Fault sealing processes in siliciclastic sediments

Abstract

The microstructure and petrophysical properties of fault rocks from siliciclastic hydrocarbon reservoirs of the North Sea are closely related to the effective stress, temperature and sediment composition at the time of deformation, as well as their post-deformation stress and temperature history. Low permeability fault rocks may develop due to a combination of processes including: the deformation induced mixing of heterogeneously distributed fine-grained material (principally clays) with framework grains, pressure solution, cataclasis, clay smear, and cementation. Fault rocks can be classified into various types (disaggregation zones, phyllosilicate-framework fault rocks, cataclasites, clay smears, and cemented faults/fractures) based upon their clay and cement content as well as the amount of cataclasis experienced. In the absence of extensive cementation, the distribution of fault rock types along a fault plane can often be predicted from a detailed knowledge of the reservoir sedimentology. The permeability of fault rocks can vary by over six orders of magnitude, depending on the extent to which the porosity reduction processes have operated. Utilizing the strong link between the petrophysical properties of fault rocks and their geohistory allows the risks associated with fault seal evaluation to be reduced.

A review of fault sealing behaviour and its evaluation in siliciclastic rocks

“…As faults/fractures may perform as conduits for fluid 294 flow, the flow behaviour of faults/fractures is sensitive to quartz precipitation 295 because within the fault zones there are both quartz sources (from 296 dissolution) and nucleation sites for potential cementation. The source for 297 cementation can be internal or external, but Fisher and Knipe (1998) pointed 298 out that natural oil/gas field examples do not always require that an external 299 fluid source controls the sealing properties of the fault zones, especially at a 300 large scale where the external fluids may not promote continuous 301 cementation for extensive sealing. 302…”

From mechanical modeling to seismic imaging of faults: A synthetic workflow to study the impact of faults on seismic

“…Although commonly represented as 2D surfaces in reservoir models, faults are actually narrow zones or volumes of highly and heterogeneously strained rocks, with petrophysical properties differing from those of the host rock (Faulkner et al, 2010 and references therein). Faults are complex and their 3D structure and rock properties distribution depend on factors such as host lithology and stratigraphy (Davatzes et al, 2005;Eichhubl et al, 2005;Bastesen and Braathen, 2010), depth of burial at time of faulting (Fisher and Knipe, 1998), initial fault array geometry and structural evolution (Childs et al, 2009), and diagenesis (Solum et al, 2010). Figure 1a illustrates the differences between an actual fault (Fig.…”

Sealing assessment of normal faults in clastic reservoirs: Modeling the petrophysical and stress attributes of faults.

“…His scheme of fault rock classification applies to both crystalline basements and sedimentary basins. For sedimentary basins, Knipe et al (1997) and Fisher and Knipe (1998) worked out a more specific classification of fault rocks based mainly on deformation processes. A modified scheme of this classification is presented below:…”