Marco Antonellini

Effect of Faulting on Fluid Flow in Porous Sandstones: Petrophysical Properties

Fault zone permeability in outcrop is quantified by detailed geologic mapping and by measurements using a minipermeameter. Deformation bands, zones of deformation bands, and slip planes are structural elements associated with successive stages in the evolution of a fault zone in porous sandstones. Deformation bands have a porosity about one order of magnitude less than the surrounding host rock and, on average, a permeability three orders of magnitude less than the surrounding host rock. The intensity of cataclasis and the clay content control the amount of permeability reduction as measured perpendicular to a band. The wall rock in proximity to slip planes can have permeabilities more than seven orders of magnitude less than the pristine sandstone. Capillary pressure wit in deformation bands is estimated to be 10-100 times larger than that in the surrounding host rock. Thus, deformation bands and slip planes can substantially modify fluid flow properties of a reservoir and have potential sealing capabilities with respect to a nonwetting phase, as evident in outcrop exposure.

Compaction bands: a structural analog for anti-mode I cracks in aeolian sandstone

Abstract We present evidence for the existence of tabular zones of localized deformation in aeolian sandstone, that accommodate pure compaction. In this sense they are analogs for anticracks or closing mode I fractures such as pressure solution surfaces or stylolites. The so called “compaction bands” are exposed in outcrops of the Jurassic Navajo Sandstone in the Kaibab monocline, Utah. They are characterized by lack of shear offset across their plane, volume loss, micro fracturing and very little grain crushing or comminution. Based on their geometry, two kinds of compaction bands are distinguished: the first kind is 0.5-1.5 cm thick and fairly straight over lengths of about 5–10 m. The second kind is 0.1-0.5 cm thick over lengths up to 2 m, and is conspicuously crooked with wavelengths of 1–5 cm and amplitudes of a few mm to a few cm. Compaction bands preferentially developed in the compressive quadrant at the tips of small faults or “deformation band faults” which suggests, together with the direction of shear along the deformation band faults, that compaction bands form perpendicular to the largest compressive stresses induced by motion along the deformation band faults. Also, the compaction bands typically occur in sedimentary layers with large grain sizes (0.3-0.8 mm) and high porosity (20–25%) whereas the deformation band faults occur in the layers with smaller grain sizes (0.05-0.25 mm) and lower (

Microstructure of deformation bands in porous sandstones at Arches National Park, Utah

Abstract At Arches National Park it is possible to distinguish three kinds of deformation bands on the basis of their distinctive microstructure: (1) deformation bands with little or no cataclasis; (2) deformation bands with cataclasis; and (3) deformation bands with clay smearing. The micromechanics of deformation band development consist of initial dilatancy followed by grain crushing and compaction. This process may be developed to different stages according to the interplay of porosity, confining pressure, clay content and amount of strain. Low porosities and low confining pressures promote the formation of dilatant bands with no cataclasis. High porosities and high confining pressures promote compaction and cataclasis. Two generations of deformation bands were documented. The older generation has little or no cataclasis and formed in relatively undisturbed sandstone probably under conditions of low confining pressure. The younger generation exhibits cataclasis, appears to be localized in proximity to major faults and seems to have developed under conditions of high confining pressure. The temporal sequence of deformation band development can be related to the regional geology of the area; where the first generation probably formed during growth of the salt anticline, and the second generation during its collapse.

Distinct element modeling of deformation bands in sandstone

Abstract We have conducted numerical experiments with the distinct element method to study factors that control the development of deformation bands in sandstone. These experiments show how sorting and initial porosity of the host rock control the development and the mode of deformation in the area of strain localization. The results of the numerical experiments are in qualitative agreement with field and microstructural observations of deformation bands at Arches National Park (Utah). In our numerical experiments sand grains are modeled as cylindrical elements that move in response to externally applied boundary conditions. Systems of elements that have a large variability in radius and/or loose packing deform at lower applied stresses than systems of elements that have a uniform radius and/or tight packing. The mode of deformation in the first kind of aggregate is particulate flow, where elements of different sizes move easily with respect to each other due to a low degree of interlocking. The mode of deformation in the second kind of aggregates is localized failure on small deformation bands. Shear bands in our numerical experiments nucleate as a zone of dilatancy and propagate via organization of dilatant zones into discrete faults. The presence of a flaw in the form of a ‘weak’ grain promotes the nucleation and propagation of shear bands.

Formation and growth of normal faults in carbonates within a compressive environment

Normal faults were initiated and grew through hierarchical formation of pressure-solution structures and their subsequent shearing in Cretaceous carbonates in the leading thrust front of Maiella Mountain, Italy. Through mapping in the field, we have documented the detailed architecture of faults with increasing slip values from a few millimeters to ∼50 m and have identified pretilting structural elements and four stages of fault development, each stage representing addition of a new structural element. The result is a conceptual model that begins with pretilting structures (bed-parallel and bed-perpendicular solution surfaces) that were reactivated in shear upon tilting of the beds at the frontal limb of the Maiella anticline. Slip on mechanical-layer boundaries and on bed-perpendicular solution surfaces resulted in oblique solution surfaces, linkage of solution surfaces, and fragmentation of rock. Oblique zones of fragmented rock in adjacent mechanical layers linked to form a continuous breccia and facilitated fault growth. These normal faults formed through mechanical processes strictly in a compressional regime.

Fault and fracture systems in a fold and thrust belt: An example from Bolivia

This paper presents an outcrop-analog study of fractures in low-porosity sandstones in the Subandean thrust belt. We analyze the evolution of fault and joint systems in these sandstones, quantify their density along the structural trend, and identify the main factors controlling their variability.We show that faults and joints occur at different scales in a hierarchical fashion, as a consequence of progressive shearing. The first generation is an orthogonal set of joints, one parallel and the other perpendicular to the bedding azimuth. Shearing along these joints transformed them into small faults and created new sets of fractures, oblique to the bedding attitude. Linkage of these small faults facilitated the formation of larger faults with significant strike-slip offset. Shearing along bedding planes created subvertical splay joints that induced the formation of conjugate normal faults. In this thrust belt, subordinate strike-slip and normal faults are concomitant products of compressive deformation.This study documents a hierarchical correspondence between spacing of structural heterogeneities and stratigraphic architecture. We measured spacings of joints and outcrop-scale faults along the backlimb of the Abra del Condor anticline. We subdivided the structural discontinuities into four main groups: joints, small faults, intermediate faults, and fault zones. Spacing of joints, small faults, and intermediate faults has a lognormal distribution, whereas spacing of fault zones shows a normal distribution. The mean of these distributions is about the same as the thickness of the confining stratigraphic intervals. Therefore, spacing and dimensions of joints and faults have a first-order relationship to the thickness of the confining stratigraphic sequences.

A Natural Analog for a Fractured and Faulted Reservoir in Dolomite: Triassic Sella Group, Northern Italy

We have used outcrops of dolomite exposed in the Triassic Sella Group of the Italian Central Dolomites as an analog for subsurface low-porosity faulted and fractured dolomite reservoirs. The Sella Group was mildly deformed at shallow burial depth (21,000 m) in a tectonic strike-slip regime during the Eocene-Miocene Alpine compression that caused the formation of joints and strike-slip faults. Because the matrix porosity in the dolomites is low (<5%) and poorly connected, joints and faults are essential to connect vugs and to provide permeability. Field observations of the Sella Group explain why many dolomite reservoirs and aquifers in strike-slip/compressive tectonic regimes are intensely jointed when they are mildly deformed. In this type of tectonic regime, in fact, pervasive jointing over a wide area accommodates small strains and is strictly associated with the formation of strike-slip faults. Our observations allow us to recognize different kinds of fault architectures that correspond to different stages of fault development. In addition, theoretical models and microscopic observations were used to estimate the petrophysical properties of the faulted and jointed dolomite. Small-offset faults (offsets up to 30 mm), characterized by en echelon arrays of joints and pockets or seams of breccia up to 10 mm wide, form areas of high permeability (100-3000 md) due to the presence of many joints and high-porosity breccia. Faults with 1-10 m offsets, characterized by a breccia zone (1-2 m in width) and associated with high joint density in the wall-rock, contain high-porosity (10%) breccia and represent areas of preferred fluid flow. Large-offset faults with offsets more than 10 m contain a wide zone of low-porosity (<1%) breccia and form potential permeability barriers. The areas adjacent to the intermediate- and large-offset faults have high permeability (100-3000 md) because of high joint densities. An important implication of the way faults develop in dolomite is the consistent relationship between the orientation of joints and faults: the faults strikes differ 15-35° from the strikes of the pervasive joint systems. Joint density also increases four to five times in the proximity of the faults. Such relationships can be used to predict the distribution and orientation of joints and faults in subsurface dolomite reservoirs.

Petrophysical study of faults in sandstone using petrographic image analysis and X-ray computerized tomography

Petrographic image analysis (PIA) and X-ray computerized tomography (CT) provide local determinations of porosity in sandstone. We have investigated small faults called deformation bands in porous sandstones using these techniques. Because the petrophysical properties of the fault rock vary at a small scale (mm scale), the ability of PIA and CT to determine porosity in small volumes of rock and to map porosity distribution in two and three dimensions is crucial. This information is used to recognize the processes involved in fault development and the different kinds of microstructures associated with dilatancy and compaction. The petrophysical study of fault rock in sandstone permits one to make predictions of the hydraulic properties of a fault and thereby evaluate the sealing or fluid transmitting characteristics of faulted reservoirs and aquifers. The results of this study indicate that faulting in sandstone alters the original porosity and permeability of the host rock: the porosity is reduced by an order of magnitude and the permeability is reduced by one to more than seven orders of magnitude for faults associated with compaction.

Climate and water budget change of a Mediterranean coastal watershed, Ravenna, Italy

It is generally difficult to quantify exactly the freshwater going in or out of the coastal watersheds along the northern Adriatic Sea because, on one hand, excess water is drained and pumped into the sea to prevent flooding but, on the other hand, water is brought onto the land from far away for irrigation. Fragmentation of water authorities makes it difficult to collect all the necessary information. Climate change and increasing salinization of the coastal aquifers make it imperative, however, to better know the quantities of freshwater involved in these small basins. The water budget of a small coastal agricultural watershed along the Adriatic Sea in Italy (The Quinto Basin near Ravenna) is presented here considering different land uses. The evaporation of open water and the evapotranspiration of wetlands, pine forests, bare soil and irrigated agriculture are calculated based on the Penman–Monteith equation and the Cropwat program. The current water budget is based on average climate data from 1989 to 2008 and drainage and irrigation data. Predictions for future evapotranspiration, net irrigation and hydrologic deficit are calculated with climate data from IPCC (The Fourth Assessment Report (AR4) 200, Climate change 2007). From the study results, the soil type may determine whether or not a crop will need more or less irrigation in the future. Regulations on land use should therefore consider which crop type can be grown on a specific soil type. Water budget analysis in scenarios A1b and A2 both show an increase of water deficits in the summer and an increase of water surplus in the winter. This is explained by the fact that a larger percentage of the rain will fall in winter and not during the growth season. The open water evaporation will decrease under future climate scenarios as a result of increased relative humidity in winter and decreased wind velocity. This may have a positive effect on the water cycle. The current irrigation is very abundant, but has beneficial effects in contrasting soil salinization and saltwater intrusion into the coastal aquifer.

Seasonal dynamic of a shallow freshwater lens due to irrigation in the coastal plain of Ravenna, Italy

Irrigation in low-lying coastal plains may enhance the formation of fresh groundwater lenses, which counteract salinization of groundwater and soil. This study presents seasonal dynamics of such a freshwater lens and discusses its influence on the salinity distribution of the unconfined aquifer in the coastal plain of Ravenna, Italy, combining field observations with numerical modeling (SEAWAT). The lens originates from an irrigation ditch used as a water reservoir for spray irrigation. The geometry of the freshwater lens shows seasonal differences because of freshwater infiltration during the irrigation season and upconing of deeper saltwater for the remainder of the year. The extent of the freshwater lens is controlled by the presence of nearby drainage ditches. Irrigation also results in a temperature anomaly in the aquifer because of the infiltration of warm water during the irrigation season. The surficial zone in the vicinity of the irrigation ditch is increased considerably in thickness. Finally, different irrigation alternatives and the influence of sea-level rise are simulated. This shows that it is necessary to integrate irrigation planning into the water management strategy of the coastal zone to have maximum benefits for freshening of the aquifer and to make optimal use of the existing infrastructure.RésuméL’irrigation dans les basses plaines côtières est susceptible de favoriser la formation de lentilles d’eaudouce, qui s’opposent à la salinisation de l’eau souterraine et des sols. Cette étude présente ladynamique saisonnière d’une telle lentille d’eau douce et discute de son influence sur la distribution desalinité dans l’aquifère à nappe libre de la plaine côtière de Ravenne en Italie, combinant desobservations de terrain avec une modélisation numérique (SEAWAT). La lentille provient d’un fosséd’irrigation utilisé comme réservoir d’eau pour l’irrigation par aspersion. La géométrie de la lentilled’eau douce montre des différences saisonnières à cause de l’infiltration d’eau douce durant la saisond’irrigation et des remontées d’eau salées pendant le reste de l’année. L’extension de la lentille d’eaudouce est contrôlée par la présence des fossés voisins. L’irrigation a aussi comme conséquence uneanomalie thermique dans l’aquifère du fait de l’infiltration d’eau à température ambiante pendant lasaison d’irrigation. L’épaisseur de la zone superficielle augmente considérablement au voisinage dufosse d’irrigation Enfin, diverses alternatives d’irrigation et l’influence de l’augmentation du niveau dela mer sont simulées. Elles montrent que l’intégration d’un schéma d’irrigation dans la stratégie degestion de l’eau en zone côtière est nécessaire pour obtenir les avantages maximum pour adoucir lanappe et faire une utilisation optimale de l’infrastructure existante.ResumenLa irrigación en planicies costeras bajas puede favorecer la formación de lentes de agua subterráneadulce, las cuales contrarrestan la salinización del agua subterránea y del suelo. Este estudio presenta ladinámica estacional de una de tales lentes de agua dulce y discute su influencia en la distribución de lasalinidad del acuífero no confinado en la planicie costera de Ravenna, Italia, combinandoobservaciones de campo con modelado numerico (SEAWAT). La lente se origina a partir de unaacequia usada como un reservorio de agua para la irrigación por aspersión. La geometría de la lente deagua dulce muestra diferencias estacionales debido a que la infiltración de agua dulce durante de laestación de irrigación y el desplazamiento vertical de agua salina más profunda durante el resto delaño. La extensión de la lente de agua dulce está controlada por la presencia de acequias de drenajecercanas. La irrigación también produce una anomalía en la temperatura del acuífero debido a lainfiltración de agua cálida durante la estación de irrigación. La zona superficial en la vecindad de laacequia de irrigación incrementa considerablemente su espesor. Finalmente, se simulan las diferentesalternativas de irrigación y la influencia del ascenso del nivel del mar. Esto muestra que la integraciónde la planificación de la irrigación dentro de la estrategia de manejo del agua de la zona costera esnecesaria para obtener los máximos beneficios y para proveer de agua dulce al acuífero y hacer un usoóptimo de la infraestructura existente.摘要低洼沿海平原灌溉可增加地下淡水透镜体的形成, 从而抵消地下水和土壤的盐度。结合野外观测和数值模拟, 本研究展示了这样的淡水透镜体的季节性动态变化, 论述了淡水透镜体对意大利Ravenna (拉文那) 沿海平原非承压含水层盐度分布的影响。透镜体源自作为喷灌储水池的灌溉渠沟。由于灌溉期间的淡水渗透及其他时间内出现的深部咸水倒锥, 透镜体的几何结构显示出季节性差异。淡水透镜体的范围受附近排水沟的控制。由于灌溉季节温水的渗透, 灌溉也导致含水层温度异常。灌溉沟渠附近的表层带厚度增加非常大。最后, 模拟了不同的灌溉替代选择及海平面上升的影响。表明, 有必要制定综合的沿海地区水管理战略灌溉规划, 以获取含水层淡化的最 大效益及优化利用现有的基础设施。ResumoA rega em planícies costeiras de zonas baixas pode induzir a formação de lentes de água doce queimpedem a salinização da água subterrânea e do solo. Este estudo apresenta a dinâmica sazonal deuma destas lentes de água doce e analisa a sua influência na distribuição da salinidade do aquíferofreático na planície costeira de Ravenna, Itália, combinando campos de observação com modelaçãomatemática (SEAWAT). A lente forma-se a partir de um açude que é usado como reservatório de águapara rega por aspersão. A geometria da lente de água doce mostra diferenças sazonais devido àinfiltração de água doce durante o período de rega e ascensão de água salgada mais profunda durante oresto do ano. A dimensão da lente de água doce é controlada pela presença de valas de drenagempróximas. A rega também induz uma anomalia de temperatura no aquífero devido à infiltração de águaquente durante o período de rega. A zona superficial nas proximidades do açude de rega aumentousignificativamente de espessura. Finalmente, foram simuladas alternativas distintas de rega e ainfluência do aumento do nível do mar. Isto demonstra que a integração do planeamento da rega noâmbito da estratégia de gestão de água da zona costeira é necessária para obter o máximo rendimentoda introdução de água doce no aquífero e para fazer o melhor uso da infraestrutura existente.