Low-permeability Sequence Research Articles

Chloride accessible porosity fractions across the Jurassic sedimentary rocks of northern Switzerland

The Jurassic clay-rich sedimentary sequence in northern Switzerland was investigated in the context of nuclear waste disposal. Tracer profiles across this sequence play an important role when assessing transport processes and long-term evolution of the environment. Negatively charged clay-mineral surfaces cause a depletion of anions in the porewater close to the surfaces compared to free porewater further away. The chloride accessible porosity fraction (fCl) is an important parameter to scale Cl bulk porewater concentrations from aqueous extraction to free porewater concentrations, which are required to establish anion profiles used for transport modelling of anionic tracers. A vast set of fCl data from advective displacement (AD), squeezing (SQ) and through-diffusion (DI) experiments allowed for method comparison and yielded insight into controlling factors of fCl across the sedimentary sequence. Two observations were made: (1) fCl varies strongly depending on lithology and partly on ionic strength; (2) at high ionic strength, the three methods yield consistent results, but at low ionic strength, fCl from DI are lower than those from AD and SQ. The observations are discussed in terms of influencing factors, including structural charge, ionic strength, rock texture, pore sizes and the fact that the different methods sample different parts of the pore space. The stronger variation of fCl from DI experiments with the ionic strength of the experimental porewater is in agreement with theoretical concepts. The volume fraction of small pores (r < 2 nm) inversely correlates with fCl; it accounts for ∼50% of the anion depleted pore volume. Two empirical approaches were tested to derive functions for fCl valid for the lithologically diverse sequence: the clay-content model uses a constant fCl at a clay-mineral content >30% and extrapolates to a fCl of 1 at zero clay content. The formation model applies formation- or member-specific average values of experimental fCl for each unit. Both approaches work reasonably well, with the formation model producing less apparent outliers of free porewater concentrations than the clay-content model. Still some scatter remains, particularly in heterogeneous and low-porosity formations. This work results in a comprehensive and coherent fCl dataset for all studied Jurassic formations and serves as a basis to interpret the profiles of anions with regard to transport across the entire low-permeability sequence.

Swiss deep drilling campaign 2019–2022: Geological overview and rock properties with focus on porosity and pore-space architecture

A recently completed deep drilling campaign, comprising 9 deep boreholes penetrating the Mesozoic sedimentary sequence of northern Switzerland, yielded >6 km of drillcore. One of the main objectives was to characterise the low-permeability sequence with the lower Jurassic Opalinus Clay in its centre, motivated by the site selection for a deep geological repository for radioactive waste. In this context, the chemical and isotopic composition of the porewater, as well as the mineralogical and petrophysical properties of the rocks, were among the main study targets. In this paper, the main objective was the characterisation of the lithologically diverse Jurassic rock sequence with focus on mineralogy, porosity and pore-space architecture. Given the large amount of data, a well constrained relationship between clay content (relating to fine-grained sheet-silicate minerals) and porosity could be established. Porosity increases with clay content along a concave curve, but in detail minor formation-specific differences of the positions relative to the best-fit curve were identified. These are attributed to the highly variable deposition rates that resulted in different times available for early diagenesis to alter the rock fabric by mineral dissolution and cementation, thereby affecting the compaction behaviour. Pore-size distributions were obtained from N2 ad-/desorption isotherms. A distinct peak at a radius of 3 nm can be clearly correlated with clay minerals, whereas limestones are dominated by pore sizes in the range of 40–100 nm, and marls show intermediate distributions. A conceptual framework is proposed distinguishing a nanometric porosity that is proportional to the clay content and a contribution of larger pores that are related to the geometric incompatibility between platy clay minerals and isometric calcite or quartz. The contribution of these larger pore is thought to explain the curvature of the clay content-porosity relationship and the more limited compaction of the clay in pressure shadows adjacent to the larger grains. A number of outliers towards high porosity at a given clay content were identified in oolites (most strongly in Fe-rich oolites), sandstones and a unit containing coral-reef material. All these units have in common the presence of competent calcite or quartz grains at the time of deposition, leading to grain-supported fabrics and therefore to a more limited compaction in the interstitial pore space.

Veins in clay-rich aquitards as records of deformation and fluid-flow events in northern Switzerland

The Jurassic clay-rich aquitard sequence underlying the Swiss Molasse Basin is locally penetrated by unusually frequent veins consisting of calcite ± celestite, in particular in the Schlattingen-1 and Oftringen drill cores. The veins are linked to extensional brittle structures, with shear senses ranging between dip-slip and strike-slip. Deformation triggered transient and localised events of fluid flow within the aquitard followed by sealing of the structures by the vein minerals. Vein calcite was dated by LA-ICP-MS analysis using the U/Pb method. While this was a demanding undertaking, given the low U and high common Pb contents of calcite, 9 significant ages could be obtained. Two groups of ages can be distinguished, namely 31–39 and 11–18 Ma. Both age groups are consistent with the known regional tectonic evolution. The older age group corresponds to the main phase of rifting in the Upper Rhine Graben, a major crustal discontinuity, whose effects were not limited to the graben proper but affected large parts of the Molasse Basin. The younger age group is linked to crustal tilting and related extensional deformation.Isotopic compositions of O and C in vein calcite from the Schlattingen-1 core show substantial variability but do not discriminate between veins of the two age groups. In contrast, 87Sr/86Sr ratios in veins show different signatures. The depth trend of 87Sr/86Sr ratios in veins fits well with that in pore water obtained by rock squeezing. The two data sets define a trend of values increasing with depth, connecting the current values in the embedding Malm and Keuper aquifer waters. This trend is best explained as a near-steady-state diffusion profile. While advective mixing of waters originating from the embedding aquifers to produce the veins, implying structures that cross cut the whole low-permeability sequence, cannot be discarded on the basis of existing data, it is considered more likely that the veins have local fluid sources. These are mixtures of local pore water with a possible contribution of water from the nearest aquifer, implying maximum vertical advection distances in the order of 50–100 m.

Pore-water evolution and solute-transport mechanisms in Opalinus Clay at Mont Terri and Mont Russelin (Canton Jura, Switzerland)

Data pertinent to pore-water composition in Opalinus Clay in the Mont Terri and Mont Russelin anticlines have been collected over the last 20 years from long-term in situ pore-water sampling in dedicated boreholes, from laboratory analyses on drillcores and from the geochemical characteristics of vein infills. Together with independent knowledge on regional geology, an attempt is made here to constrain the geochemical evolution of the pore-waters. Following basin inversion and the establishement of continental conditions in the late Cretaceous, the Malm limestones acted as a fresh-water upper boundary leading to progressive out-diffusion of salinity from the originally marine pore-waters of the Jurassic low-permeability sequence. Model calculations suggest that at the end of the Palaeogene, pore-water salinity in Opalinus Clay was about half the original value. In the Chattian/Aquitanian, partial evaporation of sea-water occurred. It is postulated that brines diffused into the underlying sequence over a period of several Myr, resulting in an increase of salinity in Opalinus Clay to levels observed today. This hypothesis is further supported by the isotopic signatures of SO42− and 87Sr/86Sr in current pore-waters. These are not simple binary mixtures of sea and meteoric water, but their Cl− and stable water-isotope signatures can be potentially explained by a component of partially evaporated sea-water. After the re-establishment of fresh-water conditions on the surface and the formation of the Jura Fold and Thrust Belt, erosion caused the activation of aquifers embedding the low-permeability sequence, leading to the curved profiles of various pore-water tracers that are observed today. Fluid flow triggered by deformation events during thrusting and folding of the anticlines occurred and is documented by infrequent vein infills in major fault structures. However, this flow was spatially focussed and of limited duration and so did not markedly affect the bulk pore-water.

北海道幌延地域における深部地球化学環境の長期変遷シナリオの構築

A methodology to estimate long-term hydrochemical evolution of deep underground is indispensable for the safe geological isolation of high-level radioactive waste (HLW). This study demonstrates the methodology by illustrating scenarios of past geological events, processes, and their interrelationships with present-day hydrochemical conditions. Besides, we infer long-term variations of groundwater chemistry at Horonobe, Hokkaido, Japan. The region is underlain mainly by Neogene to Quaternary marine sedimentary rocks, (the Wakkanai Formation (Fm) and the overlying Koetoi Fm: siliceous and diatomaceous mudstones). During various events in the geological past, such as deposition, compaction, uplift, and denudation, and the more recent Neotectonic activities in this area, highly permeable hydrogeological structures formed at depths not over than 400m below ground level in the Wakkanai Fm. The hydrogeological system can be subdivided into three hydrogeological sub-systems: (1) overlying, relatively low permeability Koetoi Fm., (2) highly permeable, upper Wakkanai Fm at depths less than 400m and (3) relatively low permeability Wakkanai Fm, at depths greater than 400m. The present-day hydrochemical conditions in each sub-system have been influenced by hydrogeological properties and hydraulic conditions over a long period. In subsystems 1 and 2, recharging with meteoric water flushed connate seawater during uplifting and denudation during the last 1Ma. In contrast, fossil seawater with one-third to one-half the salinity of present-day seawater has been preserved in subsystem 3. The relatively low permeability sequence in sub-system 3 was formed by the compaction of diatomaceous mudstone during subsidence prior to 1.0Ma. After that, changes of climatic conditions and geographical features would not have influenced groundwater flow. The groundwater chemistry evolved from seawater during long-term geochemical diagenesis in a relatively closed system. The long-term variations of salinity probably range from that of seawater to that of present-day groundwater. The occurrence of secondary minerals shows that the buffer reactions of carbonate and sulphide minerals have preserved the near neutral pH and reducing condition since initial diagenesis immediately after deposition of the rock formation.

Applying MODFLOW to wet grassland in-field habitats: a casestudy from the Pevensey Levels, UK

Abstract. Historical drainage improvements have created complex hydrological regimes in many low-lying, wet coastal grassland areas. The manipulation of ditch water levels is a common management technique to maintain important in-stream and in-field habitats in such areas. However, in wet grasslands with low soil conductivities the water table in the centre of each field is not closely coupled to variations in ditch stage. Consequently rainfall and evaporation have a greater influence on the depth to water table and water table fluctuations within each field. In-field micro-topographic variations also lead to subtle variations in the hydrological regime and depth to water table that create a mosaic of different wetness conditions and habitats. The depth, duration, timing and frequency of flooding from accumulated rainfall, surface water and standing groundwater also influence the availability of suitable in-field habitats. Land drainage models are often used for studies of wet grasslands, but tend to be more complex and require more field variables than saturated zone models. This paper applies a 3D groundwater flow model, MODFLOW, to simulate groundwater levels within a single field in a wet coastal grassland underlain by a low permeability sequence and located in the central part of Pevensey Levels, Sussex, UK. At this scale, the influence of vertical leakage and regional groundwater flow within the deeper, more permeable part of the sequence is likely to be small. Whilst available data were not sufficient to attempt a full calibration, it was found that the sequence could be represented as a single, unconfined sequence having uniform hydraulic properties. The model also confirmed that evaporation and rainfall are the dominant components of the water balance. Provided certain information requirements are met, a distributed groundwater model, such as MODFLOW, can benefit situations where greater hydrological detail in space and time is required to represent complex and subtle changes influencing the in-field habitats in wet grasslands with low permeability soils. Keywords: wetlands, hydrology,groundwater, MODFLOW

Disposal of radioactive wastes in argillaceous formations

Abstract One option presently under investigation internationally for the disposal of the hazardous by-products of the nuclear fuel cycle is the construction of underground repositories in low-permeability argillaceous rock formations. The host-rock represents the most important barrier to the migration of radionuclides to the surface environment. The performance of this geological barrier will depend on a wide variety of factors, including the degree of compaction and diagenetic alteration of the rock during burial, affecting plasticity and response to deformation, stress history which determines some important structural attributes (faults, fissures, etc.), mineralogy and chemistry which are primary factors influencing radionuclide sorption and mobility, and the thickness and lithological variability of the low-permeability sequence. Our paper provides a brief overview of these considerations and identifies a number of areas of uncertainty, including our, as yet, incomplete knowledge of the precise mechanisms of groundwater movement in clay-rich media, possible departures from Darcy's Law, the occurrence of long-term transient flow, the role of ‘coupled flow’ phenomena, and the difficult problem of gas migration within the host-formation.

Emplacement of Bitumen (Asphaltite) Veins in the Neuquen Basin, Argentina

Veins of solid bitumen (asphaltite) have been commercially exploited in the Neuquen basin, Argentina, for over 100 yr. Veins are up to 5 m wide and several kilometers in length, over a region of 15,000 km2. These veins were emplaced in fractures both parallel and at high angles to bedding, in close proximity to their source rocks in the Vaca Muerta and Agrio formations (Late Jurassic-Early Cretaceous). Two or more phases of bitumen emplacement can be recognized in several localities; structures bearing viscous oil are younger than structures having solid bitumen. Bitumen emplacement was vigorous and caused brecciation and spalling of the host rocks. The bitumen was also viscous, and supports rock debris ranging in size from sand grains up to meter-scale slabs. Brecciation, bedding-parallel injection, and wall rock impregnation suggest high fluid pressures during emplacement. High fluid pressure may have been engendered by substantial hydrocarbon generation from rich source rocks in a low-permeability sequence, and probably caused the fractures into which the bitumen migrated. The bedding-parallel veins facilitated decollement during thrusting that took place during and after bitumen emplacement. The timing of emplacement relative to thrusting and oil migration constrains bitumen emplacement to the Eocene-Oligocene.

Controls on Porosity and Permeability Distribution in the Southern Cooper Basin, Australia: Implications for Petroleum Exploration

The Cooper basin, containing fluvio-lacustrine sediments of Permo-Triassic age, is one of Australia's major onshore hydrocarbon provinces, with sales gas reserves of about 6 tcf and gas-liquids and oil reserves of 310 MMSTB. It is characterized by dominantly low-porosity, low-permeability reservoir sandstones occurring at burial depths between about 1.8 to 3.6 km subsea. Average core porosity is 10.7% and permeability is 30 md. However, over 75% of sandstones have permeabilities less than 5 md. An early phase of silicification has mainly suppressed mechanical compaction in moderate to well-sorted sandstones of point bar origin. These reservoirs are prominent on major mid-flank structures and marginal areas of the basin. Toward the interior of the basin, microporosity associated with kaolin clays becomes predominant, accounting for the generally poor quality reservoir characteristics of most Cooper basin sandstones. Significantly, two the largest gas fields in the basin, Moomba and Big Lake, produce from reservoirs dominated by microporosity. Log-derived porosity estimates may overestimate effective porosity in sandstones containing kaolin minerals. Microporosity is likely to hold relatively large irreducible water, affecting hydrocarbon reserve calculations. The spatial distribution of core porosity and permeability is variable across the basin. The permeability relationship can be attributed to grain packing andmore » orientation, differential cementation, and sedimentary facies. However, sandstones of good reservoir quality still occur within low-permeability sequences, offering deep targets for petroleum exploration. The study used a variety of tools, including optical petrography, X-ray diffraction, scanning electron microscopy, and cathodoluminescence. The results were integrated with an excess of 6000 porosity and permeability values derived from 148 field and wildcat wells.« less

Thermal Maturity, Porosity, and Facies Relationships Applied to Gas Generation and Production in Tertiary and Cretaceous Low-Permeability Sandstones, Uinta Basin, Utah

Uinta basin rocks include a thick sequence of low-permeability (tight) sandstones that contain a large volume of gas. Most known gas accumulations occur in the eastern part of the basin in the Upper Cretaceous Mesaverde Group; uppermost Cretaceous to early Eocene North Horn Formation; and the Paleocene and Eocene Wasatch, Colton, and Green River formations. Most successful tight-gas completions are in Tertiary strata, but rocks in the underlying, largely undrilled Mesaverde Group in the north-central part of the basin are potential targets as well. To characterize the gas-bearing sandstones, the authors have constructed a series of interrelated maps and cross sections showing thermal maturity, porosity, fluid pressures, and lithofacies relations for the Uinta basin. Vitrinite reflectance (R{sub m}) for each of these low-permeability sequence increases to the north and levels of thermal maturation are sufficient for gas generation over large areas of the basin. Projection of maturity values and fluid-pressure data into undrilled parts of the basin reveals the probability of a regional, overpressured, basin-entered gas accumulation, where gas is likely to be generating at present. Plots of porosity vs. R{sub m} allow prediction of porosity for Mesaverde sandstones in unexplored areas of the basin. The authors data indicates amore » large area of organic-rich, overpressured strata probably underlies the northern part of the Uinta basin. Wells drilled in the Mesaverde and lower part of the Tertiary, in the areas where R{sub m} at the base of the Mesaverde is greater than 1.1%, should have the best potential for gas production. Overpressured gas reservoirs are likely to have no free water and be enveloped by successive zones of mixed water and gas and of water only.« less