Steve Whittaker

Greenhouse gas sequestration in abandoned oil reservoirs: The International Energy Agency Weyburn pilot project

ABSTRACTCarbon dioxide sequestration in geo-logical reservoirs is being evaluated internationally as a viable means of long-term CO 2 storage. The International Energy Agency Weyburn CO 2 Monitoring and Storage Project is designed to inves-tigate the technical and economic fea-sibility of CO 2 storage in a partially de-pleted oil reservoir in conjunction with enhanced oil recovery operations. Two key elements of the project are (1) the establishment of a regional geoscience framework as a means for prediction of the long-term fate of injected CO 2 , and (2) development and application of geophysical/geochemical monitoring and verification methods to track the spread of CO 2 within the reservoir. To date, 1.90 billion m 3 of CO 2 have been injected into the reservoir, the effects of which are imaged by the various moni-toring methods. INTRODUCTIONCarbon dioxide is the primary anthro-pogenic greenhouse gas in the modern-day atmosphere and is a critical compo-nent in models of global climate change (IPCC, 2001). It is estimated that ~6 gi-gatons of carbon enters the environment annually as a result of global energy-re-lated CO

VARIATIONS IN THE NEODYMIUM AND STRONTIUM ISOTOPIC COMPOSITION AND REE CONTENT OF MOLLUSCAN SHELLS FROM THE CRETACEOUS WESTERN INTERIOR SEAWAY

Rare earth element concentrations, ϵNd(T) values, and strontium isotopic compositions of mollusc shells were used to trace variations in the neodymium and strontium isotopic composition of the epicontinental Late Cretaceous Western Interior seaway of North America. Rare earth element patterns are different in aragonite and calcite produced by the molluscs endemic to the seaway, indicating that either mineralogical control or possibly scavenging by organic films associated with the different phases of biogenic carbonate resulted in differential partitioning of the REEs from seawater during shell formation. The biogenic carbonate also may contain REEs associated with Fe-flocs trapped in the shells during growth, but these flocs cannot result in different REE patterns of aragonite and calcite produced by the molluscs. The neodymium isotopic composition of the Western Interior seaway is inferred to have varied 13 ϵ-units over 20 My as a result of incursions of seawater from the Arctic Ocean and Gulf of Mexico, river influx from tectonically active terranes, benthic diagenetic fluxes, and volcanic ash falls. Ash from a variety of volcanic centers in western North America was significant in producing rapid and marked changes in143Nd144Ndratios of the seaway, and abrupt regional variations in neodymium isotopic composition of the seaway make the construction of an accurate neodymium isotope evolution curve difficult for this basin. Strontium isotopic compositions of the mollusc shells indicate the 87Sr86Sr ratio of the Western Interior seaway was generally similar to contemporaneous oceans, although, periodically, the basin had strontium isotopic compositions distinct from contemporaneous seawater. An ϵNd(T) value of +6.2 is recorded in a molluscan shell from the Cenomanian-Turonian boundary interval, which is a time associated with a global anoxic event, biotic crisis, and elevated Ir contents in sedimentary rocks. Neither a marked Ce anomaly nor a significant variation in ThU ratios is observed in the mollusc shell for this time, nor are Ce anomalies or aberrant ThU ratios evident in mollusc shells correlative with other periods of low oxygenation of the Western Interior seaway. The high ϵNd(T) value near the Cenomanian-Turonian boundary may imply a terrestrial source for the anomalous Ir contents in strata of this age in the Western Interior Basin.

Geochemical and Nd isotopic evidence for sedimentary-source changes in the Devonian miogeocline of the southern Canadian Cordillera

Sedimentological evidence indicates that clastic sediments of the Western Canada Sedimentary Basin were derived predominantly from the north and east, an exception being the westerly sourced Sassenach Formation in the Upper Devonian. A comparison of geochemical and Sm-Nd isotopic data of the Sassenach Formation and the underlying Perdrix and Mount Hawk Formations, which have easterly sources, support a change in sedimentary provenance. The Frasnian Perdrix and Mount Hawk Formations have initial ϵ Nd values of −7 to −8.5. This range of values is higher than that of the underlying Precambrian basement and reflects a mixture of older and younger crustal detritus (Late Proterozoic to Devonian) eroded and transported from orogens in the Arctic. The overlying Famennian Sassenach Formation yields lower initial ϵ Nd values that decrease from as high as −8.5 at the base to about −11 near the top. The lower ϵ Nd values of the Sassenach Formation imply that the western crustal source of the Sassenach sediments was, on average, older than the eastern crustal sources of the underlying formations. This older western source appears to be related to uplift and reworking of older miogeosynclinal sediments in the west as a result of compression related to the Antler orogeny.

Diagenesis of an Upper Devonian carbonate-evaporite sequence; Birdbear Formation, southern Interior Plains, Canada

ABSTRACT The Frasnian Birdbear (Nisku) Formation is a carbonate-evaporite succession formed on a shallow restricted shelf along the eastern side of the Western Canada Sedimentary Basin. Low d13C values of the shelf limestones and dolostones (1 to -5) relative to coeval open marine settings reflect the restricted paleoenvironment of the shelf. Limestones are present only in the eastern part of the study area, and were altered by meteoric and burial diagenesis and do not retain marine d18O values. d18O values of replacement dolomites have limited range (-3 to -5) and may indicate dolomitization by hypersaline marine waters having low d18O values. 87Sr/86Sr ratios of the replacement dolomites are variable (0.7082-0.7085), but also suggest that reflux of Late Devonian seawater, possibly Famennian, was the principal dolomitizing mechanism in the Birdbear shelf. Most bedded anhydrites have d34S values (22 to 24 CDT) and 87Sr/86Sr ratios (ca. 0.7081) that record Frasnian seawater. Dissolution and reprecipitation of sedimentary sulfates resulted in sulfate cements having up to 2 higher d34S values and slightly higher 87Sr/86Sr ratios (ca. 0.7083). Fluids associated with anhydrite dissolution may have been derived from the overlying Famennian shelf. Late diagenetic cements in the Birdbear Shelf precipitated during the Late Cretaceous and early Tertiary during maximum burial from hot, evolved residual evaporitic brines. Residual evaporitic brines that originated on the shallow eastern Upper Devonian shelves were likely important agents of diagenesis throughout th basin.

Effects of sources and diagenesis on the isotopic and chemical composition of carbon and sulfur in Cretaceous shales

Abstract The Cretaceous Lea Park Formation of south-central Saskatchewan contains organic matter from marine and terrestrial sources in proportions that vary as a result of deposition during the Campanian Claggett transgressive-regressive cycle in the Western Interior seaway of North America. The relative proportions of organic matter from these sources were determined using a terrestrial index (TI) that compares n-alkanes considered to be of terrestrial origin,n-C25 to n-C59, to those considered to be of marine origin, n-C15 to n-C22. The C isotopic composition of Cretaceous marine organic matter is found to be near −32%o while terrestrial organic matter is near −24%.. The lower δ13C values of marine organic matter from the Lea Park Formation, as compared with those of Recent marine organic matter, are similar to the relation observed in other Cretaceous rocks; δ13C values of terrestrial organic matter, however, are similar to those of the present. During the Miocene, δ13C values of marine organic matter became similar to Recent values, and this change in isotopic composition may be related to the development of deep cold-water associated with the formation of Antarctic ice sheets. Consequences of the formation of deep cold-water may include improved exchange of CO2 between atmosphere and oceans, reduced pCO2 levels and CO2 concentrations in the photic zone of the oceans, and environmental changes sufficient to produce an alternate metabolic pathway in phytoplankton. All of these processes potentially result in higher δ13C values in marine organic matter. Pyrite formation in the Lea Park mudstones was limited by the availability of metabolizable organic matter, with the greatest amount of pyrite typically associated with organic matter having the lowest TI values. δ34Spy values range nearly 30%. and vary directly with the composition of associated organic matter. Variations in the supply of metabolizable organic matter were more important than differences in sedimentation rate in producing the range of δ34Spy values. A34S values (δ534Sseawater - δ34Spyrite) vary with the composition of organic matter and become larger when associated with more metabolizable organic matter.

Petrogenesis of 1.9 Ga limestones and dolostones and their record of Paleoproterozoic environments

Abstract Fibrous calcite cements from limestones of the Paleoproterozoic Pethei Group in northwest Canada have relatively constant δ 13 C values, near 1‰, that indicate the C exogenic cycle of ca 1.9 Ga ago had similar balances between oxidized and reduced reservoirs of carbon as has existed throughout most of the Phanerozoic. 87 Sr/ 86 Sr ratios of these cements are near 0.7051, and support a progressively increasing influx of radiogenic Sr from continental weathering into the Paleoproterozoic oceans. δ 18 O values of the cements vary from −14 to −8.5‰, although the original values of the cements were probably higher because the cements have been subseqently altered, as evidenced by their variable trace element contents. Proposed higher temperatures or substantially lower δ 18 O values of ancient seawater based on such values may be in error. Fabric retentive dolomites, generally regarded as syndepositional, also have variable δ 18 O values, the highest of which are ca 30 greater than the highest values of the fibrous calcite cements. The dolomites have more radiogenic Sr isotopic compositions indicating that they formed from fluids that interacted with rocks such as those of the clastic sediments which underlie the limestones. These fluids may have partially altered the δ 18 O values and chemical compositions of the fibrous calcite cements. Late-stage calcitic and dolomitic cements that formed from late diagenetic fluids have similar δ 18 O values and radiogenic Sr isotopic ratios as the fabric retentive dolomites. If these potentially syndepositional dolomites have had their primary isotopic compositions reset through interaction with diagenetic fluids, this would have significant implications to other studies using early dolomite to constrain ancient environmental conditions.

Geochemistry of synsedimentary cements in Early Cambrian Reefs

Abstract Synsedimentary fibrous calcite cements in reefs representing different periods of Early Cambrian time from eastern Siberia, southern Australia, and eastern Canada were analyzed to determine whether they retained information about the chemical and isotopic compositions of the seawater from which they precipitated. Although petrographically similar, the Sr, O, and C isotopic compositions as well as the Sr and Mg contents of the cements from the three areas are different. Such differences can be attributed to varying degrees of diagenesis of the cements that correlate with the tectonic history of each area: cements from the Siberian craton are the least altered, cements from near the Appalachian Orogen are partially altered and cements from the Flinders fold and thrust belt are extensively altered. Strontium contents of the fibrous cements generally increase with Mg contents, although the correlation among these two elements is more variable than that reported for Devonian or Holocene cements. Those cements with Sr Mg ratios near 0.04, which include clear fibrous calcite, have the lowest 87 Sr 86 Sr ratios and have been least altered. Strontium isotopic ratios in the least altered cements are lower than whole rock ratios and probably approach the original values of Early Cambrian seawater. δ18O values in the cements are variable because of exchange with diagenetic fluids, whereas δ13C values exhibit local variations that do not match temporal curves constructed from whole-rock data for the same regions. Thus, most of the original chemical and isotopic compositions of these cements have been variably changed through diagenesis, although limited information about the composition of Early Cambrian seawater, such as Sr and C isotopic compositions, have been retained in some samples.

Basin resources and carbon storage

Prospective sites for geological storage of carbon dioxide target sedimentary basins as these provide the most suitable geological settings for safe, long-term storage of greenhouse gases. Sedimentary basins can also host different natural resources including groundwater, oil and gas, unconventional gas, coal and geothermal energy.Understanding the nature of how these resources are distributed in the subsurface is fundamental to managing basin resource development and carbon dioxide storage. The underlying principal of the proposed workflow is to assess what basin resource – storage interactions are likely and to evaluate, at different scales, how they may be best managed. For regions having potential for resource conflicts a basin resource management plan may be required, and the appropriate regulator would need to decide on the priority of each resource and, if parallel development is not feasible, the order in which resources should be exploited.

Natural CO2 occurrences compared to anthropogenic CO2 injection sites within the Williston Basin, Central North America

Publisher Summary The CO 2 occurrences in Devonian carbonates of southwestern Saskatchewan are suggested to represent a natural analogue for the Weybum injection site due to their similar basinal environment and geological framework. On the western flank of the Williston Basin, southwestern Saskatchewan, Canada, CO 2 occurs in natural accumulations within Devonian carbonates and Cambrian silicielastics. Inert gases from some carbonate reservoirs contain greater than 80 per cent CO 2 and have sustained flow rates up to 425, 000 m 3 /day. Early estimates of this resource suggested nearly two million tons of CO 2 or about one billion m 3 of recoverable gas are present in these rocks. These natural CO 2 occurrences are found about 400 km west of the site of the IEA Weyburn CO 2 Storage and Monitoring Project that is directed toward assessing the potential for safe geological storage of anthropogenic CO 2 in Mississippian carbonates. The Devonian strata containing natural CO 2 accumulations in southwestern Saskatchewan are geologically very similar to the Mississippian beds being injected with CO 2 at the Weyburn Field.