Isotopic measurements (Sr, O, D) on formation waters from the Alberta Basin have been made, covering a stratigraphic range from Devonian to Upper Cretaceous. These measurements, combined with chemical compositional trends, give evidence for two distinct water regimes. One hydrological regime is composed of waters hosted in Devonian-Lower Cretaceous reservoirs, the other waters from Upper Cretaceous and younger sedimentary rocks. The two regimes are separated by a regional transgressive shale in the Colorado Group, the Second White Speckled Shale Formation.The waters within the Devonian-Lower Cretaceous regime exhibit a large range in 87Sr/86Sr values (0.70764).7129), but have similar Sr concentrations, regardless of host lithology. Bulk rock and late-stage diagenctic cements are less radiogenic than present brines. Importantly, brines from Devonian carbonates possess the most radiogenic Sr isotopic signatures of the waters examined. Devonian shales and/or 87 86 Cambrian shales may be sources of high Sr/ Sr ratios in the carbonate-hosted waters. Waters from the Upper Cretaceous clastic units, which have ratios as low as 0.7058, and diagenetic cements from Upper Cretaccous clastic units appear to have precipitated from fluids similar in Sr isotopic value to modern brines. High Sr concentrations in the Cretaceous clastic waters and sedimentary rocks and correspondingly low STSr/S6Sr ratios suggest that volcanism in Montana during the Cretaceous may have provided a source of sediments to the study area. Cross-formational upward water migration, superimposed on lateral fluid flow, is required to explain the geochemistry and isotopic systematics in the brines from Devonian-Lower Cretaceous reservoirs. Strontium isotope ratios and Sr contents suggest a two component mixing relation for these waters. This system of waters also exhibits 6D values characteristic of meteoric values in the Neogene, reflecting post-Laramide flushing of Tertiary waters throughout the basin, with subsequent hydrochemical isolation from more modern waters. In contrast, waters in Upper Cretaceous reservoirs have O and D isotopic compositions similar to those of present day rainfall; these, in conjunction with very dilute Sr concentrations and low Sr ratios, suggest hydrological isolation from the stratigraphically lower system.

Inorganic chemical analyses and short-chain aliphatic acid content are used to interpret the origin and compositional evolution of formation waters in the Alberta portion of the Western Canada Sedimentary Basin. Forty-three formation water samples were obtained covering a stratigraphic interval from Devonian to Cretaceous. The data show that: (1) there is a subaerially evaporated brine component that shows no apparent contribution of waters derived from evaporite dissolution; and (2) formation waters have maintained characteristics indicative of subaerially evaporated waters, despite subsequent flushing by gravity-driven meteoric waters in the basin. Formation waters are predominantly Na-CI brines that contain 4-235 g/1 total dissolved solids (TDS). Short-chain aliphatic acids (SCA) range up to 932 mg/l, with the following abundance: acetate >> propionate > butyrate. Their number varies randomly with subsurface temperature, depth, geological age and salinity. Instead, SCA distributions appear related to proximity to Jurassic and Mississippian source rocks and to zones of active bacterial SO4 reduction. Based on chemical composition, the formation waters can be divided into three groups. Group I waters arc from dominantly carbonate reservoirs and Group II from elastics. Groups 1 and II are differentiated from Group II1 in that they are composed of a brine end member, formed by evaporation of sea water beyond the point of halite saturation, that has been subsequently diluted 50-80% by a meteoric water end member. Group III waters are from elastic reservoirs and are dilute, meteoric waters that are decoupled from the more saline, stratigraphically lower, waters of Groups I and II. Group I waters have been influenced by clay mineral transformations in shales surrounding the carbonate reservoirs, ankeritization reactions of reservoir dolomites and calcites, and possible decarboxylation reactions. Group II waters indicate significant leaching reactions, particularly of feldspar and clay minerals. Group I and Group I1 waters both indicate ion exchange reactions were also possible. The waters are near equilibrium with respect to quartz, calcite, dolomite and barite, but are undcrsaturatcd with respect to evaporite minerals (halite, anhydrite). Occurrence of feldspar (predominantly albite) and kaolinite seems to control the population of the water cations. Post-Laramide invasion of meteoric waters provided an impetus for many of the diagenetic reactions in both carbonate, but especially in elastic reservoirs. Subsequent hydrochemical isolation of Group I and I1 waters from further meteoric influences occurred, resulting in pronounced mixing relations and eross-formational fluid flow replacing the once dominant lateral flow.

No abstract

No abstract

The Godthåbsfjord region of southern West Greenland comprises several terranes that were assembled between 2750 and 2550 Ma and folded during amphibolite facies metamorphism. The terranes, which are dominated by gneisses of different ages and show different preassembly metamorphic and structural histories, are (1) the Færingehavn terrane containing the 3820–3600 Ma Amîtsoq gneisses, with granulite facies metamorphism at circa 3600 Ma, (2) the Akia terrane containing the 3070–2940 Ma Nûk gneisses, with granulite‐amphibolite facies metamorphism at circa 2980 Ma, (3) the Tasiusarsuaq terrane containing circa 2900 Ma gneisses, with granulite facies metamorphism at circa 2800 Ma, and (4) the Tre Brødre terrane containing the 2800–2750 Ma Ikkattoq gneisses, with amphibolite facies metamorphism at 2800–2750 Ma. Metamorphic assemblages and structures formed prior to terrane assembly, were variably overprinted during amphibolite facies metamorphism and heterogeneous strain associated with assembly. Recognition of the region as consisting of several terranes indicates that the anatomy of some Archean high‐grade gneiss complexes may resemble that of orogens of Proterozoic and Phanerozoic age that formed as a consequence of plate tectonic processes.