Eivind Bastesen

Fault facies and its application to sandstone reservoirs

The concept of fault facies is a novel approach to fault description adapted to three-dimensional reservoir modeling purposes. Faults are considered strained volumes of rock, defining a three-dimensional fault envelope in which host-rock structures and petrophysical properties are altered by tectonic deformation. The fault envelope consists of a varying number of discrete fault facies originating from the host rock and organized spatially according to strain distribution and displacement gradients. Fault facies are related to field data on dimensions, geometry, internal structure, petrophysical properties, and spatial distribution of fault elements, facilitating pattern recognition and statistical analysis for generic modeling purposes. Fault facies can be organized hierarchically and scale independent as architectural elements, facies associations, and individual facies. Adding volumetric fault-zone grids populated with fault facies to reservoir models allows realistic fault-zone structures and properties to be included. To show the strength of the fault-facies concept, we present analyses of 26 fault cores in sandstone reservoirs of western Sinai (Egypt). These faults all consist of discrete structures, membranes, and lenses. Measured core widths show a close correlation to fault displacement; however, no link to the distribution of fault facies exists. The fault cores are bound by slip surfaces on the hanging-wall side, in some cases paired with slip surfaces on the footwall side. The slip surfaces tend to be continuous and parallel to the fault core at the scale of the exposure. Membranes are continuous to semicontinuous, long and thin layers of fault rock, such as sand gouge, shale gouge, and breccia, with a length/thickness ratio that exceeds 100:1. Most observed lenses are four sided (Riedel classification of marginal structures) and show open to dense networks of internal structures, many of which have an extensional shear (R) orientation. The average lens long axis/short axis aspect ratio is about 9:1.

Fault linkage and damage zone architecture in tight carbonate rocks in the Suez Rift (Egypt): implications for permeability structure along segmented normal faults

Abstract A field study focusing on fracture systems in a fault linkage zone from the Suez Rift, Egypt, is presented to elucidate the role of fault linkage zones in the permeability structure of segmented normal faults in tight carbonate rocks. Fracture systems in the linking damage zone show significantly increased structural complexity compared to that typical of isolated faults. The linkage zone is characterized by high fracture frequencies and multiple fracture sets of different orientations. Notably, pervasive fracture corridors strike at high angles to the fault trend and are interpreted to have formed during the latest evolutionary stages of what is interpreted as a breached relay. The structural observations indicate that along segmented normal faults in carbonate rocks, fault linkage zones represents locations of progressively increased cross- and along-fault permeability through the stages of relay growth and breaching. Our findings, in combination with previously published work, indicate that fault linkage zones represent localized conduits not only for increased fluid flow across faults, but also (vertically) within fault zones. Appreciating this has wide-ranging implications for understanding fluid transport in carbonate rocks and other naturally fractured lithologies.

Comparison of scaling relationships of extensional fault cores in tight carbonate and porous sandstone reservoirs

This paper presents a comparative analysis of the thicknesses–displacement relationships of fault cores in non-/low-porous carbonate and porous sandstone reservoirs. Fault thickness data were collected from extensional faults in mainly Cretaceous continental sandstones and Late Cretaceous–Paleogene marine carbonates exposed along the eastern flank of the Suez Rift, Egypt. The dataset consists of 730 thickness measurements, of which 313 are from 68 faults in carbonate rocks, and 417 are from 120 deformation band/microfaults and faults in sandstones. These data show that the increase in fault core thickness with displacement for the two lithologies is, overall, similar in log–log space, consistent with a power-law trend with an exponent of 0.5. However, for smaller faults, cores in carbonates are generally thicker than those in sandstones. The observation of nearly equal core thickness–displacement relationships for larger faults (>10 m displacement) suggests that increased displacement reduces the geomechanical influence of lithology. In a statistical analysis of bins of fault displacement, small displacement faults seem to follow a skewed distribution towards smaller fault core thicknesses, moderate displacement faults have a normal distribution, while large displacement faults have a distribution skewed towards large core thicknesses. Each thickness distribution can be estimated with a mean and standard deviation. Fault core thickness variations are caused by interplaying factors such as fault geometric development, spatial and temporal positioning of shear and volumetric strains, and rheological changes. The variability in core thickness and intrinsic architecture of fault cores for any given location will cause significant variation in across-fault fluid flow, even for the same juxtaposition style.

Evolution and structural style of relay zones in layered limestone–shale sequences: insights from the Hammam Faraun Fault Block, Suez rift, Egypt

A fully breached relay zone was investigated to gain insight into relay growth and breaching in layered limestone–shale sequences. Associated fracture patterns were analysed to gain a qualitative understanding of fault- and damage-zone evolution and fracture-related fluid conduits associated with fault overlap zones. Internally, the relay zone is characterized by dilatant brittle deformation in mechanically strong limestone of the stratigraphically upper part of the ramp, and down-dip shear along extensional detachments in stratigraphically lower and mechanically weak shale layers at the base. The linking damage zone is characterized by multi-directional fracture patterns, including fracture corridors at high angles to the main (bounding) faults. The causal relationship between fault growth and the complex fracture patterns lies in the interaction and progressive rotation of the local stress fields of overlapping or linking fault segments. Increasingly complex fracture patterns may therefore be expected during growth and linkage of fault segments. The observed fracture patterns, in concert with complex juxtaposition relations generated by dipping relay beds, indicate that fault linkage points in carbonate rocks represent localized conduits for cross-fault as well as vertical along-fault fluid flow. These have implications for hydrocarbon migration pathways and for reservoir connectivity during production from carbonate reservoirs.

Do deformation bands matter for flow? Insights from permeability measurements and flow simulations in porous carbonate rocks

We investigate the permeability and flow effects of deformation bands in porous granular carbonate rocks in Malta and use results from flow simulations to discuss the practical implications of deformation bands in carbonate and siliciclastic reservoirs rocks in general. Image- and laboratory-based analyses of deformation bands show permeabilities that are 1 – 2 orders of magnitude lower than the adjacent host rocks. Small-scale outcrop-based flow models (1 × 1 m) focus on the effect of deformation band on flow at the scale of individual bands. Two-phase flow simulations (water displacing oil) show that at the local scale a decrease in deformation band permeability led to increasing flow complexity, reduced and irregular waterfront propagation and reduction in sweep efficiency. A reduction in host rock permeability is associated with increased sensitivity to deformation bands. In low-permeable host rocks, a single magnitude-order reduction of deformation band permeability significantly delays flow, whereas in higher-permeable host rocks the effect is less pronounced. Hence, in some cases, deformation bands may represent a significant impediment to flow already when they are only 1 – 2 orders of magnitude less permeable than host rock. Consequently, deformation bands may have greater practical implications than previously thought, particularly in reservoir rocks with moderate to low host rock permeability.