Outcrop Fracture Research Articles

Unravelling the influence of heterogeneities and abstraction on groundwater flow and solute transport in a fractured carbonate aquifer, Sicily, Italy

The migration of a groundwater contaminant plume consisting of light nonaqueous phase liquids (LNAPLs) along the Ionian coastline of Sicily, Italy, has been reported to follow a trajectory that is inconsistent with the regional hydraulic gradient. The influence of several faults affecting the fractured carbonate bedrock aquifer, and groundwater abstraction from a well, were hypothesized to be responsible for the anomalous trajectory of the contaminant plume. A conceptual hydrogeological model was developed for the study area that incorporated structural information derived from geophysical surveys and the mapping of fractures in bedrock outcrops. This conceptual model was incorporated into numerical groundwater flow and solute transport models to simulate the groundwater transport of the light nonaqueous phase liquids. Four model scenarios representing different levels of complexity were tested to assess the relative influence of geological heterogeneity and groundwater abstraction on the migration of the contaminant plume. Results show that underground major discontinuity systems, invoking the presence of the faults in the model domain accounted for the observed migration of the contaminant plume, act as conduits for groundwater flow. Conversely, groundwater abstraction from a well was found to result in relatively minor, localized impacts to the migration of the contamination plume. This study demonstrates the importance of incorporating geological heterogeneity into groundwater modelling and environmental risk assessments associated with the storage of LNAPLs.

Introducing standardized field methods for fracture-focused surface process research

Abstract. Rock fractures are a key contributor to a broad array of Earth surface processes due to their direct control on rock strength as well as rock porosity and permeability. However, to date, there has been no standardization for the quantification of rock fractures in surface process research. In this work, the case is made for standardization within fracture-focused research, and prior work is reviewed to identify various key datasets and methodologies. Then, a suite of standardized methods is presented as a starting “baseline” for fracture-based research in surface process studies. These methods have been shown in pre-existing work from structural geology, geotechnical engineering, and surface process disciplines to comprise best practices for the characterization of fractures in clasts and outcrops. This practical, accessible, and detailed guide can be readily employed across all fracture-focused weathering and geomorphology applications. The wide adoption of a baseline of data collected using the same methods will enable comparison and compilation of datasets among studies globally and will ultimately lead to a better understanding of the links and feedbacks between rock fracture and landscape evolution.

Matching method between nanoparticle displacement agent size and pore throat in low permeability reservoir

Nano-particles possess desirable attributes such as small particle size, excellent injectivity, and migration performance, making them highly compatible and adaptable for addressing the water flooding requirements of the low-permeability oil reservoir. When selecting an oil displacement agent for enhancing water flooding and improving oil recovery, factors such as injectivity and migration need to be carefully considered. In this study, through a comprehensive analysis of the mechanism and technical characteristics of nano-particle oil displacement agents, the plugging and profile control mechanisms recognized by the mainstream of nano-particles are elucidated. By examining various elements including outcrop fractures, natural micro-fractures, artificial support fractures, and dynamic monitoring data, a reevaluation of the dominant channel scale governing water drive in low permeability reservoirs is conducted, thereby defining the target entities for profile control and flooding operations. Drawing upon Darcy’s percolation law and leveraging enhanced oil recovery techniques based on the classical Kozeny equation, a profile control and flooding mechanism is proposed that focuses on increasing the specific surface area of polymer particles while simultaneously reducing reservoir permeability. This innovative approach establishes a novel matching method between nano-polymer particles and the diverse media found within the reservoir. Lastly, the application of nanoparticle flooding technology in Changqing Oilfield is presented, highlighting its practical implementation and benefits.

Discontinuity development patterns and the challenges for 3D discrete fracture network modeling on complicated exposed rock surfaces

Natural slopes usually display complicated exposed rock surfaces that are characterized by complex and substantial terrain undulation and ubiquitous undesirable phenomena such as vegetation cover and rockfalls. This study presents a systematic outcrop research of fracture pattern variations in a complicated rock slope, and the qualitative and quantitative study of the complex phenomena impact on three-dimensional (3D) discrete fracture network (DFN) modeling. As the studies of the outcrop fracture pattern have been so far focused on local variations, thus, we put forward a statistical analysis of global variations. The entire outcrop is partitioned into several subzones, and the subzone-scale variability of fracture geometric properties is analyzed (including the orientation, the density, and the trace length). The results reveal significant variations in fracture characteristics (such as the concentrative degree, the average orientation, the density, and the trace length) among different subzones. Moreover, the density of fracture sets, which is approximately parallel to the slope surface, exhibits a notably higher value compared to other fracture sets across all subzones. To improve the accuracy of the DFN modeling, the effects of three common phenomena resulting from vegetation and rockfalls are qualitatively analyzed and the corresponding quantitative data processing solutions are proposed. Subsequently, the 3D fracture geometric parameters are determined for different areas of the high-steep rock slope in terms of the subzone dimensions. The results show significant variations in the same set of 3D fracture parameters across different regions with density differing by up to tenfold and mean trace length exhibiting differences of 3–4 times. The study results present precise geological structural information, improve modeling accuracy, and provide practical solutions for addressing complex outcrop issues.

Multiscale mapping and scaling analysis of the censored brittle structural framework within the crystalline bedrock of southern Finland

Fracture studies commonly lack data for the length range between 10 m to 1 km. For this reason, scaling laws are required to extrapolate fracture properties, for example in discrete fracture network models. This study focused on analysis and correlation of topology, orientation and length distribution of multiscale fracture datasets to assess their scalability. The used datasets comprise UAV-derived photogrammetric models from natural outcrops and lineaments mapped using airborne LiDAR, bathymetry and aerogeophysical data, in several contrasting scales and resolutions. This study highlights challenges in acquiring uncensored and coherent brittle structural datasets from source data characterized by a large span of resolutions between the remote sensing datasets and models of the fractured outcrop. In specific, collected data was found to be potentially biased and affected by uncertainties related to both the censoring by sedimentary cover and the scale of observation. Our results revealed differences between lineament and outcrop fracture orientations, as well as difficulties in assessing topological parameters from lineament datasets. The 1:200000 resolution was found best suited to the mapping of lineament length and resulted in a length distribution power law exponent of -1.92. For outcrop fractures that are less than 2 m long, the lognormal length distribution provided the only good fit to our data, while the longer outcrop fractures fitted relatively well with a power law exponent of -2.26.

Study on Calculation Method of Heat Exchange Capacity and Thermal Properties of Buried Pipes in the Fractured Rock Mass-Taking a Project in Carbonate Rock Area as an Example

Fractures are developed in carbonate rock areas, and the fracture water flow significantly influences the heat exchange between buried pipes and the rock mass by induing heat convection, providing the carbonate rock area a strong heat exchange capacity and preferable conditions for shallow geothermal development and utilization. In this paper, the calculation method of heat exchange capacity of buried pipes based on fracture distribution characteristics is proposed and deduced, featuring such advantages as quick speed and low cost. Taking an actual project in carbonate rock area as an example, the heat exchange capacity of buried pipes was obtained by the following two methods: in-situ thermal response test and calculation based on fracture distribution characteristics. In the thermal response test, the initial ground temperatures of the two test holes were 15.18 °C and 12.72 °C. By fitting the linear equation of time and average temperature with a linear thermal source model, the heat exchange capacities were 57.21 W/m and 58.22 W/m, the thermal conductivities were 3.56 W/(m·K) and 2.32 W/(m·K), the thermal diffusivities were 1.71 × 10−6 m2/s and 1.12 × 10−6 m2/s, and the volume specific heat capacity was 2.08 × 106 J (m3·K). The test results indicated that the thermal property parameters of rock and soil mass were higher than those of other areas, with obvious wide-range distribution characteristics. Through the statistical analysis of outcrop fracture characteristics, combined with the cube law to calculate the fracture water flow and convective heat transfer, an alternative method for the calculation and optimization of buried pipe heat transfer in fractured rock mass area is also proposed in this paper. According to the measured fracture distribution characteristics of the field outcrop, the heat exchange capacities of the two holes were 57.26 W/m and 58.56 W/m, which were basically consistent with the thermal response test values and verified the reliability of the calculation method of heat exchange capacity of buried pipes based on fracture distribution characteristics.

Geometric properties and scaling laws of the fracture network of the Ypresian carbonate reservoir in central Tunisia: Examples of Jebels Ousselat and Jebil

The fracturing network has a significant impact on fluid circulation and productivity in fractured buried reservoirs. In this work, we present the results of the analysis of outcrop fracture networks in the Ypresian nummulitic limestone hydrocarbons reservoir, located in Central Tunisia. The study is based on outcrops in two anticlinal structures of different extensions and geometries: Jebels Ousselat and Jebil, in the North-South Chain of Central Tunisia. The fracture network was characterized by the analysis of lineament attributes, derived by photo-interpretation of the fracturing data at the regional scale. Herein, we demonstrate that the distribution of fracture lengths (L), treated as distinct sets or taking in consideration the whole directions, fits a power law. Likewise, the spacing (E) between fractures of length (L), for each distinct set, adjusts to a law of the same type. Statistical distributions of lineament length and spacing follow power laws, highlighting the fractal nature of the fracture network. Further, the coefficient of variation of lineament spacings and the characteristic exponent of the power-law of lineament length indicate that the fracturing process is intense in the region and the fracture network reaches a mature stage of development. Fractal analysis of the fractures network of the Ypresian Reservoir in Jebels Ousselat and Jebil displays a self-similar nature of these parameters over a scale range extending from 0.7 cm to 22.4 cm. The obtained geometric parameters (scaling laws) indicate that the network has reached an advanced stage of development (distribution of fracture lengths in power-law) and is fractal (self-similar).

The fracture-controlled carbonate Brejões Karst System mapped with UAV, LiDAR, and electroresistivity in the Irecê Basin - Brazil

Understanding how fractures influence fluid flow in carbonate rocks is essential to understanding karst evolution, oil reservoirs, and aquifer exploitation. The present study investigates karst fracture geometry at the surface and subsurface in the Irecê Basin, a Neoproterozoic epicontinental cratonic basin in the São Francisco Craton, Brazil. We used an Unmanned Aerial Vehicle (UAV) to map both fractures in three outcrops and valley geometries, Light Detection And Ranging (LiDAR) to map 3500 m of cave conduits, and Electrical Resistivity Tomography (ERT) to identify karst zones between 90 and 110 m deep. Our results indicate that the Brejões Karst System is comprised of dissolution and collapse features such as caves, dolines, pavements, and karst valley. The quantitative analysis of subseismic fractures (<50 m long) identified 4398 fractures in Outcrop A, 2458 in Outcrop B, 4374 in Outcrop C, and 7172 in 18 cross-sections of the Brejões Cave. These fractures form a corridor with four main sets striking N–S, NNE–SSW, NW–SE, and E–W, where the fracture system is essential to cave connectivity, permeability, and development. The highest frequency of fractures coincides with the main cave axes, where the intersection of the fracture/fault are sites prone to dissolution. The ERT profiles indicated a high density of geophysical anomalies that correspond to enlarged fractures in the epikarstic zone that extend between the surface and approximately 30 m deep. The cave infill impacts the cave characterization and leads to underestimating cave dimensions. This study may contribute to an underestimation of the role played by subseismic scale fractures in karst evolution in gently deformed settings.

A new subsampling methodology to optimize the characterization of two-dimensional bedrock fracture networks

This paper introduces a new subsampling method to determine the empirical relationships between the areal sampling coverage and the topological-geometric parameters resulting from characterization of two-dimensional outcrop fracture networks. We further developed these relationships into correlations between the total sampling coverage and variance of the characterization result, hence providing insight to the objective uncertainties related to fracture network characterization. The analyses were conducted using two new open-source Python packages: fractopo and fractopo-subsampling, designed for fracture network analysis and subsampling, respectively. We conducted the study on the well-exposed crystalline outcrops of Getaberget, Åland Islands, Finland, where a total of 42499 fracture traces were manually digitized from 13 circular target areas. For the purposes of subsampling, we conducted fracture network characterization for randomly located and sized subsample areas, which locate within the larger target areas.Based on our subsampling results we provide recommendations for the preliminary optimization of areal coverage used in outcrop fracture sampling and the use of our subsampling method for assessing the precision related to the areal fracture network characterization in other previously uncharacterized areas. As an example, we recommend using a total sampling area of 8000 m2 with 8 circular sampling areas to define the power-law exponents of fracture traces when conducting outcrop fracture network characterization with drone-based methodology done with similar initial sampling setup in comparable geological environments, as this coverage has shown an acceptable level of precision.

Paleostress inversion and outcrop fracture analysis of the brittle deformation along Main Boundary Thrust (MBT), NW Himalaya, Pakistan

Paleostress inversion and outcrop fracture analysis of the brittle deformation along Main Boundary Thrust (MBT), NW Himalaya, Pakistan

The joint sets on the Lilstock Benches, UK. Observations based on mapping a full resolution UAV-based image

Outcrop studies of fracture networks are important to understand fractured reservoirs in the subsurface, but complete maps of all fractures in large outcrops are rare due to limitations of outcrop and image resolution. We manually mapped the first full-resolution UAV-based, Gigapixel dataset and DEM of the wave-cut Lilstock Benches in the southern Bristol Channel basin, a classic outcrop of layer-bound fracture networks in limestones. We present a map of the patterns and age relationships of successive sets of joints in dm-thick limestone layers separated by claystone beds. Using interpretation criteria based on crosscutting relationships, abutting and joint length, up to eight successive sets of joints were mapped. Results show that joint geometry and interrelations are fully resolved in the whole outcrop. Different joint sets have unique characteristics in terms of shape, orientation, spatial distribution and cross-cutting relations. The presence of low-angle crossings and junctions of joints suggest periods of partial joint sealing and reactivation. The dataset and interpretations are proposed as an outline for large scale, complete fracture network mapping to test digital fracture network models.

Mechanical stratigraphy in Mesozoic rocks of the San Juan Basin: Integration of stratigraphic and structural terms and concepts

We characterize relationships between stratigraphy and natural fractures in outcrops of Mesozoic strata that rim the San Juan Basin in New Mexico and Colorado. These outcrops expose fluvial and shallow-marine siliciclastic deposits and calcareous mudstones deposited in a distal marine setting. We focus primarily on a regionally extensive fracture set formed during the Eocene to minimize localized tectonic effects on fracture development. Where possible, we supplement our observations with wireline log- or laboratory-derived measurements of rock properties. Our goals are twofold: 1) to illustrate how direct integration of data and concepts from stratigraphy and structural geology can lead to better fracture characterization, and 2) to develop thought processes that will stimulate new exploration and development strategies. Genetic beds form one scale of stratification in the outcrops we describe. For example, sandstone beds can be arranged into coarsening and thickening upward successions that are the depositional record of shoreline progradation. In fluvial settings, cm- to dm-scale sandstone beds can also be part of m-scale single-storey channel complexes that, themselves, can be arranged into amalgamated channel complexes 10s of m thick. In these and other settings, it is important to distinguish between beds and features that can be defined via wireline logs because it is the former (cm- to dm-scale) that are usually the primary control the distribution of natural fractures. The extension fractures we describe are typically bed-bound, with bedding being defined by lithology contrasts and the associated changes in elastic properties. Fracture spacing distributions are typically lognormal with average spacing being less than bed thickness. Although mechanical bedding and depositional bedding are commonly the same, diagenesis can cut across bed boundaries and complicate this relationship, especially where lithologic contrasts are small. Deposits from similar depositional environments which undergo different diagenetic histories can have substantially different mechanical properties and therefore deform differently in response to similar imposed stresses.

Paleostress and outcrop fracture analysis along Himalayan Foothills (Eastern Salt Range), Potwar Plateau, NW Himalaya, Pakistan

The Salt Range (SR) is an ENE-WSW trending fault bend fold that form the range front of the Himalayan fold and thrust belt in Pakistan. The research is carried out in the Eastern Salt Range (ESR) with the objective to determine the paleostress inversion from reduced stress tensors by using fracture data. The surface morphology of the fold is predominantly shaped by Eocene limestone that provide best exposure of joint surfaces that can be used for kinematic and dynamic analysis. We adopted a classical circle inventory method in the field and collected orientation data (dip amount, direction, and strike) from 7 localities (outcrop stations) of the Eocene Sakesar Limestone. Three prominent fracture trends are present in the study area namely, FS-1; (E-W), FS-2; (ENE-WSW) and FS-3; (NNE-SSW). For stress analysis and data processing we used the Tensor Program of Delvaux and Sperner (2003) and calculated seven stress fields by the Right Dihedron Method. The orientations of the principal stress axes (ϭ1, ϭ2, ϭ3) and Stress Ratio (R) depicts ϭ1(Shmax) and ϭ2 are sub-horizontal while ϭ3 is vertical in all stress tensors. The paleostress results show that ϭ1(Shmax) oriented NNE-SSW belonging to a compressive regime. It is suggested that ϭ1 developed more or less perpendicular to the trend of SRT and other relevant structures in the Potwar Plateau.

Effect of fracture characteristics on history matching in the Qamchuqa reservoir: a case study from Iraq

Carbonate reservoirs in the Middle East hold a significant proportion of the remaining conventional oil reserves, which most of them were classified as naturally fractured. These reservoirs are well known for their low recoveries compared with their counterpart clastic unfractured reservoirs. Therefore, boosting their recoveries can assure to sustain the energy supply to the global oil markets. In this work, an example from the Middle East has been used to evaluate the impact of the fracture characteristics on the reservoir performance using four decades of production history. The characteristics of natural fractures have a remarkable impact on fluid flow behaviour in fractured reservoirs. Besides, these characteristics, such as fracture intensity, determines the matrix block dimensions, which in turn, influence the matrix recovery. However, it is challenging to estimate the fracture characteristics accurately due to the scarcity of the fracture data from subsurface measurements. Therefore, outcrop analogue data can be utilised to enhance the fracture modelling in fractured reservoirs and to fill the data gap between the seismic scale and well scale. In this work, we have integrated the outcrop fracture data with the core observations to improve the fracture modelling in the studied reservoir. Furthermore, a sensitivity study was carried out to investigate the impact of flow curves, gravity drainage mechanism, and its parameters, as well as the fracture storativity on the history matching in fractured reservoirs. Excellent matching results were obtained for the majority of the wells, as indicated by the low mismatch values. The obtained results highlighted the importance of fractures on the fluid flow behaviour, and hence the reservoir performance. Therefore, fracture properties should not be abusively modified using multipliers and local adjustment to avoid nullifying the reservoir characterisation efforts and degrade the simulation model capability for a future prediction.

Connectivity evaluation of fracture networks considering the correlation between trace length and aperture

Connectivity evaluation of fracture networks is important in the design, assessment, and development of reservoirs in various engineering applications involving geothermal exploitation and the petroleum industry. By employing nonhomogeneous Poisson distribution and annealing arithmetic, this study generates fracture networks that fit well with actual outcrop fracture data. Based on Allard's definition of the connectivity index, a weighted factor is introduced, and an extended connectivity evaluation method is proposed to consider the effects of the aperture and its correlation with trace length. The results of the analysis show that the extended method improves the accuracy and reliability of connectivity evaluation compared with the traditional method. Moreover, the extended method is effective and accurate at predicting potential preferential flow paths in a practical example of the Dragon and Tiger Mountain (Jiangxi, China), and can better show the anisotropy with a change in the aperture. Hence, the proposed method extends the function of connectivity analysis and can benefit well location optimization in geothermal or petroleum exploration.

Developmental characteristics and controlling factors of natural fractures in the lower paleozoic marine shales of the upper Yangtze Platform, southern China

This work elucidates the developmental characteristics and controlling factors of natural fractures, and their effects on the shale gas content in the lower Paleozoic marine shales of the upper Yangtze Platform, southern China. These objectives have been achieved by systematic observations and descriptions of fractures in outcrops and drilling cores, as well as experimental tests on samples from the corresponding fractured intervals. The types of natural macro-fractures observed in both the Niutitang and Wufeng-Longmaxi shales mainly include vertical tension fractures, high-angle shear fractures, bed-parallel slip fractures and bedding fractures, with bedding fractures accounting for the largest proportion. In addition, Field Emission-Scanning Electron Microscopy (FE-SEM) images show that interlayer, inter-particle, intra-particle and organic matter-associated fractures are the primary types of micro-fractures. Tectonism is one of the dominant factors in fracture development, but other factors also have a strong influence, such as TOC content, mineral composition and shale-bed thickness. Under similar tectonic settings, the development of fractures is mainly influenced by the TOC content. For the Wufeng-Longmaxi shale, fracture abundance is positively correlated with the TOC content. However, the TOC content in Niutitang shale displays a positive correlation with fracture density only when the TOC content is less than 6%; when the TOC content is greater than 6%, the correlation is shown to be negative. Shale with higher quartz or lower clay contents exhibits greater fracture development. Furthermore, the shale-bed thickness has a negative influence on the fracture development, which can be essentially attributed to the change in mineral composition of shale. Natural fractures provide reservoir spaces for free gas and are favorable for the desorption of adsorbed gas; therefore, the better shale fractures are developed, the greater the gas content will likely be.

Estudo hidrogeológico e geofísico do aquífero fissural no Campus da UFMT-Cuiabá-MT

Os aquíferos fissurais representam uma das mais difíceis questões da Hidrogeologia, por se tratar de aquíferos anisotrópicos e heterogêneos, nos quais a agua subterrânea circula por fissuras nas rochas, principalmente estruturas rúpteis geradas por ação tectônica, como fraturas e falhas. O Campus da UFMT, em Cuiabá, se sobrepõe a um aquífero fissural instalado em rochas metassedimentares do Grupo Cuiabá, tal aquífero até o momento, não foi devidamente estudado. Esta pesquisa visou definir as características hidráulicas e estruturais do aquífero; determinar as direções preferenciais do fluxo hídrico subterrâneo; e definir pontos favoráveis à futuras perfurações. As direções preferenciais do fluxo hídrico subterrâneo na área de estudo, foram determinadas através de ensaios geofísicos pelo Método da Resistividade com arranjo Dipolo-Dipolo; em conjunto com medições das atitudes das fraturas em afloramentos rochosos, além de fotointerpretação a partir de fotografias aéreas em escala 1:8.000. As direções assim definidas para o fluxo subterrâneo foram N30-40E, N30-40W e N60-70W. As características hidráulicas dos poços instalados no campus da UFMT foram definidas através de uma analise dos relatórios de testes de bombeamento disponíveis nos arquivos desta universidade. Ficou constatado que os poços apresentam uma variação em suas vazões específicas, sendo estas; 0,054 m3/h/m, 0,227 m3/h/m e 0,67 m3/h/m. Tal heterogeneidade é característica dos aquíferos fissurais.

Effect of transverse faults on fracture characteristics and borehole instability in the Asmari reservoir of Zagros folded belt zone, Iran

In this paper, the effect of transverse faults on the orientation, density and fractures apertures and amount of mud loss is investigated in Asmari Formation as a major oil reservoir in Zagros folded belt zone, Iran. This carried out on three selected anticlines as the surface Kuh-e-Kamarab, and subsurface Marun and Aghajari oil fields. Subsurface data on fracture characteristics were obtained from Image logs, mud losses, and reported drilling data. These are compared with outcrop fracture characteristics gathered from fieldwork and satellite image interpretation. The results show that fracture orientations are almost identical in both outcrops and subsurface data, whereas the fractures density and aperture are different along and across the folded layers and are generally governed by their location with respect to the transverse faults. The closest sector to the faults displays the highest fracture density and greater aperture. Quantitative analysis of fractures from the Kuh-e-Kamarab Anticline indicates average fracture density almost two times greater close to the Izeh-Hendijan transverse (IZH) fault than the other parts of the fold. In the Marun and Aghajari oil fields, fracture density shows three to five times greater close to the transverse faults. Analysis of the Breakout (BO) and Drilling Induce Tensile Fractures (DITF) indicate that the directions of maximum horizontal stress (SHmax) in the wells located in the bent part of the Marun field consistent with the stress state of the Mn transverse fault. Moreover, the SHmax orientation of wells in the southern periclinal closure of the Aghajari Field is also affected by the IZH transverse fault. The result of Iso-mud loss maps in the oil fields reveals that the severe mud loss occurred in areas close to the transverse faults.

2D spatial analysis of the natural fractures in the organic-rich Qusaiba Shale outcrop, NW Saudi Arabia

Natural fracture is the main control on production from unconventional shale reservoirs as it serves as a drainage conduit and promotes the efficiency of the hydraulic fracturing technique. The Qusaiba Shale (QS), particularly its basal unit was recently identified as a potential unconventional shale gas reservoir in Saudi Arabia. This study presents a comprehensive and systematic description as well as statistical analysis of the natural fractures that cut through the QS outcrop in the Tabuk Basin, NW Saudi Arabia. These analyses were used to identify the factors that control the development of a fracture system in this organic-rich Shale. Three main approaches including areal sampling, scanline and topology analyses were utilized to collect fracture measurements. A total of 495 fracture traces were measured. The fracture length ranges between 7 cm and 8.2 m. The measured fracture traces are classified into four groups; subvertical to vertical, high-angle, low angle and bedding to subparallel fracture traces. Using twenty-one scan lines, the total number of fracture traces that cut through are 374, with average fracture intensity ranging from 0.9 to 3.7 fracture/meter. Four main fracture sets trending E-W, NE-SW, NW-SE and NNW-SSE were identified. The measured fractures vary from cemented (59%), partially filled (38%) to open-mode (3%). The cement in fractures is either gypsum or calcite. Applying the topology method analysis, the total number of nodes is 811, where the isolated nodes (I-type) account for 522, the termination nodes (Y-type) accounts for 222, and the cross-cutting nodes (X-type) are only 83. Microfractures are sharp with high-angle, closely spaced, open-mode fractures with lithology-induced terminations. The small vertical to sub-vertical fracture traces are terminated against the more resistant silty laminae. Pyrite shows positive correlation with the linear fracture density. The high amount of pyrite in association with high quartz and subordinate feldspars contents in the studied interval might have enhanced the brittleness of the lithologies and facilitated fracture propagation. Tectonically, two main fracture types exist in the studied section; shear and extensional fractures. The strike of the shear fractures in the study area is NE-SW and NW-SE directions. The open-mode, vertical long fractures cutting through the entire outcrop indicate an extension stress regime, which has already shaped the Tabuk Basin with multiple NW-SE rift structures.

Mineral Filling Pattern in Complex Fracture System of Carbonate Reservoirs: Implications from Geochemical Modeling of Water-Rock Interaction

Much research has been conducted on physical and numerical modeling that focus on stress state and structural controls on subsurface geofluid flow, yet very few attempts have been made to discover and quantify the mineral precipitation/dissolution kinetics in complex fracture system such as Tarim Basin of China. We conducted a geochemical simulation study using the outcrop fracture networks in Ordovician carbonate rocks in Tabei Outcrop Area of Tarim Basin. Structural analysis, filling analysis within the fracture networks and surrounding rocks were used to constrain the generation and geochemical evolution of the geofluids. Using an advanced reactive transport simulation platform TOUGHREACT, a pertinent thermodynamic system was applied to establish the geological model of the fracture-surrounding rock, where the corresponding calcium carbonate (CaCO3) solution was configured to replace the deep saturated hydrothermal fluids. Different types of mineral parameters were considered with material balance and phase equilibrium calculation to perform numerical simulation of multi-field, e.g., pressure field, temperature field, seepage field and chemical field under formation conditions. The simulation results were consistent with field observations. The major findings of this simulation study include: (1) Along with fluid injection, local dissolution occurred within the fractures and matrix, but with the gradual saturation of calcium ions and the increasing pH value, considerable calcite precipitation occurred. (2) The dissolution/precipitation in different fractures was mainly affected by their structure and physical properties, resulting in changes in fluid flow rate, temperature, pressure and ion concentration over time. (3) In the same group, the degree of mineral filling of small-aperture fractures, low-angle fractures and shallow fractures was significantly higher than other types of fractures. (4) The better the connectivity between reticular fractures and the higher the linear density of fractures, the lower the mineral filling degree. (5) Dissolution phenomenon strengthened within large-aperture conjugated fractures gradually along the flow direction. The proposed methodologies in this study can be applied to model effective fracture filling of other deep reservoirs.