Large-Scale Underpressuring in the Mississippian-Cretaceous Succession, Southwestern Alberta Basin

The hydrodynamic regime of formation waters in the post-Devonian sedimentary succession was studied for an area of about 120,000 km(2) in southwestern Alberta using approximately 15,000 drillstem tests and 13,000 formation-water analyses. The salinity of formation waters generally increases both northward and with depth from 5000 mg/L to more than 100,000 mg/L. Based on flow characteristics and driving mechanisms, the sedimentary succession can be divided into two megahydrostratigraphic groups overlain by an unconfined aquifer at the top. The Mississippian-to-Mannville (Cretaceous) hydrostratigraphic group is basically an open hydrodynamic system dominated by aquifers. The flow of formation waters is driven by basin-scale topography, and part of a basin-scale flow system with recharge at high elevations in the south and southwest discharge at low elevations in the north-northeast. The Cretaceous Colorado-to-Edmonton hydrostratigraphic group is largely a closed hydrodynamic system dominated by aquitards. The flow in aquifers is driven westward downdip, toward the thrust and fold belt, by large-scale underpressuring caused by erosional rebound in thick shales. In places, pressures reach lower values than those corresponding to the lowest basin elevation located far to the north. This flow system is in a transient state of mechanical and hydrodynamic adjustment to the present topography. The different flow pattern in the two megahydrostratigraphic successions has consequences for hydrocarbon exploration in terms of secondary migration paths and possible hydrodynamic entrapment of hydrocarbons

Hydrogeology of Formation Waters, Northeastern Alberta Basin

The hydrogeological study of formation waters in the northeastern part of the Alberta basin (defined as the area from 55 to 58-degrees-N and from 110 to 114-degrees-W) is based on information from 12,4-5 wells, 3187 formation-water analyses, 2531 drill-stem tests, and 452,030 core-plug analyses. Because the study area, covering approximately 76,000 km2 is located at the feather edge of the basin, local topography and physiographic features, particularly the Athabasca River system, exert a strong influence on the flow of formation waters in most of the aquifers. Generally, temperature seems to be the main controlling factor on salinity distributions. The salinity of formation waters increases in the vicinity of evaporitic beds, and decreases close to the surface because of mixing with fresh meteoric water introduced through local flow systems.The Lower and Middle Devonian pre-Prairie aquifer systems, beneath the regionally extensive Prairie aquiclude, are characterized by regional topographically-driven flow updip to the northeast. This updip flow is opposed by buoyancy forces caused by salinity increase with temperature downdip to the southwest. The post-Prairie Devonian aquifers are characterized by transitional flow regimes. Because of erosion at the sub-Cretaceous unconformity and outcrop at the Athabasca River, local physiographic influences are superimposed on basin-scale regional flow in these aquifers. Hydraulic communication between the Beaverhill Lake-Cooking Lake and Grosmont aquifers is inferred in places caused by Cooking Lake reefs penetrating the intervening shales of the Lower Ireton aquitard. Finally, the Cretaceous aquifers all can be described as having local flow regime characteristics with no buoyancy effects as a result of recharge in topographically high areas and discharge in low regions along the valleys of the Athabasca River system.The flow of formation waters in northeastern Alberta played an important role in the formation of the huge Athabasca oil sands deposits. Hydrocarbons that migrated into the area from the west were trapped into local reservoirs, and biodegraded and washed by fresh meteoric water introduced by local now systems. Environmentally, the subsurface hydrogeology in the area imposes specific constraints on waste disposal in deep formations mostly because of the absence of a thick, continuous regional aquitard and because most aquifers, subcrop at shallow depth or crop out and discharge along the valleys of the Athabasca River system and at the basin edge

Deep Waste Injection, Western Canada - Analysis of Data and Requirements for Numerical-Simulation

The application of numerical models to deep waste injection problems requires stringent preparation of data from a variety of sources. These include physiography, topography, stratigraphic picks used to define the geological framework, and drill stem test, core analyses, and analyses of formation water chemistry used to characterize the hydrogeological regime. The Alberta Geological Survey has assembled various purchased and in-house developed software. This paper presents the techniques and associated software used for the spatial analysis and interpretation of various data types required for simulating deep injection of residual water. -from Author

Regional Subsurface Hydrogeology, Peace River Arch Area, Alberta and British-Columbia

The Peace River Arch area is defined as the area from 54 to 58-degrees-N and 114-degrees-W (5th Mer) to the eastern margin of the disturbed belt. It covers about 220,000 km2 of northwestern Alberta and northeastern British Columbia. The hydrogeological database comprises stratigraphic information from more than 22,000 wells, 450,000 core analyses in Alberta, 42,000 bottomhole temperatures, 21,000 drillstem tests, and 30,000 formation water analyses. This information was variously interpreted, culled for erroneous values and interpolated into a regional framework comprising 60 porosity-modified lithostratigraphic units (32 aquifers, 23 aquitards and 5 aquicludes).The hydrogeological information is presented as 1) a series of maps of porosity distribution, permeability populations, potentiometric surfaces (freshwater basis) and formation water salinity for selected hydrostratigraphic units, 2) cumulative frequency plots for K(Max), and 3) strike and dip cross-sections showing variations of hydraulic head and formation water salinity.There are three hydrogeological groups separated by two regionally significant aquitards. The lower Paleozoic hydrogeological group lies below the Ireton aquitard and is characterized by regional flow. The flow has been modified where it is affected by drawdown from the Grosmont aquifer, which lies above the Ireton aquitard. Formation water salinity is enhanced near the Lower Devonian aquiclude as a result of salt solution.The upper Paleozoic-lower Mesozoic hydrogeological group lies below the Clearwater-Wilrich aquitard and extends down to the Ireton aquitard, or where this is absent to the Precambrian aquiclude. This group is mainly characterized by regional flow except at shallower depths adjacent to the sub-Cretaceous unconformity where lowered salinity and an intermediate flow regime predominate.The upper Mesozoic-Cenozoic hydrogeological group includes all hydrostratigraphic units above the Clearwater-Wilrich aquitard. Flow is intermediate in character below the Colorado aquitard, and local in character above it. Salinity of formation water is effectively always 35-degrees-C/km. As a result of both the low permeability of the rocks and the low flow velocity of the formation waters, the temperature distribution in the sedimentary rocks depends on the terrestrial heat flow and basin thickness, and reaches > 140-degrees-C at depths of 4100 m. Because of the low flow velocity of subsurface fluids, the salinity of formation waters is mainly controlled by temperature, and hence shows a direct relation with depth, except where influenced by contact with evaporites or downward moving meteoric water

Flow of Formation Waters in the Cretaceous-Miocene Succession of the Llanos Basin, Colombia

This study presents the hydrogeological characteristics and flow of formation waters in the post-Paleozoic succession of the Llanos basin, a mainly siliciclastic foreland sub-Andean sedimentary basin located in Colombia between the Cordillera Oriental and the Guyana Precambrian shield. The porosity of the sandy formations is generally high, in the range of 16-20% on average, with a trend of decreasing values with depth. Permeabilities are also relatively high, in the 10(2) and 10(3) md range. The salinity (total dissolved solids) of formation waters is generally low in the 10,000-20,000 mg/L range, suggesting that at least some strata in the basin have been flushed by meteoric water. The shaly units in the sedimentary succession are weak aquitards in the eastern and southern parts of the basin, but are strong in the central-western part. The pressure in the basin is close to or slightly subhydrostatic. The underpressuring increases with depth, particularly in the central-western area. The flow of formation waters in the upper units is driven mainly by topography from highs in the southwest to lows in the northeast. Local systems from the foothills and from focal topographic highs in the east feed into this now system. The flow of formation waters in the lower units is driven by topography only in the southern, eastern, and northern parts of the basin. In the central-western part, the flow is downdip toward the thrust-fold belt, driven probably by pore-space rebound induced by erosional unloading, which also is the cause of underpressuring. Hydrocarbons generated in the Cretaceous organic-rich, shaly Gacheta Formation probably have migrated updip and to the north-northeast, driven by buoyancy and entrained by the topography-driven flow of formation waters; however, the downdip flow of formation waters in Cretaceous-Oligocene strata in the central-western part of the basin could have created conditions for hydrodynamic entrapment of hydrocarbons

Geothermal Regime and Thermal History of the Llanos Basin, Colombia

The Llanos basin is a siliciclastic foreland sub-Andean sedimentary basin located in Colombia between the Cordillera Oriental and the Guyana Precambrian shield. Data on bottom-hole temperature, lithology, porosity, and vitrinite reflectance from all 318 wells drilled in the central and southern parts of the basin were used to analyze its geothermal regime and thermal history.Average geothermal gradients in the Llanos basin decrease generally with depth and westward toward the fold and thrust belt. The geothermal regime is controlled by a moderate, generally westward-decreasing basement heat flow, by depositional and compaction factors, and, in places, by advection by formation waters. Compaction leads to increased thermal conductivity with depth, whereas westward downdip flow in deep sandstone formations may exert a cooling effect in the central-western part of the basin. Vitrinite reflectance variation with depth shows a major discontinuity at the pre-Cretaceous unconformity. Areally, vitrinite reflectance increases southwestward in Paleozoic strata and northwestward in post-Paleozoic strata. These patterns indicate that the thermal history of the basin probably includes three thermal events that led to peaks in oil generation: a Paleozoic event in the southwest, a failed Cretaceous rifting event in the west, and an early Tertiary back-arc event in the west. Rapid cooling since the last thermal event is possibly caused by subhorizontal subduction of cold oceanic lithospheric plate

Pressure distribution in a reservoir affected by capillarity and hydrodynamic drive: Griffin Field, North West Shelf, Australia

The effects of capillarity in a multilayered reservoir with flow in the aquifer beneath have characteristic signatures on pressure-elevation plots. Such signatures are observed for the Griffin and Scindian/Chinook fields of the Carnarvon Basin North West Shelf of Australia. The Griffin and Scindian/Chinook fields have a highly permeable lower part to the reservoir, a less permeable upper part, and a low permeability top seal. In the Griffin Field there is a systematic tilt of the free-water level in the direction of groundwater flow. Where the oil-water contact occurs in the less permeable part of the reservoir, it lies above the free-water level due to capillarity. In addition to these observable hydrodynamic and capillary effects on hydrocarbon distribution, the multi-well pressure analysis shows that the gas-oil contacts in the Scindian/Chinook fields occur at different elevations. For both the Griffin and Scindian/Chinook fields the oil pressure gradients from each well are non-coincident despite continuous oil saturation, and the difference is not attributable to data error. Furthermore, the shift in oil pressure gradient has a geographical pattern seemingly linked to the hydrodynamics of the aquifer. The simplest explanation for all the observed pressure trends is an oil leg that is still in the process of equilibrating with the prevailing hydrodynamic regime