Outcrop Fracture Research Articles

A new methodology to train fracture network simulation using multiple-point statistics

Abstract. Natural fracture network characteristics can be establishes from high-resolution outcrop images acquired from drone and photogrammetry. Such images might also be good analogues of subsurface naturally fractured reservoirs and can be used to make predictions of the fracture geometry and efficiency at depth. However, even when supplementing fractured reservoir models with outcrop data, gaps will remain in the model and fracture network extrapolation methods are required. In this paper we used fracture networks interpreted from two outcrops from the Apodi area, Brazil, to present a revised and innovative method of fracture network geometry prediction using the multiple-point statistics (MPS) method. The MPS method presented in this article uses a series of small synthetic training images (TIs) representing the geological variability of fracture parameters observed locally in the field. The TIs contain the statistical characteristics of the network (i.e. orientation, spacing, length/height and topology) and allow for the representation of a complex arrangement of fracture networks. These images are flexible, as they can be simply sketched by the user. We proposed to simultaneously use a set of training images in specific elementary zones of the Apodi outcrops in order to best replicate the non-stationarity of the reference network. A sensitivity analysis was conducted to emphasise the influence of the conditioning data, the simulation parameters and the training images used. Fracture density computations were performed on selected realisations and compared to the reference outcrop fracture interpretation to qualitatively evaluate the accuracy of our simulations. The method proposed here is adaptable in terms of training images and probability maps to ensure that the geological complexity in the simulation process is accounted for. It can be used on any type of rock containing natural fractures in any kind of tectonic context. This workflow can also be applied to the subsurface to predict the fracture arrangement and fluid flow efficiency in water, geothermal or hydrocarbon fractured reservoirs.

Outcrops as guides to subsurface natural fractures: Example from the Nikanassin Formation tight-gas sandstone, Grande Cache, Alberta foothills, Canada

Abstract Outcrop studies are essential for understanding subsurface fractures, because some key attributes such as length and height distribution cannot be measured using wellbores. Discrepancies between outcrop and subsurface fracture attributes arise because loading paths due to burial, exhumation, and thermal paths differ. Consequently, even where rock type and structural setting are close matches, differences may exist in mechanical properties and fracture growth. Here we investigate petrologic criteria and disseminated arrays of quartz-filled microfractures as tools for comparing and contrasting fractures in outcrop with those as sampled by core for tight gas sandstones. Fracture cements are key diagnostic criteria for proving correspondence between outcrop and core. We offer guidelines on selecting suitable outcrop analogs that differ markedly from current practice. Our illustration uses outcrops and core samples of Late Jurassic to Early Cretaceous Nikanassin Formation, a tight gas sandstone, but aspects of the outcrop vetting procedure can be applied to other rock types. In some core and outcrops, Nikanassin Formation fractures contain quartz and ankerite cement assemblages, crack-seal and rind textures, and aperture size distributions that show that the outcrop and subsurface localities we examined share the same fracture stratigraphy and similar diagenesis. These outcrops can thus be used to measure attributes including those that are unlikely to be captured in core, such as fracture height and length distributions, relative and absolute fracture intensity with structural position, and degree of fill by synkinematic cement. In our example, absolute fracture intensity and distribution of postkinematic carbonate cements are site specific and differ between outcrop and core, but in ways that can be quantified.

Outcrop fracture network characterization for unraveling deformation sequence, geomechanical properties distribution, and slope stability in a flysch sequence (Monte Venere Formation, Northern Apennines, Italy)

A detailed characterization of outcrop fracture networks in the turbiditic flysch sequence of the Monte Venere Formation (Northern Apennines) together with in situ measurements of rock strength using the Schmidt hammer provided important insights into the sequence of deformation and the slope stability conditions. The inferred sequence of structure formation from oldest to youngest is bedding-parallel cleavage, veins and normal faults, joints, and strike-slip faults (sheared joints). Alteration halos around fractures (joints, splay joints, strike-slip faults, and some normal faults) point out that these structures were conductive to meteoric water during uplift and erosion in the Holocene. Calcite-filled veins without alteration halos are considered local barriers to fluid flow and diffusion. Bedding thickness controls rock fracturing characterization parameters in the Monte Venere Formation. Reactivation in shear of pre-existing structures, however, causes the formation of splay joint clusters that locally increase fracture density contributing to degrade the mechanical strength of the rock. These localized clusters are apparent in detailed outcrop maps but they are usually not detected by the rock fracturing characterization parameters. Our data also imply that the presence of bedding-parallel cleavage is more important than layer thickness in controlling the rock compressive strength and ultimately the peak shear strength along a potentially sliding surface. This study takes closer look at landslide formation in a sloped flysch sequence under Mediterranean climate conditions and allowed to consider a conceptual model for landslide occurrence in which structural discontinuities and meteoric water flow through fracture networks are main triggering factors.

Numerical estimation of fracture network permeability based on GEOFRAC model for groundwater modeling in a tin mine

Abstract Numerous geological research studies and mining operations have proved that fracture is one of the important factors controlling groundwater flow, mineralization, and ore distribution in metallic deposits. Most current approaches to groundwater flow simulation of naturally fractured media rely on the calculation of equivalent permeability tensors from a discrete fracture network (DFN). This study is aimed at developing a rational two-dimensional DFN by GEOFRAC, a geostatistical method of fracture direction and locations of sample data from a tin mine in the Gaosong area, Gejiu city, southwest China, and utilizing 3,724 outcrop fractures sampled on the ground of mountain Gaosong. Principal inputs of the DFN are density, direction, and continuity of disks that constitute a fracture plane. Fractures simulated by GEOFRAC were validated in that their directions corresponded well with those of the sample fractures. The permeability tensor of each modeling grid was then calculated based on the fracture network constructed. The results showed that GEOFRAC is valuable for two-dimensional DFN modeling in mines and other fracture-controlled geological phenomena, such as groundwater flow and slope failure.

Comparing fracture statistics from outcrop and reservoir data using conventional manual and t-LiDAR derived scanlines in Ca2 carbonates from the Southern Permian Basin, Germany

Abstract Natural fracture networks strongly control hydrocarbon flow paths in tight carbonate reservoirs. An improved understanding of their geometries regarding orientations and distribution may result in reduced uncertainties in reservoir modeling and well planning. Outcrop analogs provide supplementary information about fracture networks below seismic resolution. We introduce a suitable analog for a gas-producing Zechstein reservoir in the Stassfurt carbonates (Ca2) in the Southern Permian Basin, northern Germany. Dolomite represents the main lithology in outcrop and reservoir rocks, which were deposited on a similar carbonate platform slope environment, and both locations were subject to the same events of diagenetic and tectonic overprint. The scope of this study is the evaluation of fracture characteristics by a direct comparison of three similar datasets of manual outcrop and digital outcrop data with borehole resistivity image log data from a horizontal gas development well. Manually measured fracture data of the exact same digital scanlines in the outcrop are used to successfully validate terrestrial laser scanning (t-LiDAR). T-LiDAR data is used to generate a high-resolution digital outcrop model, and we introduce a novel workflow, firstly applied to carbonate rocks, to detect fractures from that data set along artificial horizontal wells following E&P industry best practice. Results of both outcrop and subsurface data suggest W – E orientations for well path planning due to dominant northerly striking open tectonic fracture directions. Higher fracture intensities (P10 values describing the amount of fractures intersecting the scanline) of conventional scanline measurements of 4.3 m−1 indicate a bias in the t-LiDAR (P10: 2.6 m−1) dataset inversely related to limitations in fracture detection abilities along the horizontal well. As for the horizontal well (P10: 2.7 m−1), fractures paralleling the borehole appear systematically underrepresented. Fractures with almost perpendicular orientation to the outcrop wall can only be occasionally detected in the t-LiDAR data set due to their limited exposed surface. However, outcrop fracture characteristics are in good correlation with subsurface results and thus, can help reducing uncertainties in reservoir characterization during field development and well planning.

The developmental characteristics of natural fractures and their significance for reservoirs in the Cambrian Niutitang marine shale of the Sangzhi block, southern China

The great potential that southern China has for marine shale gas resources is well known, yet shale gas production is very small. Exploration and development of southern China has lagged because of the complicated structural history of this region. Thus, it is important to recognize the tectonic history of this region and to carefully describe the developmental characteristics of natural fractures associated with the tectonic evolution of southern China. In this paper, we discuss the developmental characteristics of natural fractures based on the description and statistical analysis of natural fractures in outcrops, cores and thin sections, and from several experimental tests. Fractures examined in outcrop appear to be principally fold-fault related, regional tectonic, bedding or weathering fractures. Fractures observed in core are mainly shear fractures, tension fractures, and shear-tension fractures. Most of these fractures are sealed by calcite. Micro fractures and pores are well developed. Analysis of the cross-cutting relation of micro fractures suggests that the study area has been subject to at least two tectonic movements, consistent with our investigation of core. Fracture development appears to have been strongly influenced by structural evolution. However, fracture density displays a weak negative correlation with organic matter content. Further, fracture density appears to correlate positively with quartz content. Vertical variations of mineralogy appear to have influenced fracture generation and, therefore, hydrocarbon migration within the Niutitang Shale.

LIDAR-based fracture characterization and controlling factors analysis: An outcrop case from Kuqa Depression, NW China

Fracture is of primary importance to the natural gas production capacity from many tight sandstone reservoirs in the Kuqa foreland basin, NW China, but the orientation, size and plane porosity of the fractures in the subsurface is difficult to measure directly. Terrestrial light detection and ranging (LIDAR) surveys can offer factual information of outcrop-based research efforts to characterize fracture development laws and controlling factors. In this paper, utilizing a multi-level covering, we obtain the three dimensional (3D) point cloud data from a LIDAR survey launched at a typical outcrop. Matching with high-resolution digital photos and artificial measured information, the 3-D positions of natural fractures are extracted strictly in the data volume section. Furthermore, the fracture and reservoir model can be founded based on the systematic sampling and laboratory analysis, while a variety of accurate facture parameters can be obtained. It is founded that three groups of shear fractures are mainly developed in two periods with large inclination, short trace length and small spacing of normal distribution. Its patterns has provided a literal distribution of penetrating fracture zone and interlayer fracture zone with a single and a double set of advantage orientation respectively. It turned out that the fracture development scale is controlled by lithology, layer thickness, maximum principle paleostress and rock composition with a good exponential relationship. Our work could provide a workflow linking outcrop fracture observations to the 3D model of subsurface fracture prediction and extend modeling capability in other outcrop studies.

Quantifying opening-mode fracture spatial organization in horizontal wellbore image logs, core and outcrop: Application to Upper Cretaceous Frontier Formation tight gas sandstones, USA

The Upper Cretaceous Frontier Formation is a naturally fractured gas-producing sandstone in Wyoming. Regionally, random and statistically more clustered than random patterns exist in the same upper to lower shoreface depositional facies. East-west- and north-south-striking regional fractures sampled using image logs and cores from three horizontal wells exhibit clustered patterns, whereas data collected from east-west-striking fractures in outcrop have patterns that are indistinguishable from random. Image log data analyzed with the correlation count method shows clusters ∼35 m wide and spaced ∼50 to 90 m apart as well as clusters up to 12 m wide with periodic inter-cluster spacings. A hierarchy of cluster sizes exists; organization within clusters is likely fractal. These rocks have markedly different structural and burial histories, so regional differences in degree of clustering are unsurprising. Clustered patterns correspond to fractures having core quartz deposition contemporaneous with fracture opening, circumstances that some models suggest might affect spacing patterns by interfering with fracture growth. Our results show that quantifying and identifying patterns as statistically more or less clustered than random delineates differences in fracture patterns that are not otherwise apparent but that may influence gas and water production, and therefore may be economically important.

Analysis of lineaments within the Wajid Group, SW Saudi Arabia, and their tectonic significance

The Wajid Group is a Cambro-Permian sedimentary succession in southwest Saudi Arabia. This group is a well-known groundwater aquifer in the Wadi Al-Dawasir and Najran areas. The group also represents siliciclastic hydrocarbon reservoirs in the Rub' Al-Khali Basin. The Wajid Group is exposed in an area extending from Wadi Al-Dawasir southward to Najran city. This study aims to map and characterize the lineament traces of the Wajid Group outcrops. Landsat-8 OLI/TIRS satellite images with 30-m resolution, Spot-5 satellite images with 2.5-m resolution and SRTM digital elevation models (DEM) with 30-m resolution were used for lineament trace detection. Those lineament traces supplemented by aeromagnetic lineaments detected from reduced to pole magnetic anomaly map of the studied outcrop. Multi-scale lineament trace maps were generated, and the lineament datasets, including orientation and length, were analyzed statistically. Eight lineament trace trends were identified including NW-SE, NNW-SSE, N-S, NNE-SSW, NE-SW, ENE-WSW, E-W, and WNW-ESE. The northerly, northwesterly, and northeasterly trending lineament traces are predominant. The lineament trace lengths are generally followed the power law distribution. The lineament trace trends were validated through field investigation of the Wajid Group outcrop. The reported outcrop fracture trends are consistent with major lineament trace trends. Lineaments within the Wajid Group outcrop are also consistent with those of the southern portion of the Arabian Shield. The results of this study provide insight into the tectonic origin of the Wajid Group outcrop lineaments, and understanding of the lineaments distribution which can help to predict the fluid flow behavior within the groundwater fractured aquifers or hydrocarbon fractured reservoirs in Rub’ Al-Khali Basin.

Benefits of maximum likelihood estimators for fracture attribute analysis: Implications for permeability and up-scaling

The success of any predictive model is largely dependent on the accuracy with which its parameters are known. When characterising fracture networks in rocks, one of the main issues is accurately scaling the parameters governing the distribution of fracture attributes. Optimal characterisation and analysis of fracture lengths and apertures are fundamental to estimate bulk permeability and therefore fluid flow, especially for rocks with low primary porosity where most of the flow takes place within fractures. We collected outcrop data from a fractured upper Miocene biosiliceous mudstone formation (California, USA), which exhibits seepage of bitumen-rich fluids through the fractures. The dataset was analysed using Maximum Likelihood Estimators to extract the underlying scaling parameters, and we found a log-normal distribution to be the best representative statistic for both fracture lengths and apertures in the study area. By applying Maximum Likelihood Estimators on outcrop fracture data, we generate fracture network models with the same statistical attributes to the ones observed on outcrop, from which we can achieve more robust predictions of bulk permeability.

A region-growing approach for automatic outcrop fracture extraction from a three-dimensional point cloud

Conventional manual surveys of rock mass fractures usually require large amounts of time and labor; yet, they provide a relatively small set of data that cannot be considered representative of the study region. Terrestrial laser scanners are increasingly used for fracture surveys because they can efficiently acquire large area, high-resolution, three-dimensional (3D) point clouds from outcrops. However, extracting fractures and other planar surfaces from 3D outcrop point clouds is still a challenging task. No method has been reported that can be used to automatically extract the full extent of every individual fracture from a 3D outcrop point cloud. In this study, we propose a method using a region-growing approach to address this problem; the method also estimates the orientation of each fracture. In this method, criteria based on the local surface normal and curvature of the point cloud are used to initiate and control the growth of the fracture region. In tests using outcrop point cloud data, the proposed method identified and extracted the full extent of individual fractures with high accuracy. Compared with manually acquired field survey data, our method obtained better-quality fracture data, thereby demonstrating the high potential utility of the proposed method.

Extraction of three-dimensional fracture trace maps from calibrated image sequences

The routine application of digital survey technologies such as terrestrial lidar and photogrammetry to the characterization of fault and fractures in outcrop over the past decade has resulted in major advances in terms of the efficiency of discontinuity data acquisition. However, the reliance upon mesh- and point-cloud–based analysis approaches means that data sets obtained from these sources commonly offer heavily abstracted views of the measured fracture network due to the limited resolution of the input model. Here, we pre­sent an alternative approach that combines conventional two-dimensional (2D) image analysis with ray-tracing techniques to extract three-dimensional (3D) fracture trace maps from photogrammetrically calibrated image sequences. These 3D trace objects may be interrogated to obtain fracture network properties (trace length, intensity, and connectivity), with probabilistic methods used to estimate fracture orientation for high collinearity traces. Our approach possesses a number of advantages over existing digital surface reconstruction-based methods, with the use of a 2D pixel-based approach allowing established image-processing routines (e.g., edge detection/connected components analysis) to be applied to the characterization of fracture and fault properties. Moreover, the innately high resolution of the input images results in practically lossless 3D fracture trace representation, limiting truncation effects. As a result, the method is capable of resolving local varia­bility in higher-order fracture properties such as fracture intensity, which are difficult to derive using existing approaches. We demonstrate the approach on pervasively faulted Permian age exposures of the Vale of Eden Basin, UK.

The characteristics of open fractures in carbonate reservoirs and their impact on fluid flow: a discussion

Permeability in fractured carbonate reservoirs is very heterogeneous due to fracturing at different scales superimposed on inherent textures from deposition and diagenesis. Observations of fractures in core and outcrop indicate that flow in open fractures in carbonate rock tends to be channelled rather than through fissures. Most of the flow takes place along a few dominating channels in the fracture plane, whereas most of the fracture plane is not effective for fluid flow. The formation of flow channels is caused by a combination of mechanical and, in particular, diagenetic processes. Single extension fractures occur as partly open or vuggy fractures, and their hydraulic properties are controlled by dissolution and cementation. Single shear fractures are typically open at local steps in the fault plane controlled by shearing along irregular fracture surfaces. Fault damage zones tend to be concentrated at fault tips, intersections, pull-aparts and overlap zones that represent areas of dilation. These damage zones represent elongated features in three dimensions with a high fracture density that will result in channelled flow at reservoir scales. The effect of channelled flow should be taken into account during evaluation of fractured carbonate reservoirs and when building dynamic flow models.

Fracture distribution along an Upper Jurassic carbonate ramp, NE Spain

Understanding the distribution of natural fractures in sedimentary systems is of high relevance for the exploration and production of fluids in the subsurface. This study focuses on a Kimmeridgian mixed siliciclastic-carbonate ramp system, which is part of the Jurassic limestones outcropping in the northeastern Iberian Chain. The study area is located north of the Ricla village, fifty kilometers southwest of Zaragoza. The outcrop stretches over six kilometers in length, it allows for recording detailed fracture patterns and facies variations. A GIS-based software-package 'DigiFract' is used to measure and digitize fractures in vertical outcrops. Fracture orientation measurements obtained from vertical and horizontal outcrops are used to create a conceptual three-dimensional image of the fracture distribution of the mixed ramp system. The fracture data are processed by integrating outcrop logs and sample-data obtained from thin-sections and rock property analysis. The continuous facies belts of the shallow low-angle ramp system show limited lateral variations. The studied sediments are subdivided in four main facies tracts (FT's): (FT-1) Bioclastic siltstones, (FT-2) alternating sandy limestones and marls, (FT-3) cross-bedded and channelled oolitic-bioclastic sand- and grainstones and (FT-4) coralgal float-to mudstones with eventites. Measured fracture orientations of the vertical and horizontal outcrops are identical throughout the entire exposure. Two main fracture sets are identified, the first set (Set 1) has a N–S direction and the second set (Set 2) has a NE–SW direction. The lateral homogeneity of the sedimentary system, thus facies, layer thickness and slope angle, can be translated to the observed fracture patterns. Within one single facies belt, fractures tend to behave the same in the proximal, middle and distal part of the ramp system. However, vertical facies variations are an important factor for the measured vertical fracture-heterogeneity. Fine-grained mud-supported facies correspond to periods of a sea-level highstand; coarse-grained cemented facies on the other hand are related to a sea-level lowstand. The physical contrast of the sediments caused by sea-level fluctuations forces fractures to solely concentrate in the brittle layers. Fracture density and termination patterns observed on this ramp stand in contrast to fracture geometries observed in flat-topped carbonate platforms. Lateral facies heterogeneity and platform anatomy of flat-topped carbonate platforms are key parameters for the eventual fracture distribution. For the studied mixed ramp-type system the vertical facies variations are key parameter.

Characterization of natural fracture systems: Analysis of uncertainty effects in linear scanline results

The exploitation of hydrocarbon reserves in naturally fractured reservoirs composed of different types of rocks has drawn considerable attention from the fracture characterization research community because of the importance of fractures to the prediction of fluid flow. One of the most common methods for rapidly analyzing fracture features is the scanline technique, which provides an estimate of fracture density and frequency. Despite the confidence provided by the systematic use of this method, errors and uncertainties caused by sampling biases exist. The problems caused by these uncertainties can detrimentally affect the construction of a computational model due to misleading trends. This study evaluated the uncertainty caused by sampling biases in the scanline data of opening-mode fractures in outcrops of naturally fractured Aptian laminated limestone from the Crato Formation, Araripe Basin, northeastern Brazil. The Monte Carlo method was chosen to introduce random values into the sampled values, which enabled us to verify the importance of errors in the accuracy of the method of representing the fracture network. In this study, errors and uncertainties were grouped into one parameter, termed the coefficient of uncertainty, which was defined as the ratio between the uncertainties, created by the errors and artifacts introduced artificially, and the original scanline data. The propagation of errors and uncertainties in the scanline data to the coefficients of the corresponding power law were determined. This evaluation can be applied in the construction of more reliable geomechanical models using analog geological models for naturally fractured reservoirs.

Calibrating discrete fracture-network models with a carbonate three-dimensional outcrop fracture network: Implications for naturally fractured reservoir modeling

Modeling naturally fractured reservoirs requires a detailed understanding of the three-dimensional (3D) fracture-network characteristics, whereas generally only one-dimensional (1D) data, often suffering from sampling artifacts, are available as inputs for modeling. Additional fracture properties can be derived from outcrop analogs with the scanline method, but it does not capture their full two-dimensional (2D) characteristics. We propose an improved workflow based on a 2D field-digitizing tool for mapping and analyzing fracture parameters as well as relations to bedding. From fracture data collected along 11 vertical surface outcrops in a quarry in southeast France, we quantify uncertainties in modeling fracture networks. The fracture-frequency distribution fits a Gaussian distribution that we use to evaluate the intrinsic fracture density variability within the quarry at different observation scales along well-analog scanlines. Excluding well length as a parameter, we find that 30 wells should be needed to fully (i.e., steady variance) capture the natural variability in fracture spacing. This illustrates the challenge in trying to predict fracture spacing in the subsurface from limited well data. Furthermore, for models with varying scanline orientations we find that Terzaghi-based spacing corrections fail when the required correction angle is more than 60°. We apply the 1D well analog data to calculate 3D fracture frequency using stereological relations and find that these relations only work for cases in which the orientation distribution is accurately described, as results greatly vary with small changes in the orientation distribution.

Modeling of the Dammam outcrop fractures: Case study for fracture development in salt-cored structures

ABSTRACT The exposed Cenozoic carbonates of the Dammam Dome are studied to: (1) characterize fractures and associated structures; (2) interpret the fracture mechanism; and (3) gain insights into fracture development within dome-like structures in the subsurface of the Arabian Gulf region. The fieldwork is integrated with structural analysis of the near-surface horizons mapped from interpretations of 3-D reflection seismic and borehole logs. Fractures are mapped from the outcrops of the middle limestone unit of the Eocene Rus Formation. The outcrops are concentrated in the central, northern and western areas of the Dammam Dome. The fractures are interpreted as opening-mode, bed-bounded joints that form orthogonal sets in most areas. The primary (older) joint set (J1) developed in long lineaments, some of which can be traced for over 300 m across entire exposures. The J1 set is found to be broadly consistent in its trend over the dome, indicating that formation of J1 fractures was systematic and not influenced by local structural anomalies (including karst collapse) formed during the Miocene to Recent. The trend of the J1 set does not correlate with the NE-SW compressional orientation of regional stresses associated with the Zagros Orogeny. Field data interpretation, allied with analysis of dome’s growth and curvature, suggest that the overall joint pattern reflects the growth of the strata as a dome. In addition, the joint density is controlled by structural position on the dome and mechanical stratigraphy. The study results provide a first-order conceptual fracture model for the subsurface reservoirs to guide future development.

A field sampling strategy for semivariogram inference of fractures in rock outcrops

The stochastic continuum (SC) representation is one common approach for simulating the effects of fracture heterogeneity in groundwater flow and transport models. These SC reservoir models are generally developed using geostatistical methods (e.g., kriging or sequential simulation) that rely on the model semivariogram to describe the spatial variability of each continuum. Although a number of strategies for sampling spatial distributions have been published in the literature, little attention has been paid to the optimization of sampling in resource- or access-limited environments. Here we present a strategy for estimating the minimum sample spacing needed to define the spatial distribution of fractures on a vertical outcrop of basalt, located in the Box Canyon, east Snake River Plain, Idaho. We used fracture maps of similar basalts from the published literature to test experimentally the effects of different sample spacings on the resulting semivariogram model. Our final field sampling strategy was based on the lowest sample density that reproduced the semivariogram of the exhaustively sampled fracture map. Application of the derived sampling strategy to an outcrop in our field area gave excellent results, and illustrates the utility of this type of sample optimization. The method will work for developing a sampling plan for any intensive property, provided prior information for a similar domain is available; for example, fracture maps or ortho-rectified photographs from analogous rock types could be used to plan for sampling of a fractured rock outcrop.

Evolution of fracture porosity and permeability during folding by cataclastic flow: Implications for syntectonic fluid flow

The Canyon Range Syncline, central Utah, folded and continued to tighten by cataclastic flow, where fracture-bound blocks, defined by a distributed network of mesoscale (outcrop) fracture sets slid past each other. Thin zones of microscale cataclasite coat many of the fracture-bound blocks9 surfaces. Different generations of fracture sets used to accommodate cataclastic flow have been unraveled using crosscutting relationships and are used to track different stages of the syncline9s folding history. Many of the fracture sets preserve evidence for fluid flow (such as iron-oxide precipitates) at different stages of folding. The number of generations of quartzite and iron-oxide cataclasite zones preserved along the mesoscale fractures within the Canyon Range Syncline is used here, in conjunction with mesoscale crosscutting relationships, to develop a three-dimensional kinematic model for fracturing and potential fluid flow during folding. This study shows that there is no relationship between porosity and permeability with degree of deformation, i.e., amount of folding. Also, slight lithological variations play a large role in the geometry of the interconnected fracture network.

Karst conduit flow in the Cambrian St. Lawrence Confining Unit, southeast Minnesota, USA

Southeastern Minnesota’s karst lands support numerous trout streams created by Paleozoic bedrock springs. Several of the Paleozoic bedrock units are recognized as karst aquifers. Recent field investigations have discovered sinking, and one losing, streams in the Cambrian St. Lawrence Formation. The siliciclastic-dominated St. Lawrence Formation has historically been viewed as a confining unit and is designated as such in the Minnesota Water Well Code. Stream sinks in the St. Lawrence Formation have been identified in five different streams, and there are numerous other streams crossing the St. Lawrence that have yet to be investigated. Dye traces have been conducted on the five streams. At all of the sites, dye was recovered at springs emanating from the base of the St. Lawrence Formation. The initial dye breakthrough occurs quickly, but the breakthrough curves have tails that continue for months to over a year. Breakthrough travel velocities from the sinks to the springs are 35–750 m/day. The five dye tracing sites are geographically separated by 65 km. The subcrop of the St. Lawrence Formation is found throughout southeastern Minnesota and southwestern Wisconsin. The St. Lawrence Formation has well-developed macroporosity; most of the formation is moderately to well cemented with dolomite and contains abundant horizontal and vertical fractures in outcrop and subcrop conditions. Volumetrically minor dolomite beds with centimeter-scale solution cavities are common in the lowermost part of the formation. These attributes, combined with rapid groundwater flow as determined by dye tracing, supports the conclusion that this purported confining unit has karst conduit flow properties in subcrop conditions.