Permeable Lithologies Research Articles

CAI and microtextures of low-grade metamorphosed conodonts related to lithological and geological controls

Textures, microtextures, and CAI of early Permian conodonts recovered from three sections of the La Cueva Limestone and the Mina México Formation from Sonora, northwestern Mexico, are herein reported. Nearly uniform values of CAI 5 (a few at CAI 5.5) indicate low-grade metamorphic conditions in the study area. Metamorphic effects in conodonts are demonstrated by the development of the granular microtexture (recrystallization). It is documented herein that facies of reduced porosity and permeability due to compaction and cementation yielded the most recrystallized conodonts. Observations have revealed a correlation between the external cast microtexture and the micrite crystals found in mudstone to packstone lithologies, with no exceptional requirements for sorting, porosity, and permeability for its development. Furthermore, it has been noted that the development of small and large apatite crystal microtextures primarily occurs in highly porous and permeable lithologies, such as packstone and grainstone. Additionally, most of the sparry calcite cements appear to precipitate after the formation of the apatite overgrowths microtextures during intermediate and deep burial diagenesis. These results on conodont microtextures are useful in the recognition of regional tectonothermal events.

Preliminary identification of areas suitable for Sustainable Drainage Systems and Managed Aquifer Recharge to mitigate stormwater flooding phenomena in Rome (Italy)

This paper proposes a preliminary and large-scale survey methodology to identify areas suitable for indepth analysis for the application of Sustainable Drainage Systems and Managed Aquifer Recharge. These techniques are frequently applied to increase the natural infiltration capacity of water into the ground and their effectiveness depends on the local hydrogeological and morphological characteristics. The study area is the city of Rome where the aim is to mitigate the problems related to rainwater which, in case of extreme events, struggles to infiltrate into the ground, overloads the undersized drainage systems, and floods the urban space.The proposed method involves GIS geospatial analysis of the permeability of outcropping lithologies, the digital elevation model, and the piezometric levels of the aquifers. To identify the suitable zones, areas characterised by high permeability and a piezometric level that would confer a volumetric capacity to possibly store even large quantities of water, without triggering possible problems of water table rise, were identified. Data were divided into classes and indexed to compare and overlap them. Furthermore, the final result was compared with the urban flooding phenomena and the soil permeability map of Rome. The results of the performed analysis show that the preliminary suitable conditions to apply SuDS and MAR in Rome are widespread. The geological setting of the city is characterised by permeable lithologies in many places with an effective infiltration potential that would allow rainwater to infiltrate the subsoil and reach the first available aquifer.

Mapping the spatial distribution of fossil geothermal manifestations and assessment of geothermal potential of the Tangyin rift, Southeast of Taihang Mountain in China

The spatial prediction of geothermal sites along the southeast side of Taihang Mountain in the Tangyin rift is a critical goal in the development of renewable energy resources in cities such as Hebi in Henan province, which has significant potential for geothermal energy production due to the existence of deep faults and Cenozoic volcano. The study aimed to delineate areas with high geothermal manifestations for detailed geothermal exploration and well-test drilling. To define favorable zones for geothermal potentiality, multi-source datasets were chosen to generate evidence thematic layers that demonstrate the distribution of geothermal anomalies such as geologic maps, structural maps, Land Surface Temperature maps (LST), and hydrothermal mineral alterations maps. The remote sensing and Geographical Information System (GIS) approaches were integrated to map fossil geothermal manifestations and appraise the promising geothermal zones from Landsat 8 OLI/TIRS and Advanced Spaceborne Thermal Emission and Reflection Radiometer-Digital Elevation Model (ASTER-DEM) dataset using the Analytic Hierarchy Process (AHP), which is one of the Mult-criteria Decision Making methods. Results show LST values ranging from 22.6°C to 45.4°C, with the highest values found along the faults and volcanic areas. The hydrothermal alteration minerals discriminated from band ratios of 4/2, 6/5, and 6/7 for iron oxide, ferrous minerals, and clays reveal the presence of high amount of montmorillonite and illite minerals along the faults and volcanoes which are related to hot fluid circulation from subsurface. Structural analysis shows that the major structural trend follows the NNE Tangdong and Tangxi faults, with high density on the west side of the Tangxi fault where there is good exposure of outcrop but less fault density on the east side due to sedimentation in a low land which may conceal geological structures. Lastly, all the thematic layers of geothermal factors were reclassified, ranked, and integrated by the weight overlay logic model based on the influence of geothermal suitability to map potential areas for geothermal resources whereby 5 zones were obtained ranging from very poor to very good. The promising potential sites are mostly found along the highly deep structures of the major Tangxi fault crosscutting the permeable lithology of carbonate rocks and around the Hebi volcanoes. The distribution of hot springs and geothermal wells of the Hebi field were correlated and verified against the potential geothermal anomalies map. The overall integrated method proposes an advanced technique for designing the Tangyin rift’s exploration plan and development actions for geothermal resources utilization, which can be used for exploration of any unveiling geothermal potential sites.

Multi-criteria decision support for geothermal resources exploration based on remote sensing, GIS and geophysical techniques along the Gulf of Suez coastal area, Egypt

The spatial prediction of geothermal sites along the Gulf of Suez coastal area, is an essential goal as an attempt for the development of renewable energy sources in Egypt. Remote sensing, geophysical methods and Geographic Information System (GIS) were integrated to appraise the promising geothermal zones. Hot-springs and wells data were collected initially in database for modeling the potential zones through the Bayesian statistical approach. Multi-source datasets were selected to derive factor layers indicating the distribution of geothermal energy such as topographic and geologic maps, Bouguer gravity anomaly and aeromagnetic reduced-to-the-pole maps, seismic data, satellite images, earthquake activities, and Bottom Hole Temperature (BHT) from oil and gas wells. Eight evidence layers were derived, analyzed, and integrated in a GIS platform to develop a Geothermal Potential Model (GPM). The representative ranks and weights were estimated to the spatial evidence layers and their classes respectively based on information value (IV) and weights-of-evidence (WoE) analyses. The eight evidence layers include proximity to major fault, lineaments density, land surface temperature, density of seismic activities, heat flow, temperature gradient at different depths, Curie point depth, and Bouguer anomaly. A geothermal potential map was created and classified the study area into five relatively zones, varied from very poor to very good potential. The very good potential sites are distributed mainly in the highly deep-structured and permeable lithologies. The new promising sites are expected to be in Ras Matarma, Ras Badran, Abu Rudies, Belayim, Abu Durba, El-Tur and some sites in southwestern shoreline of Gulf of Suez. The geothermal potential map was correlated and tested against the distribution of thermal wells and hot-springs. The holistic integrated approach suggests an innovative technique to design management plan and development actions along Gulf of Suez coastal area.

Self Potential (SP) and Resistivity Methods of Discriminating Lithologies in a Landslide Areas of Ohafia Town, Nigeria

This research presents the application of geophysical self-potential (SP) and resistivity methods in evaluating subsurface phenomenon in Ohafia Area. Two communities were covered with three profile lines of 200 m spread occupied in each location. The data were acquired using ABEM SAS 300C Terrameter and later processed with EarthImager 1D inversion modelling software. Self-Potential values ranged from -107.8 to 112.3 mV in Amaekpu and -100 to 150 mV in Elu area. Positive SP values indicated lithologies with low permeability while negative SP values showed permeable lithologies. Six geologic layers were inferred for Amaekpu area while seven layers were inferred from Elu area. Layer 1, 3 and 5 are low permeable lithologies while Layer 2, 4 and 6 are permeable intervals in Amaekpu. In Elu, Layers 1, 3, 5 and 7 are having low permeability while Layers 2, 4 and 5 are highly permeable lithologies. Lithologic intervals inferred from inverted resistivity data which include sands, sandstones and gravels conformed with regions having negative SP values. Similarly, depth intervals identified as topsoil, siltstones and clay conformed with positive SP values.

Comparative field study of shallow rhyolite intrusions in Iceland: Emplacement mechanisms and impact on country rocks

Shallow silicic intrusions are known to exist in many active volcanoes and can fuel both eruptions and hydrothermal fields. However, our knowledge of magma intrusions remains far from complete, and processes occurring at intrusion margins are poorly understood. In this field-based study, we characterise four shallows, dissected rhyolitic intrusions at three sites in Iceland (Njarðvík-Dyrfjöll, Krafla and Húsafell central volcanoes). We focus on the relationship between intrusion emplacement mechanisms and country rock response, employing scanline mapping of fractures and in-situ rock property measurements (hardness and permeability) along transects from the intrusion margins to damaged and undamaged country rocks.We identify various scenarios of shallow intrusion emplacement style, based upon their diverse geometry and lithofacies architecture. Additional information from rock properties and characteristics of fractures and vesicles, indicates that initial country rock properties strongly influence the emplacement style. We identify two discrete types of country rock response to magma injection. The matrix permeability of weak, porous and permeable lithologies (conglomerate and hyaloclastite) is reduced by >1 order of magnitude adjacent to intrusions due to pore occlusion. Stronger and denser, low-permeability lithologies (basalt and welded ignimbrite) undergo a decrease in hardness by a factor >2 related to an up to fivefold increase in fracture density, with no significant change in matrix permeability.Our observations highlight the importance of robust characterisation of the mechanical properties of caldera-filling or geothermal reservoir formations, for appropriate forecasting of magma mobility, geophysical data interpretation, and geothermal resources characterisation.

Thermal history and basin evolution of the Moatize - Minjova Coal Basin (N'Condédzi sub-basin, Mozambique) constrained by organic maturation levels

Kerogen concentrates obtained from Lopingian (Late Permian) to Upper Triassic mudrock lithologies of seven coal exploration boreholes, drilled in the Moatize – Minjova Coal Basin (N'Condédzi sub-basin, Mozambique), were studied by means of vitrinite reflectance (VR), spore fluorescence and spore colour, in order to constrain the thermal history and basin evolution by organic maturation levels. VR increases with depth, indicating organic maturation related to sediment burial for most of the boreholes. Modelled VR data indicate a regional palaeogeothermal gradient between 35 and 40 °C/km. Lower Jurassic doleritic intrusions observed in three boreholes had only local thermal effects without affecting the regional palaeogeothermal gradient. Two boreholes located near the basin margin show high palaeogeothermal gradients suggesting thermal processes other than heating due to burial were involved. These processes may have involved hot diagenetic fluids circulating through fault zones and/or permeable lithologies, locally elevating geothermal gradients. Circulation of these fluids was induced by lithostatic pressure due to rapid rates of sedimentation. These high sedimentation rates lead to the accumulation of a thick succession (over 2000 m) of Lopingian (Late Permian) to Upper Triassic siliciclastic sediments. All the organic maturation indices measured and the age of the successions indicate that organic maturation occurred during or after Late Triassic times. However, the presence of reworked Permian palynomorphs into Upper Triassic sediments and the absence of Middle Triassic sediments indicate an exhumation and erosion of Permian strata in Middle Triassic times. The organic maturation levels of the reworked palynomorph population are considerably higher than the indigenous Upper Triassic population, indicating that they attained higher burial temperatures prior to being reworked.

Fluids Along the Plate Interface Influencing the Frictional Regime of the Chilean Subduction Zone, Northern Chile

AbstractThe plate interface beneath the Mejillones Peninsula in Northern Chile is characterized by anomalous seismogenic behaviors, with seismic and aseismic slip, and low coupling values. We analyze this zone through the seismicity pattern and a 3‐D tomography model. We identify high VP/VS values within the oceanic crust and in the lower continental crust, which we interpret as hydrated zones rich in fluids. These zones are correlated with the Mejillones fracture zone and with highly permeable lithologies of the lower continental crust, which allow a greater accumulation of fluids at the plate interface beneath the Mejillones Peninsula. Additionally, these areas exhibit a high rate of seismicity and concentrated swarms and repeaters. We propose that the presence of fluids controls the anomalous seismogenic behavior along the plate interface beneath the Mejillones Peninsula.

The 2013/14 Thames Basin Floods: Do Improved Meteorological Forecasts Lead to More Skillful Hydrological Forecasts at Seasonal Time Scales?

Abstract The Thames basin experienced 12 major Atlantic depressions in winter 2013/14, leading to extensive and prolonged fluvial and groundwater flooding. This exceptional weather coincided with highly anomalous meteorological conditions across the globe. Atmospheric relaxation experiments, whereby conditions within specified regions are relaxed toward a reanalysis, have been used to investigate teleconnection patterns. However, no studies have examined whether improvements to seasonal meteorological forecasts translate into more skillful seasonal hydrological forecasts. This study applied relaxation experiments to reforecast the 2013/14 floods for three Thames basin catchments with different hydrogeological characteristics. The tropics played an important role in the development of extreme conditions over the Thames basin. The greatest hydrological forecasting skill was associated with the tropical Atlantic and less with the tropical Pacific, although both captured seasonal meteorological flow anomalies. Relaxation applied over the northeastern Atlantic produced confident ensemble forecasts, but hydrological extremes were underpredicted; this was unexpected with relaxation applied so close to the United Kingdom. Streamflow was most skillfully forecast for the catchment representing a large drainage area with high peak flow. Permeable lithology and antecedent conditions were important for skillfully forecasting groundwater levels. Atmospheric relaxation experiments can improve our understanding of extratropical anomalies and the potential predictability of extreme events such as the Thames 2013/14 floods. Seasonal hydrological forecasts differed from what was expected from the meteorology alone, and thus knowledge is gained by considering both components. In the densely populated Thames basin, considering the local hydrogeological context can provide an effective early alert of potential high-impact events, allowing for better preparedness.

Spectroscopic studies of dissolved organic matter in a heavily modified Mediterranean and ancient coastal lake

DOM has a vital role in the environmental fate of numerous contaminants including trace metals and organic compounds. The aim of this research is the spectroscopic characterization of the DOM of sediment samples collected from the shoreline of a coastal and heavily modified Mediterranean lake, the appraisal of DOM nature and origin, the evaluation of impacts from lithology and land uses, the comparison of the relative effectiveness of spectroscopic approaches for chemical classification and the final combination of the results provided from each approach. UV–Vis absorption spectra suggest the presence of low molecular weight and a photochemical bleaching of chromophoric DOM. Spectral slope ratio had a negative correlation with autochthonous production and DOM molecular weight. Fluorescence spectra showed that DOM is mainly freshly produced and has an autochthonous origin. Chromophores are of a rather simple structure deriving from the degradation of plant components. The contribution of submerged springs in organic matter input was uncovered. FT-IR spectroscopy revealed the presence of aliphatic and aromatic chains. The presence of carboxylic, phenolic, alcoholic and polysaccharide groups was supported. A correlation of recent autochthonous material with relative depletion in hydroxyl, carboxyl, alcoholic and polysaccharide groups was found. The morphological particularity of the area with its permeable lithology and the presence of karstic springs, in combination with the land uses of the catchment area hosting heavy industrial, agricultural and urban activities, in addition to the high archaeological and ecological importance of the wetland and its surrounding area, may require the application of a sound management and environmental protection scheme.

Controls on hydrothermal fluid flow in caldera-hosted settings: Evidence from Lake City caldera, USA

Silicic caldera volcanoes are often associated with hydrothermal systems economically important for electricity generation and localization of ore deposits. Despite their potential importance, the poor exposure that is typical in caldera settings has limited the number of detailed studies of the relationship between caldera structures and fluid flow. We use field mapping, outcrop scale scanline transects, and petrographic analyses to characterize fault rocks, alteration, and veins in the well-exposed 22.9 Ma Lake City caldera fossil hydrothermal system. The caldera margin consists of relatively straight segments linked by more structurally complex intersections; these structural intricacies produce a zone of deformation that can reach >300 m wide. Structural analyses show that the wide (up to ∼60 m) fault core of the ring fault contains abundant subparallel veins, with orientations similar to that of the caldera margin. Smaller displacement faults inside the caldera generally have narrow (<1 m), hydrothermally cemented fault cores with more variably oriented veins in the surrounding damage zone. These findings at Lake City illustrate that fluid flow is controlled by lithology and the location and displacement of faults, e.g., ring fault versus intracaldera fault. Fault connectivity is another key control. We propose a conceptual model where fluid flow in caldera-hosted settings is influenced by: (1) the presence of favorable lithologies (proximity to magmatic intrusions and/or the presence of permeable lithologies), (2) a high density of faults and fractures, and (3) favorable orientations of faults and fractures that promote the formation of discontinuity intersections.

Groundwater level responses to precipitation variability in Mediterranean insular aquifers

Groundwater is one of the largest and most important sources of fresh water on many regions under Mediterranean climate conditions, which are exposed to large precipitation variability that includes frequent meteorological drought episodes, and present high evapotranspiration rates and water demand during the dry season. The dependence on groundwater increases in those areas with predominant permeable lithologies, contributing to aquifer recharge and the abundance of ephemeral streams. The increasing pressure of tourism on water resources in many Mediterranean coastal areas, and uncertainty related to future precipitation and water availability, make it urgent to understand the spatio-temporal response of groundwater bodies to precipitation variability, if sustainable use of the resource is to be achieved. We present an assessment of the response of aquifers to precipitation variability based on correlations between the Standardized Precipitation Index (SPI) at various time scales and the Standardized Groundwater Index (SGI) across a Mediterranean island. We detected three main responses of aquifers to accumulated precipitation anomalies: (i) at short time scales of the SPI (<6months); (ii) at medium time scales (6–24months); and at long time scales (>24months). The differing responses were mainly explained by differences in lithology and the percentage of highly permeable rock strata in the aquifer recharge areas. We also identified differences in the months and seasons when aquifer storages are more dependent on precipitation; these were related to climate seasonality and the degree of aquifer exploitation or underground water extraction. The recharge of some aquifers, especially in mountainous areas, is related to precipitation variability within a limited spatial extent, whereas for aquifers located in the plains, precipitation variability influence much larger areas; the topography and geological structure of the island explain these differences. Results indicate large spatial variability in the response of aquifers to precipitation in a very small area, highlighting the importance of having high spatial resolution hydro-climatic databases available to enable full understanding of the effects of climate variability on scarce water resources.

Contribution of Geophysics to the Management of Water Resources: Case of the Ariana Agricultural Sector (Eastern Mejerda Basin, Tunisia)

In Tunisia, the Mejerda basin is the most important agricultural sector. Identification of additional water sources and good management of current and new resources is required to maintain and increase its productivity. The present study concerns the Ariana region (north-east of Tunisia) covering the eastern part of Mejerda basin. Lithological columns, well logs, vertical electrical soundings, and gravity data were analyzed and interpreted to give more precise information for exploitation of the water resources of the Ariana region. Lithological columns, well logs, and geoelectrical models reveal that the permeable Quaternary alluvial deposits, the Upper Campanian–Lower Maestrichtian, Albian and Barremian limestones, and Valanginian–Hauterivian sandstones are the main aquifers in Ariana. Comparison of geoelectrical cross sections to the Bouguer anomaly map of the Ariana region reveals tectonic influence on the geometry of the aquifers. The folded structures attributed to the Tortonian compressive stress and the tectonic movements in N–S, E–W, NW–SE, and NE–SW directions have resulted in the compartmentalization of Ariana’s aquifers into subsided and raised blocks. This reconstituted geometry of the region influenced the depth of burial of permeable lithologies and inhibited groundwater circulation in some localities.

Looking through drumlins: testing the application of ground-penetrating rada

AbstractGround-penetrating radar (GPR) is becoming a commonly applied technique in geomorphology. However, its use in the study of subglacial bedforms has yet to be fully explored and exploited. This paper presents the results of a GPR feasibility study conducted on a drumlinized terrain in Cumbria, UK, where five drumlins were investigated using multiple radar antenna frequencies. The site was selected for the presence of nearby bedrock outcrops, suggesting a shallow drumlinized diamict–bedrock contact and a permeable lithology. Despite the clayey sediment and unfavourable weather conditions, a considerable penetration depth of ~12 m was achieved when using a 50 MHz antenna, with a separation of 1 m, trace spacing of 1 m and 128-fold vertical stack. Results indicate that the drumlinized diamict is in direct erosional contact with the bedrock. While the internal drumlin geometry is generally chaotic on the stoss side, evidence of layering dipping downflow at an angle greater than the drumlin surface profile was found on the lee side. The inter-drumlin areas comprise ~4 m of infill sediment that masks part of the original drumlin profile. Overall, this study indicates that GPR can be deployed successfully in the study of glacial bedform sedimentary architecture.

Conditions and implications for compaction band formation in the Navajo Sandstone, Utah

Observations from quartz-rich eolian Navajo Sandstone in the Buckskin Gulch site in southernmost Utah show that pure compaction bands only occur in sandstones where current porosity > 0.29 ± 3, permeability > 10 ± 7 darcy, and grain size > 0.4 mm – properties restricted to the lower and most coarse-grained and well-sorted parts of grain flow units within the dune units. Hence a direct correlation between stratigraphy and band occurrence has been established that can be used to predict deformation band occurrences in similar sandstone reservoirs. We show that the pure compaction bands formed perpendicular to a subhorizontal σ 1, bisecting conjugate sets of shear-enhanced compaction bands. The latter bands locally developed into shear-dominated bands that transect entire dune units, suggesting that an increase in the amount of simple shear promotes band propagation into less porous and permeable lithologies. Stress considerations indicate that, as a continuous and overlapping sequence of events, pure compaction bands in quartz-rich Navajo Sandstone initiated at 10–20 MPa (∼1 km depth), followed by shear-enhanced compaction bands that locally developed into more stratigraphically extensive shear-dominated bands. The rare combination of special lithologic and stress conditions may explain why pure compaction bands are rarely observed in naturally deformed sandstones.

Significance of retrograde hydration in collisional metamorphism: A case study of water infiltration in the Kokchetav ultrahigh-pressure metamorphic rocks, northern Kazakhstan

Petrological analysis reveals the volume of aqueous fluids infiltrated into high-pressure–ultrahigh-pressure metamorphic (HP–UHPM) rocks during exhumation of the Kokchetav massif, Kazakhstan. The mode and extent of retrograde hydration in the massif are highly variable depending on lithology. The volume of aqueous fluids infiltrated during retrograde hydration was calculated based on a comparison of the peak and present mineral assemblages obtained by observations and thermodynamic calculations. Eclogite and metapelite absorbed about 1.0 wt.% and 0.2 wt.% of water during retrograde metamorphism, respectively, whereas orthogneiss and leptite showed no change in water content despite nearly complete retrograde recrystallisation. Based on the change in water content and the spatial coverage of each lithology, the volume of aqueous fluids infiltrated during retrograde metamorphism of the HP–UHPM unit was calculated as 0.1% of the total mass of the massif. This amount of fluid is fully covered by the dehydration of the underlying Daulet Suite during contact metamorphism by the emplacement of the HP–UHPM unit at the mid-crustal level. Our results suggest that the fluid infiltration at the exhumation stage is tectonically inevitable, and even a small amount of water infiltration less than 1 wt.% effectively enhances retrograde recrystallisation in permeable lithologies such as pelitic and felsic schists/gneisses.

Modelling Primary Hydrocarbon Expulsion in a Synthetic Sedimentary Section: Relative Contributions of Edge Conditions and other Geological Factors

The effects of numerical edge conditions and of the relative contributions of geological processes or factors to primary hydrocarbon migration are studied using the GEOPETII program developed by the Basin Modeling Group at USC. Lateral edge conditions can have important effects on the results of modelled sedimentary sections. The absolute mass of an oil or gas accumulation varies with the section length modelled, while more vertical grids (pseudo-wells) along a section result in less lateral continuity of accumulated oil and gas. A modelled section with a highly permeable lithology at the two lateral edges produces lower excess pressure between the three left-edge pseudo-wells and between the three right-edge pseudo-wells, and tends to have less oil and gas accumulated in the section center. If the modelled section has a low-permeability lithology at the two edges, the opposite effect is produced. A modelled section which has a thick basal layer, high permeability, and relatively few horizontal subdivisions produces less excess pressure in a shale source rock above the basal layer, and has smaller oil and gas accumulations in a reservoir unit above the source rock. By contrast, a modelled section with more subdivisions, and a smaller time-step for each layer calculation, produces higher excess pressures and tends to have more oil and gas accumulated in reservoirs above the source rock. Accordingly, the ratio of the thickness deposited in a time-step to the subdivision thickness used in the calculation should be kept small (less than 0.4 for the code used here). Twelve major processes or factors control oil and gas expulsion: (1) buoyancy; (2) the solubility of hydrocarbons in connate waters; (3) the saturation of irreducible water in a source rock; (4) capillary pressure; (5) critical pressure for fracturing a source rock; (6) the viscosity of hydrocarbons; (7) the relative permeabilities of oil and gas; (8) the lithology-dependent permeability ratio in the vertical to horizontal direction; (9) the total organic carbon in the source rock; (10) the kerogen type; (11) the ages of the source-rock strata; and (12) the palaeoheat flux of the area. The Shongliao Basin, China has been intensively drilled, and two volumes of downhole data are available. Those data were used to set limits on the ranges of parameter values and on geological factors. In this way, an evaluation is given of the relative importance of the twelve contributions influencing primary hydrocarbon expulsion. Both separate-phase gas-expulsion efficiency and the absolute mass amount of gas expelled from the source rock increase with increases in: buoyancy, hydrocarbon saturation in irreducible water in the source rock, capillary pressure, relative permeability, total organic carbon, source-rock age, and palaeoheat flux. However, gas expulsion efficiency and the amount of gas expelled decrease with increases in: hydrocarbon solubility in mobile waters in the source rock, critical fracture pressure, the viscosity of the hydrocarbons, and with changes in kerogen type to a more oil-prone variety. The object of the work presented here is to investigate simultaneously all the processes thought to control primary hydrocarbon expulsion. The Shongliao Basin in China was chosen as a case study because copious, basin-wide data are available.

TEMPERATURE CONTROLLED POROSITY/PERMEABILITY REDUCTION, FLUID MIGRATION, AND PETROLEUM EXPLORATION IN SEDIMENTARY BASINS

Findings mainly from the Norwegian Continental Shelf indicate that at temperatures greater than approximately 60°C, internally sourced quartz cementation and diagenetic clay become important porosity and permeability reduction factors, respectively. Porosity loss due to quartz cementation in sandstones and siltstones proceeds mainly independent of effective stress or fluid pressure. Porosity loss rates approach those required to generate high fluid overpressures at approximately 120°C, which can result in hydrofracturing of the overlying low permeability shales. In thick low permeable sediment sequences, the probability of km scale vertical fluid migration increases. These thermally driven processes are capable of generating and sustaining fluid overpressure and facilitate fluid migration for up to tens of millions of years. Unlike mechanical compaction models, this model predicts that porosity loss and therefore fluid migration will continue despite the buildup of overpressure, even during periods of no sedimentation. In sealed compartments, fluid flow from more deeply buried high permeable lithologies will occur by hydrofracturing of overlying low permeable lithologies, preferentially along near vertical faults if present. When hydrofractures are induced from high permeable sediments, they will propagate vertically through overlying low permeable sediments, unless they enter lithologies with sufficient permeability and volume to bleed off the fracture fluid propagation pressure. Quantitative analysis of the mineral reactions causing porosity loss cannot only identify sediments in thermal zones which are expelling fluids, but also lithologies in thermal zones which are likely to receive those fluids, in addition to the timing and rates of fluid migration. The model has important applications for evaluating petroleum exploration risks and the potential for remigrated hydrocarbon plays both at basin scale and prospect level.

A physically-related regional model for extreme discharges in Israel

Israel is a small country which experiences wide variations in magnitudes of extreme discharges. A consistent model has been constructed for prediction of such discharges throughout the country. Extreme flow characteristics, geographical proximity, lithology, soils, and rainfall properties are the major factors in the delineation of relatively homogeneous regions within the country. For each region, discharge-area relationships are formulated in association with low exceedance probabilities. These relationships follow at-site predictions which have been prepared by fitting the log Pearson type III distribution to annual maxima series of peak discharges. For catchments larger than 100 km2 in area, the differences between the regional and the at-site predictions are small. Relatively high extreme discharges are found for the arid area where rainfall depth is low, for an area of steep slopes, for areas of low permeable lithology and soils, and for areas where the fraction of intense rainfall in the total depth of precipitation is high. For large arid catchments, the discharge- area relationships exhibit a negative trend. The model is simply applicable and appears suitable for other semiarid and arid areas.

Structural controls on formation of the Hollinger-McIntyre Au quartz vein system in the Hollinger shear zone, Timmins, southern Abitibi greenstone belt, Ontario

The Hollinger-MeIntyre Au quartz-carbonate-pyrite vein system, the largest known in the Superior province ( approximately 995 t Au), is localized within a 450- to 600-m-wide, northeast-southwest-trending, ductile-brittle shear zone, the Hollinger shear zone. This shear zone is characterized by a strong east-northeast-striking foliation that dips 80 degrees S and a prominent elongation-stretching lineation plunging 60 degrees to the east. Kinematic and strain indicators such as folded veins and fractures, asymmetrical strain shadows, lineations, and asymmetries of foliation trajectories indicate complex movement; however, a reverse, oblique (dextral) component of displacement appears to have been dominant during and after the main period of Au quartz vein mineralization.Successive stages of vein formation were largely synchronous with shear zone development since dilation occurred in secondary shear fractures that broadly reflect the shear zone stress system. At the scale of a stope or individual vein structure, vein morphologies commonly reflect local development of arrays of tension gash veins followed by vein formation by hydraulic fracturing along Reidel (R) shear fractures, and then principal displacement fractures (D). This process resulted in the characteristic combination of irregular quartz veins with superimposed banks of parallel veins and stringers slightly oblique to foliation. Veins parallel to foliation are thought to have resulted from a combination of reorientation of early veins and hydraulic fracturing along shear zone foliation planes.On a broader scale the position and plunge of the most productive central ore zone is controlled by the occurrence of favorable lithologic units (e.g., permeable Central subgroup breccias), favorable stratigraphic contacts, interflow horizons, and competency contrasts, combined with large-scale anisotropies, in this case, the position and attitude of the quartz-feldspar porphyry intrusions within the shear zone, particularly the Pearl Lake porphyry.The Hollinger-McIntyre vein system is best interpreted as having developed under lithostatic pressure; fluid migration was localized within a more extensive shear zone by the presence of earlier felsic intrusions and permeable lithologies. Vein orientations and morphologies were then controlled by dilation of a variety of secondary shear and tension fractures which formed in response to late reverse dextral movement. The Hollinger shear zone is superimposed on an earlier fabric which also affects Timiskaming sediments; mineralization is thus post-Timiskaming in age and similar in relative timing and controls to other examples of Au quartz vein mineralization in the southern Abitibi greenstone belt such as the Kirkland Lake, Larder Lake, and Val d'Or camps. The 2673 (super +6) (sub -2) Ma age for an albitite dike which immediately predates Au quartz vein mineralization in the McIntyre mine corresponds to the late Timiskaming phase.The Hollinger shear zone is one of three alignments, two of which are known to be ductile-brittle shear zones, which relate most of the major known Au quartz vein systems in the Timmins area (e.g., Dome mine).