Conny Zeeb

Evaluation of sampling methods for fracture network characterization using outcrops

Outcrops provide valuable information for the characterization of fracture networks. Sampling methods such as scanline sampling, window sampling, and circular scanline and window methods are available to measure fracture network characteristics in outcrops and from well cores. These methods vary in their application, the parameters they provide and, therefore, have advantages and limitations. We provide a critical review on the application of these sampling methods and apply them to evaluate two typical natural examples: (1) a large-scale satellite image from the Oman Mountains, Oman (120,000 m2 [1,291,669 ft2]), and (2) a small-scale outcrop at Craghouse Park, United Kingdom (19 m2 [205 ft2]). The differences in the results emphasize the importance to (1) systematically investigate the required minimum number of measurements for each sampling method and (2) quantify the influence of censored fractures on the estimation of fracture network parameters. Hence, a program was developed to analyze 1300 sampling areas from 9 artificial fracture networks with power-law length distributions. For the given settings, the lowest minimum number of measurements to adequately capture the statistical properties of fracture networks was found to be approximately 110 for the window sampling method, followed by the scanline sampling method with approximately 225. These numbers may serve as a guideline for the analyses of fracture populations with similar distributions. Furthermore, the window sampling method proved to be the method that is least sensitive to censoring bias. Reevaluating our natural examples with the window sampling method showed that the existing percentage of censored fractures significantly influences the accuracy of inferred fracture network parameters.

The Jabal Akhdar dome in the Oman Mountains: Evolution of a dynamic fracture system

The Mesozoic succession of the Jabal Akhdar dome in the Oman Mountains hosts complex networks of fractures and veins in carbonates, which are a clear example of dynamic fracture opening and sealing in a highly overpressured system. The area underwent several tectonic events during the Late Cretaceous and Cenozoic, including the obduction of the Samail ophiolite and Hawasina nappes, followed by uplift and compression due to the Arabia-Eurasia convergence. This study presents the results of an extensive tectonic survey, and correlates subseismic-scale structures in Jabal Akhdar (faults, fractures, veins and stylolites) with the main tectonic events in the Northeastern Arabian plate. As some of the studied formations host large oil reserves in neighboring areas, determining the relative timing of these events in the exhumed rocks is important to understand hydrocarbon distribution and fracture patterns in these reservoirs. The formation of early veins and stylolites in the Oman Mountains is followed by top-to-the-South layer-parallel shearing that may be associated with the obduction of the Samail and Hawasina nappes. This compressional tectonic event is followed by normal (dip-slip) to oblique-slip faults and veins. Top-to-the-Northeast layer-parallel shearing, which corresponds to the first stage of exhumation of the autochthonous rocks offsets these structures. Our new data indicate that this first phase of events is overprinted by complex strike-slip networks of veins and fractures, as well as by the reactivation and onset of seismic-scale faults. Strike-slip structures belong to three distinct events. The first one (NW-SE-oriented compression) is probably associated with the oblique collision of the Indian plate against the Arabian platform during the Late Campanian to the Mid Eocene. The second event (E-W-oriented compression) is likely to have been formed during the Late Oligocene-Middle Miocene during uplift. The last event (NE-SW-oriented compression) probably took place during the Miocene-Pliocene. Structures of the first two strike-slip events have the same orientation as seismic-scale faults observed in the subsurface of Oman and Abu Dhabi. In addition, increasing vein intensity towards the top of the autochthonous formations in the Oman mountains, as well as the small angle between conjugate vein sets, indicate that high fluid pressures that are thought to be present during strike-slip deformation.

Fracture network evaluation program (FraNEP): A software for analyzing 2D fracture trace-line maps

Fractures, such as joints, faults and veins, strongly influence the transport of fluids through rocks by either enhancing or inhibiting flow. Techniques used for the automatic detection of lineaments from satellite images and aerial photographs, LIDAR technologies and borehole televiewers significantly enhanced data acquisition. The analysis of such data is often performed manually or with different analysis software. Here we present a novel program for the analysis of 2D fracture networks called FraNEP (Fracture Network Evaluation Program). The program was developed using Visual Basic for Applications in Microsoft Excel(TM) and combines features from different existing software and characterization techniques. The main novelty of FraNEP is the possibility to analyse trace-line maps of fracture networks applying the (1) scanline sampling, (2) window sampling or (3) circular scanline and window method, without the need of switching programs. Additionally, binning problems are avoided by using cumulative distributions, rather than probability density functions. FraNEP is a time-efficient tool for the characterisation of fracture network parameters, such as density, intensity and mean length. Furthermore, fracture strikes can be visualized using rose diagrams and a fitting routine evaluates the distribution of fracture lengths. As an example of its application, we use FraNEP to analyse a case study of lineament data from a satellite image of the Oman Mountains.

2D or not 2D: Are two dimensions enough to accurately model convective fluid flow through faults and surrounding host rocks?

In many studies of water-rock interaction, convective fluid flow has been invoked to explain diagenetic processes, metamorphism, or metal precipitation. Fluid convection in faults is increasingly recognised as an important mechanism for fluid flow, heat transfer, and mass transport in hydrothermal systems, particularly in consolidated and crystalline rocks. Convection is influenced not only by heat transport processes within the fault but also by lateral heat transfer to and from the surrounding rock mass. There is often a close spatial relationship between major ore deposits and regional scale faults. Most numerical studies simulate free convection in 2D only. This is because fluid patterns are more easily recognised with less complicated geometries, less computational time is required, or because some computer codes are restricted to two dimensions. Using the finite difference simulation code SHEMAT, a series of numerical simulations of thermally driven fluid flow have been carried out to investigate the difference in the fluid flow patterns in 2D and 3D models for the same geological architecture. SHEMAT solves coupled problems involving fluid flow, heat transfer, species transport, and chemical water-rock interaction on a Cartesian grid. In SHEMAT, the different flow, transport, and reaction processes can be selectively coupled. The results of this study show that 2D and 3D models of convection in hydrothermal systems produce significantly different results. In many cases 2D models represent an oversimplification, and conclusions reached from such investigations are likely to be irrelevant. In the case of planar high permeability regions, such as faults and permeable stratigraphic units extending along strike, 2D and 3D modelling outcomes vary significantly. Hence 3D models are absolutely essential to describe the flow field in these cases. An exception is incorporation of an impermeable basement, resulting in 2D convection patterns identical to observed 3D fluid flow fields, but only if vertical fault permeability equals horizontal host rock permeability. Conceptual exemptions are 2D models of high permeability regions with close to radial or linear symmetries, such as damage zones between fault jogs or at fault intersections, giving reasonable results in 2D.