Jim Underschultz

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°N and from 110 to 114°W) is based on information from 12,475 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 beca se 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 retaceous 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 flow 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.

Evaluating hydrocarbon trap integrity during fault reactivation using geomechanical three-dimensional modeling: An example from the Timor Sea, Australia

Three-dimensional (3-D) coupled deformation and fluid-flow numerical modeling are used to simulate the response of a relatively complex set of trap-bounding faults to extensional reactivation and to investigate hydrocarbon preservation risk for structural traps in the offshore Bonaparte Basin (Laminaria High, the Timor Sea, Australian North West Shelf). The model results show that the distributions of shear strain and dilation as well as fluid flux are heterogeneous along fault planes inferring lateral variability of fault seal effectiveness. The distribution of high shear strain is seen as the main control on structural permeability and is primarily influenced by the structural architecture. Prereactivation fault size and distribution within the modeled fault population as well as fault corrugations driven by growth processes represent key elements driving the partitioning of strain and up-fault fluid flow. These factors are critical in determining oil preservation during the late reactivation phase on the Laminaria High. Testing of the model against leakage indicators defined on 3-D seismic data correlates with the numerical prediction of fault seal effectiveness and explains the complex distribution of paleo- and preserved oil columns in the study area.

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 km2 in southwestern Alberta using approximately 15,000 drill-stem 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 megahydro-stratigraphic 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 f ow system with recharge at high elevations in the south and southwest and 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 an possible hydrodynamic entrapment of hydrocarbons.

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 de-creasing values with depth. Permeabilities are also relatively high, in the 102 and 103 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 t e 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 local topographic highs in the east feed into this flow 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 underpressu ing. 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 CretaceousOligocene 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 robably 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.

Regional-scale porosity and permeability variations, Peace River Arch area, Alberta, Canada

This study examines the large-scale variability of porosity and permeability of the sedimentary rocks in the Phanerozoic succession in the Alberta part of the Peace River arch area of the Western Canada sedimentary basin. The study is based on about 450,000 core analyses at approximately 22,000 wells in an area of more than 165,000 km{2}. Plug-scale porosity and permeability values are scaled up to the well scale by hydrostratigraphic unit, resulting in two sets of about 16,000 values each for porosity and permeability, unevenly distributed both areally and with depth. The permeability frequency distributions are lognormal for most of the units or parts of the units. The regional-scale variability of porosity and permeability is quite high, between 1 and 38% for porosity, and 0.001 md and 3 d for permeability. The clastic units of the foreland basin exhibit a relatively high correlation between permeability and porosity. Several areal trends and patterns are identified for groups of hydrostratigraphic units, patterns that change gradually from one group to another. It is hypothesized that the observed variability is caused by the dominance of the Peace River arch, carbonate deposition, or compaction at various times throughout the evolution of the basin. Based on the predominant controlling factor, the geological history can be divided into four periods: arch influence during the Early to Middle Devonian, reefal carbonate-deposition influence during the Middle to Late Devonian, passive margin influence during the Late Devonian to Middle Jurassic, and orog nic influence since the Middle Jurassic.

Pressure and fluid dynamic characterisation of the Dutch subsurface

This paper presents and discusses the distribution of fluid and leak-off pressure data from the subsurface of onshore and offshore Netherlands in relation to causes of formation fluid overpressure and the permeability framework. The observed fluid pressure conditions demonstrate a clear regional difference between the southern and the north and north-eastern part of the study area. In the southern area, formation fluid pressures are close to normal and well below measured leak-off pressures. In the north, formation fluids are overpressured and may locally even approach the measured leak-off pressures. The regional differences in fluid overpressure can, in large part, be explained by differences in geologic framework and burial history. In the south, relatively low rates of sedimentary loading and the presence of relatively permeable sedimentary units have led to the currently observed normally pressured conditions. In the northern area, relatively rapid Neogene sediment loading plays an important role in explaining the observed overpressure distributions in Cenozoic mudstones, Cretaceous Chalk and Rijnland groups, and probably also in Jurassic units. The permeability framework of the northern and north-eastern area is significantly affected by Zechstein and Triassic salt deposits and structures. These units are characterised by very low permeability and severely restrict fluid flow and pressure dissipation. This has created hydraulically restricted compartments with high overpressures (for example overpressures exceeding 30 MPa in the Lower Germanic Trias Group in the Terschelling Basin and Dutch Central Graben).

Conducting comprehensive analyses of potential sites for geological CO2 storage

Publisher Summary This chapter illustrates that geological storage of CO2 may provide a solution to the problem of reducing anthropogenic emissions of greenhouse gases to the atmosphere. To accurately appraise a potential site in terms of its suitability for CO2 storage, a comprehensive workflow analyzing the detailed geological and geophysical characteristics of the site needs to be undertaken. In particular, potential sites need to be evaluated geologically in terms of their injectivity, containment, and capacity. Injectivity can be assessed by a review of the reservoir quality and by a detailed sequence stratigraphic model to estimate the likely flow-unit geometries and connectivity. Containment can be assessed via an analysis of the seal capacity (using MICP), and by assessing the possible migration pathways and trapping mechanisms from the structural geometry and stratigraphic architecture. In addition, integration of hydrodynamic analysis of formation water flow systems and geomechanical studies of fault stability and sustainable pore fluid pressures with the stratigraphic interpretation provide vital confirmation of the containment potential.

Tectonic loading in the Canadian Cordillera as recorded by mass accumulation in the Foreland Basin

The geometry of the Alberta foreland basin is generally the result of lithospheric loading in the adjacent Cordillera. Sedimentation within the Foreland basin was quantified by calculating mass accumulation rates to constrain loading events in time and space. Rates of mass accumulation for Cretaceous units in the basin can be classified into low, moderate, and high categories and may related to deformation events in the Canadian Cordillera. The rates of accumulation show a distinct cyclic pattern when plotted against time. There were two major episodes of rapid sedimentation from early Aptian to late Campanian time, each preceded by a period of relative quiescence. The first lasted from approximately 115 to 108 Ma. It shows a succession of rapid, moderate, and rapid sedimentation events. The second episode of rapid sedimentation lasted from approximately 95 to 90 Ma, and is characterized by three rapid sedimentation events separated by short intervals of quiescence. Order-of-magnitude changes in sediment accumulation in the foreland basin can be related to major deformation events in the Cordillera. Furthermore, the deformation can be located in time and space and evaluated in terms of its relative intensity. This delineation of deformation indicates that the docking of foreign terranes does not necessarily coincide with their final accretion and the associated tectonic loading.

The hydrodynamics of fields in the Macedon, Pyrenees, and Barrow Sands, Exmouth Sub-basin, Northwest Shelf Australia: identifying seals and compartments

The Barrow Group strata (Macedon Member, Pyrenees Member, and Barrow Group sandstones) of the Exmouth Sub-basin host significant accumulations of gas and liquid hydrocarbons. There is currently oil production from the Macedon sandstone at the Enfield Field and ongoing development drilling at the Stybarrow Field. Active appraisal and exploration is underway, including the multi-field Pyrenees Development. In the course of assessing these discoveries, BHP Billiton and its joint-venture partners have undertaken a hydrodynamic study in order to better understand the sealing mechanisms, the position of free-water levels (FWLs), and the likelihood of compartmentalisation within the discoveries. Whilst the region is faulted with a predominant south-west-north-east grain, the potentiometric gradient is surprisingly flat indicating that the individual sands are hydraulically well connected. Other than the Macedon Gas Field, there is no pressure data that indicate intra-formational seals have been breached. Thus, top and bottom seal capacity is probably not limiting the pool sizes. Rather, structural spill points and fault seal capacity appear the significant factors in determining pool geometry, with the underlying aquifer being regionally connected around fault tips. On the field-scale, the flat hydraulic gradient allows for the calculated FWLs to have a high confidence. Pressure data from the hydrocarbon phases indicate that in some cases, fault zones may compartmentalise a field into multiple pools. These areas are then targeted for additional focused geological analysis to reduce uncertainty in field compartmentalisation. The Macedon Gas Field, on the eastern edge of the play fairway, marks a change in the trapping character with intra-formational and fault seals having been breached resulting in a single continuous gas pool despite internal structural complexity. Stybarrow and Laverda-Skiddaw clearly occur as separate accumulations and the Stybarrow data define a single oil column in contrast to the potentially compartmentalized Laverda-Skiddaw field. Stybarrow represents an anomalously large oil column relative to other fields in the area and it is located on the low hydraulic head side of a sealing fault.