Abstract
This paper presents a new schematic model for generation and timing of multiple phases of solid bitumen throughout the continuum of organic matter maturation in source and tight reservoir ...rocks. Five distinct stages in the evolution of solid bitumen are proposed: (1)
diagenetic solid bitumen
(or
degraded bituminite
), which is not a secondary maceral resulting from the thermal cracking of kerogen. Instead it is derived from degradation of
bituminite
in the diagenesis stage (Ro < 0.5%); (2)
initial-oil solid bitumen
, is a consolidated form of early catagenetically generated bitumen at the incipient oil window (Ro ~ 0.5–0.7%); (3)
primary-oil solid bitumen
is derived from thermally generated bitumen and crude oil in the primary oil window (Ro ~ 0.7–1.0%); (4)
late-oil solid bitumen
(solid-wax) is derived from the waxy bitumen separated from the mature paraffinic heavy oil in the primary- and late-oil windows; and (5)
pyrobitumen
, which is mainly a non-generative solid bitumen, is evolved from thermal cracking of the remaining hydrocarbon residue and other types of solid bitumen in the dry gas window and higher temperature (Ro > 1.4%). This model shows concurrence of multi-populations solid bitumen with oil, bitumen, and other phases of fluid hydrocarbon residue during most of the maturity continuum.
Tight-gas and shale-gas systems can undergo significant depressurization during basin uplift and erosion of overburden due primarily to the natural leakage of hydrocarbon fluids. To date, geologic ...factors governing hydrocarbon leakage from such systems are poorly documented and understood. Here we show, in a study of produced natural gas from 1,907 petroleum wells drilled into a Triassic tight-gas system in western Canada, that hydrocarbon fluid loss is focused along distinct curvilinear pathways controlled by stratigraphic trends with superior matrix permeability and likely also structural trends with enhanced fracture permeability. Natural gas along these pathways is preferentially enriched in methane because of selective secondary migration and phase separation processes. The leakage and secondary migration of thermogenic methane to surficial strata is part of an ongoing carbon cycle in which organic carbon in the deep sedimentary basin transforms into methane, and ultimately reaches the near-surface groundwater and atmosphere.
A 6.7million year record of marine sedimentation through the Permo Triassic Biotic Crisis shows notable shifts in mercury concentrations that varied over an order of magnitude (0.002 to 0.116μg/g). ...Mercury concentrations correlate strongly with total organic carbon, showing both parallel increasing and decreasing trends over geologic time scales. Results suggest that changes in bioproductivity strongly influence mercury sequestration into marine sediment. This may provide insight to potential marine response to modern anthropogenic Hg loading, and supports models that suggest enhanced bioproductivity that due to recent climate has increased Hg sequestration over time.
•The Permo–Triassic Biotic Crisis has significant fluctuations in mercury.•Hg strongly correlates with total organic carbon.•A natural buffering system limits impact of transient Hg deposition events.•Extreme volcanic events can overload marine systems.
Unconventional petroleum systems go through multiple episodes of internal hydrocarbon migration in response to evolving temperature and pressure conditions during burial and uplift. Migrated fluid ...signatures can be recognized using stable carbon isotope and PVT compositional data from produced samples representative of in-situ petroleum fluids. Such samples, however, are seldom collected due to operational complexity and high cost. Here, we use carbon isotope and PVT data from co-produced hydrocarbon gas and liquid to provide evidence for widespread migration of gas-condensate in the Montney unconventional petroleum system of western Canada. Extended C
-C
isotopic profiles exhibit convex upward signatures with C
-C
maxima at low molecular weight, and increasing or nearly uniform signatures at high molecular weight. Additionally, recombination PVT compositional data show C
-C
condensate concentrations are higher than expected for unmodified oils. The combined convex upward and increasing or uniform isotopic signatures are interpreted as mixing profiles formed by the introduction of high-maturity gas-condensate (C
-C
) to shallower zones with in-situ hydrocarbon fluids of lower thermal maturity. The recognition of widespread gas-condensate migration adds to the complex history of internal hydrocarbon migration within the Montney tight-petroleum system including previously identified migration episodes of early oil and late-stage methane-rich gas.
In this study of the Triassic Montney tight gas siltstone play in the Western Canadian Sedimentary Basin petrophysical measurements of drill-core samples (porosity, pore throat size, water saturation ...and grain size) are integrated with Rock-Eval TOC data, organic petrography observations and SEM imaging to show that reservoir quality in the gas window is strongly influenced by the pervasive presence of pore-occluding solid bitumen (and pyrobitumen at higher thermal maturity). The solid bitumen formed as a pore-filling liquid oil phase that was diagenetically and thermally degraded with further burial and increase in temperature. The proportion of solid bitumen filling the intergranular paleopore network can be expressed as bitumen saturation, and this attribute is found to be the dominant control on pore throat size and absolute permeability. The samples with low bitumen saturation and large pore throat radius (>0.01μm) have water saturations that generally increase as pore throat size diminishes, a relationship consistent with capillary theory for conventional water wet conditions. The samples with high bitumen saturation and small pore throat radius (<0.01μm), on the other hand, have abnormally low water saturation, a condition inconsistent with capillary theory for conventional water wet rocks. The coincidence of small pore throat size, low water saturation and high bitumen saturation is attributed to the presence of well-connected nanopores within the devolatilized, solid bitumen and the hydrophobic nature of the bitumen. Siltstones in economic portions of the Montney tight gas fairway have porosity mostly in the range of 3 to 7%. The results of this study show that reservoir quality in this economically key porosity range is influenced more strongly by bitumen saturation than by conventional determinants of porosity and permeability such as grain size, sorting, clay content and cementation. The concept of pore-occluding solid bitumen as an important negative control of reservoir quality elucidated here for Montney siltstones likely has application to the technical and economic evaluation of other tight gas plays particularly those in indirect basin-centered gas accumulations.
•The proportion of solid bitumen filling the paleo-pore network can be expressed as bitumen saturation.•Bitumen saturation is the dominant control on pore throat size distribution.•Solid bitumen formed as a migrated oil phase that was thermally degraded with further burial.•Solid bitumen can have a major control on reservoir quality of tight gas reservoirs.
•Explore the effect of thermal maturity Mo enrichment and speciation in black shale.•Assess the role of thermal sulfate reduction (TSR) during Mo remobilization.•Identify two Mo species: i) ...molybdenite, and ii) Mo(VI)-OM in overmature samples.•Discuss the consequences for the use of Mo as the paleo-redox proxy in black shale.
Molybdenum (Mo) concentrations in sedimentary records have been widely used as a method to assess paleo-redox conditions prevailing in the ancient oceans. However, the potential effects of post-depositional processes, such as thermal maturity and burial diagenesis, on Mo concentrations in organic-rich shales have not been addressed, compromising its use as a redox proxy. This study investigates the distribution and speciation of Mo at various thermal maturities in the Upper Ordovician Utica Shale from southern Quebec, Canada. Samples display maturities ranging from the peak oil window (VRo∼1%) to the dry gas zone (VRo∼2%). While our data show a significant correlation between total organic carbon (TOC) and Mo (R2=0.40, n=28, P<0.0003) at lower thermal maturity, this correlation gradually deteriorates with increasing thermal maturity. Intervals within the thermally overmature section of the Utica Shale that contain elevated Mo levels (20–81 ppm) show petrographic and sulfur isotopic evidence of thermochemical sulfate reduction (TSR) along with formation of recrystallized pyrite.
X-ray Absorption Fine Structure spectroscopy (XAFS) was used to determine Mo speciation in samples from intervals with elevated Mo contents (>30 ppm). Our results show the presence of two Mo species: molybdenite Mo(IV)S2 (39±5%) and Mo(VI)-Organic Matter (61±5%). This new evidence suggests that at higher thermal maturities, TSR causes sulfate reduction coupled with oxidation of organic matter (OM). This process is associated with H2S generation and pyrite formation and recrystallization. This in turn leads to the remobilization of Mo and co-precipitation of molybdenite with TSR-derived carbonates in the porous intervals. This could lead to alteration of the initial sedimentary signature of Mo in the affected intervals, hence challenging its use as a paleo-redox proxy in overmature black shales.
The end-Triassic mass extinction (ETME) is thought to have been caused by voluminous, pulsed volcanic activity of the Central Atlantic Magmatic Province (CAMP). Over the last decades, various ...geochemical signals and proxy records, including δ13C, pCO2, iridium and other platinum-group elements, mercury, polycyclic aromatic hydrocarbons (PAH), charcoal and SO2, have been directly or indirectly attributed to CAMP magmatism. Here, we compile these various records in a stratigraphic framework to present a cohesive chain of events for the CAMP and the end-Triassic mass extinction. Mercury and iridium anomalies in sediments indicate that CAMP activity commenced prior to the onset of the marine extinctions (as marked by the last occurrence of the Triassic ammonoid Choristoceras marshi or closely related species), and a negative δ13C excursion in organic matter (the Marshi CIE). This CIE may be explained by input of light carbon to the atmosphere from CAMP lavas of the Tiourjdal and Prevalent groups. Pedogenic carbonate below and above the Prevalent group in North America indicates a more than twofold increase in atmospheric pCO2. Subsequent n-alkane C-isotopes, and stomatal pCO2 data seem to indicate a temporary cooling after the Marshi CIE, which is consistent with climate models incorporating volcanic emissions of both CO2 and SO2. Records of excess iridium and Hg/TOC indicate intensified magmatism during the extinction interval. Tectonic and perhaps epeirogenic (i.e. doming due to rise of magma) activity is suggested by the occurrence of multiple and widespread seismites in Europe. Atmospheric pCO2 proxies indicate global warming, which culminated contemporaneously with the Spelae CIE. Global warming is corroborated by increased wildfire activity testified by charcoal and pyrolytic PAH records. Increased isotopic ratios of Os and Sr from sections that record global ocean signatures suggest increased weathering of continental crust likely due to climatic changes. Just prior to the increase in pCO2 from stomatal proxy data, fossil plants exhibit SO2-induced damage indicating excess sulfur dioxide deposition priot to and across the Triassic–Jurassic boundary. At the same time, increased ratios of heavy molecular PAHs (coronene/benzo(a)pyrene) in sediments suggest metamorphism of organic sediments also occurred across the Triassic–Jurassic boundary. These proxies may suggest that thermogenic release of light carbon and sulfur from sill intrusions in the Trans-Amazonian basins, where both evaporate- and organic-rich sediments are known to have been intruded, may have played an important role during the course of the ETME. Geochemical traces of magmatism, i.e. Ir and Hg, appear to have gradually disappeared during the Hettangian, suggesting that later phases of CAMP were less voluminous. Stomatal proxy data from Greenland and n-alkane C-isotope data from the UK, together with oxygen isotope data from carbonate fossils in the UK, may indicate that the global warming at the Spelae CIE was succeeded by another short-term cooling event. A gradual decrease in δ13C culminated at the top-Tilmanni CIE, marking the beginning of a long-term steady state with more negative C-isotope values than prior to the ETME. At this time, terrestrial ecosystems appear to have stabilized globally and ammonoids had begun to rediversify.
•The CAMP volcanism left geochemical traces in the sedimentary record.•Hg- and Ir-records suggest that the volcanism commenced prior to the ETME.•Global warming and weathering intensified during the later part of the ETME.•PAHs, and SO2-damages may indicate thermogenic emissions from CAMP.•All geochemical traces of CAMP seem to disappear in the middle Hettangian.
This paper on core samples collected from the Triassic Montney Formation tight gas reservoir in the Western Canadian Sedimentary Basin (WCSB) illustrates that operationally-defined S1 and S2 ...hydrocarbon peaks from conventional Rock–Eval analysis may not adequately characterize the organic constituents of unconventional reservoir rocks. Modification of the thermal recipe for Rock–Eval analysis in conjunction with manual peak integration provides important information with significance for the evaluation of reservoir quality. An adapted method of the analysis, herein called the extended slow heating (ESH) cycle, was developed in which the heating rate was slowed to 10°C per minute over an extended temperature range (from 150 to 650°C). For Montney core samples within the wet gas window, this method provided quantitative distinctions between major organic matter (OM) components of the rock. We show that the traditional S1 and S2 peaks can now be quantitatively divided into three components: (S1ESH) free light oil (S2aESH) fluid-like hydrocarbon residue (FHR), and (S2bESH+residual carbon) solid bitumen (more refractory, consolidated bitumen/pyrobitumen).
The majority of the total organic carbon (TOC) in the studied Montney core samples consists of solid bitumen that represents a former liquid oil phase which migrated into the larger paleo-intergranular pore spaces. Physicochemical changes to the oil led to the precipitation of asphaltene aggregates. Subsequent diagenetic and thermal cracking processes further consolidated these asphaltene aggregates into “lumps” of solid bitumen (or pyrobitumen at higher thermal maturity). Solid bitumen obstructs porosity and hinders fluid flow, and thus shows strong negative correlations with reservoir qualities such as porosity and pore throat size.
Although the FHR fraction constitutes a small portion of the total rock mass and volume in Montney samples it has important implications for reservoir quality. This fraction represents a thin film of condensed, heavy molecular hydrocarbon residue covering surfaces of the present-time pore spaces and may represent the lighter component of the paleo-oil that migrated into tight interstices in the Montney reservoir. The FHR fraction potentially plays an important role in wettability alteration by creating hydrophobic matrix pore networks in portions of the reservoir that were not already filled with solid bitumen.
•Introduce an extended slow heating (ESH) Rock–Eval method for characterization of organic matter fractions in rock•Investigate role of various organic matter fractions in the reservoir quality of Montney tight gas siltstone•Propose a theory describing accumulation of solid bitumen from migration of paleo oil into the reservoir
The Cambro‐Ordovician interval marks a significant transition from extinction to bio‐diversification in deep time. However, the relationship of bio‐transition to volcanism, commonly characterized by ...mercury (Hg) systematics in sedimentary records, has not been examined. We present the first Cambro‐Ordovician Hg systematics from the Scandinavian Alum Shale. Our results show pronounced Furongian Hg enrichments, coupled with positive Δ199Hg, Δ200Hg, and Δ201Hg values and negative Δ204Hg values that we ascribe to atmospheric Hg transport over long‐distances, while Early Ordovician Hg anomalies, characterized by near‐zero mass‐independent isotope values, indicative of submarine source. Our findings are supported by two new proxies: molybdenum‐Hg and vanadium‐δ202Hg co‐variations, demonstrating Hg systematics were strongly influenced by changes in source and depositional conditions. Constrained by a synchronous atmospheric‐tectonic‐oceanic model, we hypothesize Furongian subaerial volcanism contributed to global extinction and oceanic anoxia, whereas Early Ordovician submarine volcanism concurrent with ocean water upwelling promoted the nascent bio‐diversification.
Plain Language Summary
The late Cambrian‐Early Ordovician interval is a crucial time that bridges the Cambrian extinction and Great Ordovician Bio‐diversification events. The former is associated with 50% decrease in genera, whereas the latter displays threefold increase in species. Volcanism is associated with extinction and bio‐development events throughout Earth's history. Prior works investigated potential biogeochemical controls that could have supported the Cambro‐Ordovician bio‐transition, but none explored the role of volcanism. We, for the first time, examine Hg abundance ratios and isotopes in the Scandinavian Alum Shale core across this boundary. Two novel molybdenum‐Hg and vanadium‐δ202Hg models are proposed to improve our interpretation of the geochemical records about the effects of volcanism on environmental changes during this enigmatic transition. Constrained by a synchronous atmospheric‐oceanic‐tectonic model, our results demonstrate that late Cambrian subaerial volcanism contributed to oceanic anoxia and extinction, whereas Early Ordovician submarine volcanism and water upwelling led to the subsequent bio‐radiation.
Key Points
Mercury is associated with organic matter in carbonaceous Alum shale deposited under sulfidic conditions
Late Cambrian‐Early Ordovician mercury was released by volcanism that also triggered major environmental change
Mercury mass independent fractionation isotopes suggest late Cambrian subaerial volcanism but Early Ordovician submarine source
Core samples from petroleum wells are costly to obtain, hence drill cuttings are commonly used as an alternative source of rock measurements for reservoir, basin modelling, and sedimentology studies. ...However, serious issues such as contamination from drilling mud, geological representativeness, and physical alteration can cast uncertainty on the results of studies based on cuttings samples. This paper provides a unique comparative study of core and cuttings samples obtained from both vertical and horizontal sections of a petroleum well drilled in the Canadian Montney tight gas siltstone reservoir to investigate the suitability of cuttings for a wide range of geochemical and petrophysical analyses. The results show that, on average, the bulk quantity of kerogen or solid bitumen measured in cuttings is comparable to that of the core samples. However, total organic carbon (TOC) measurements are influenced by oil-based drilling mud (OBM) contamination. Solvent-cleaning of cuttings has been shown to effectively remove OBM contamination in light, medium, and heavy range hydrocarbons and to produce similar kerogen/solid bitumen measurements to that of core samples. Similarly, pyrolysis methods provide an alternative to the solvent-cleaning procedure for analysis of kerogen/solid bitumen in as-received cuttings. Microscopic study substantiates the presence of significant contamination by OBM and caved organic and inorganic matter in the cuttings, which potentially influence the bulk geochemistry of the samples. Furthermore, minerals in the cuttings display induced micro-fractures due to physical impacts of the drilling process. These drilling-induced micro-fractures affect petrophysical properties by artificially enhancing the measured porosity and permeability.
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