation is greater for G. prisca disseminated varieties compared to the stromatolitic maceral variety. Aromatic C—H absorption (3050 cm −1 ) per unit area for disseminated A and B alginite increases at approximately the same rate with increasing thermal maturity. In contrast the aliphatic C absorption bands (2920 and 2850 cm −1 ) decrease with increasing maturity in G. prisca disseminated A alginite whereas aliphatic absorption per unit area increases in disseminated B alginites with increasing thermal maturation. The aliphatic absorption bands also increase with increasing thermal maturation for stromatolitic G. prisca , but in a pattern unlike disseminated B. A hydrocarbon generation model for G. prisca alginite proposes that the molecular structure of the disseminated B variety undergoes a transformation from an alkyl long chain structure into a more cyclic bitumen-like maceral prior to peak generation. In contrast, the smaller, thin-walled, disseminated A alginite,representing the early growth stage, probably generates free bitumen from diagenesis through to catagenesis. This is supported by a reduction in aliphatics with increasing thermal maturity and by the formation of oily-bitumen that evolves from the alginite during petrographic analysis. The stromatolitic G. prisca alginite remains morphologically very rigid throughout the diagenetic range of thermal maturities.

A comparison of temperate and boreal peats from Ontario, Canada : possible modern analogues for Permian coals

Abstract Peat profiles were sampled from a raised bog and an interior fen located in the boreal James Bay Lowlands, Northern Ontario, and a cold-temperate swamp and a cold-temperate bog in the Great Lakes–St. Lawrence Lowlands, southern Ontario. An examination of the micromorphology and maceral compositions of the peats indicates that considerable differences exist between environments. The boreal bog and fen both consist predominantly of relatively unaltered textinite (commonly Sphagnum-derived), with higher liptinite contents in the fen profile attributable to the more diverse vegetation. Both the cold-temperate swamp and bog show high levels of humification at the surface, possibly partially attributable to recent climatic warming trends causing a decline in water table elevation. In all profiles, funginite was persistent, ranging from 2 to 10% of the total maceral content, indicating that aerobic conditions generally prevailed at the surface at the time of deposition. The influence of depositional environment and vegetation type on the resulting peat was born out in plots of Tissue Preservation Index vs. Gelification Index. The petrographic study also demonstrated that none of the profiles contained high inertodetrinite/semifusinite contents comparable to those within cold-climate cratonic Permian Gondwana coals. Since the oxidation processes leading to the formation of inertinite are considered to take place at the peat stage, if the peats studied were to contain a similar maceral assemblage upon coalification, inertinite precursors may be present in the peat. The lack of petrographic similarity between the Holocene peats and the Permian coals may be related to a number of factors. The decrease in water table elevation possibly responsible for the inertinization of the Permian coals may have occurred following the cessation of peat formation, not concurrently with accumulation, and is potentially attributable to long-term factors such as climatic warming. The change in the peat-forming vegetation (from Permian Glossopteris- and Gangamopteris-dominated communities to Holocene Sphagnum and coniferous vegetation) alters the characteristics of the initial botanical starting materials, and some aspects of the ecology, particularly trophic and pH conditions, which control the post-depositional alteration of the plant materials and the resulting maceral assemblages.