Barbara Tilley

Gas isotope reversals in fractured gas reservoirs of the western Canadian Foothills: Mature shale gases in disguise

Isotopically reversed gases (13C methane 13C ethane 13C propane) occur in fractured mixed clastic-carbonate reservoirs of the Permian and the Triassic in the foothills at the western edge of the Western Canada sedimentary basin (WCSB). The 13C methane values (–42 to –24), gas dryness, and organic maturity (Ro 2.2) are indicative of mature gases, and gas maturity generally increases with reservoir age and from the southeast to the northwest. The 13C ethane values range from 44 to 25, with the less negative values in isotopically normal gases to the northeast of the gas fields we studied. To explain the gas isotope reversals observed in the WCSB foothills, we adopt the concept of a closed-system shale, in which simultaneous cooking of kerogen, oil, and gas yields gas with light 13C ethane and heavy 13C methane. This gas was released from shales and trapped in fractured folds of brittle clastic-carbonate rocks during deformation and thrust faulting of the Laramide orogeny, creating some of the most prolific gas pools. These gases are actually mature shale gases. Local high abundances of H2S and CO2 are most likely the products of thermochemical sulfate reduction (TSR) reactions in anhydrite-rich interbeds and underbeds that admixed to the released shale gas during the tectonic event. No evidence exists that TSR is responsible for the isotope reversals. Variations in 13C ethane are likely caused by local differences in thermal history, the timing of gas release from shale, and the timing of the fault and fold development. Less negative 13C ethane values (resulting in isotopically normal gases) to the northeast of the fields and in the underlying Devonian carbonates likely reflect a more open shale system where the earliest generated gas was lost. We suggest that isotopic reversals are restricted to closed-system maturation, and that their magnitude may be related to the relative volume of gas retained in shales.

Diagenesis and isotopic evolution of porewaters in the Alberta Deep Basin: The Falher Member and Cadomin Formation

Abstract Diagenesis and porewater evolution in the Alberta Deep Basin have been studied by combining results from petrologic, stable isotope, and fluid inclusion analyses of diagenetic minerals. Four stages of diagenesis and porewater evolution, corresponding to burial and subsequent uplift and erosion, have been identified in the Falher Member of the Spirit River Formation and the Cadomin Formation. Stage 1 (deposition and burial) is marked by early precipitation of hematite, siderite, or chlorite and the dissolution of unstable detrital grains, followed by later albitization, precipitation of illite and local calcite cement, and pressure solution of chert grains. During this stage, δ18O values of porewaters in the Falher Member increased from −10 ± 3%. to +3%. Infiltration of saline fluids from pre-Cretaceous units may have produced porewaters in the Cadomin Formation with δ18O values as high as +4 to +7%. Stage 2 (maximum burial and relief) is dominated by precipitation of quartz druse in conglomerates and horizontal fractures. Porefluids during Stage 2 had δ18O values of +3%. at 190°C (2–3 wt% dissolved solids) in the Falher Member and at 150°C in the Cadomin Formation. During Stage 3 (uplift and erosion) precipitation of dickite occurred in both the Falher Member and the Cadomin Formation from evolved meteoric water as temperatures and δ18O values of the porewater decreased. In addition, precipitation of ankerite and calcite was initiated in the Falher Member as the influx of evolved meteoric water continued. Finally, in Stage 4 (maximum generation of methane from interbedded coals), methane saturation of the porespaces marked the end of diagenesis in the down-dip, gas-saturated part of the Deep Basin.

Controls on Hydrocarbon Accumulation in Glauconitic Sandstone, Suffield Heavy Oil Sands, Southern Alberta

Hydrocarbon distribution in the Lower Cretaceous Glauconitic sandstone in the Suffield area of southeastern Alberta is controlled by three factors: sedimentology, structure, and mineralogy. The Glauconitic sandstone consists of six lithological facies interpreted to represent the lower-middle shoreface, middle shoreface, upper shoreface-foreshore, backshore, marsh, and lagoonal zones of a progradational, barrier-island system. Sediment deposited in the foreshore zone (laminated sandstone facies) has the best reservoir qualities: good porosity, low clay content, and good lateral continuity. The bioturbated, argillaceous sandstone, deposited in the backshore zone, has poor reservoir qualities: low porosity and high clay content with only isolated porous zones. Tidal inlet a d/or later stage fluvial channel deposits cutting through the sandstone trend form discontinuities in the reservoir. The hydrocarbon trapping mechanism is stratigraphic but with some structural influence. Deep faults, active during the deposition of upper Mannville sediments, caused differential subsidence and local thickening of sediment. This activity resulted in the apparent lateral juxtaposition of different facies. Parts of the Glauconitic sandstone form an exceptionally thick beach-shoreface sequence (up to 45 m or 148 ft thick). Faulting of sub-Cretaceous units may have controlled the rate of subsidence and the amount of sediment accumulation during deposition of the Glauconitic sandstone. The abundance of clay, mostly kaolinite, largely controls reservoir quality. Argillaceous backshore sandstones, which contain abundant detrital kaolinite, are poor reservoirs; clean foreshore deposits are good reservoirs. Porosity and permeability are only slightly reduced in the clean sandstone by formation of diagenetic phases such as kaolinite and quartz. During the wet forward-combustion recovery process, migration of kaolinite and dissolution-reprecipitation of silica could cause formation damage.

DV-Xα calculations of charge exchange between D+ and TiC, TiCxO(1-x), and SrO surfaces during low energy D+ scattering

Electronic structures for D+ ( H+) interacting with surface clusters of TiC (001), oxygenated TiC (001) ( TiCx O(1-x )), and SrO (001) have been calculated using self-consistent-charge discrete variational Xα methods, and the results used to successfully explain local charge exchange during low energy D+ scattering from the anionic component of the surfaces. The calculations show that the character of the surface peak formed in the D+ energy spectrum is target specific and is largely controlled by the size of the energy difference between the highest occupied molecular orbital (HOMO) and the antibonding molecular orbital containing the majority of the H 1s character (H-MABO). Calculations also demonstrate that the energy difference between the HOMO and the H-MABO is dependent on the charge of the cluster, which can be modified in covalent and metallic bonds by electron flow from atoms outside the cluster.