Abstract
Abstract Deformation bands significantly alter the local petrophysical properties of sandstone reservoirs, although it is not known how the intrinsically variable characteristics of sandstones (e.g. grain size, sorting and mineralogy) influence the nature and distribution of deformation bands. To address this, cataclastic deformation bands within fine- and coarse-grained Triassic Sherwood Sandstone at Thurstaston, UK were analysed, for the first time, using a suite of petrographical techniques, outcrop studies, helium porosimetry and image analysis. Deformation bands are more abundant in the coarse-grained sandstone than in the underlying fine-grained sandstone. North- and south-dipping conjugate sets of cataclastic bands in the coarse-grained sandstone broadly increase in density (defined by number/m 2 ) when approaching faults. Microstructural analysis revealed that primary grain size controls deformation band density. Deformation bands in both coarse and fine sandstones led to significantly reduced porosity, and so can represent barriers or baffles to lateral fluid flow. Microstructural data show preferential cataclasis of K-feldspar grains within the host rock and deformation band. The study is of direct relevance to the prediction of reservoir quality in several petroleum-bearing Lower Triassic reservoirs in the near offshore, as deformation band development occurred prior to Carboniferous source-rock maturation and petroleum migration.
Highlights
The oil and gas industry has expressed a growing interest in deformation bands because they are subseismic, tabular zones of strain localization that can cause large changes to a reservoir’s petrophysical properties (Ballas et al 2013)
The presence of deformation bands at Thurstaston has been documented (Knott 1994; Beach et al 1997); ; this paper provides the first detailed analysis of deformation band distribution, as well as a petrographical description and interpretation
There is a negligible difference in the mineralogy of the deformation band and the host rock
Summary
The oil and gas industry has expressed a growing interest in deformation bands because they are subseismic, tabular zones of strain localization that can cause large changes to a reservoir’s petrophysical properties (Ballas et al 2013). At the Anschutz Ranch East Field, Wyoming, USA, deformation bands separate clean sandstones and bitumen-stained sandstones, implying a strong impact on oil and gas movement (Solum et al 2010). Deformation bands in this study will be classified by the predominant deformation mechanism: disaggregation; phyllosilicate smear; cataclasis; and solution and cementation (Fossen et al 2007). Solution bands are produced when chemical compaction, or pressure solution, is the dominant process; they commonly form at shallow depths and contain minimal cataclasis (Fossen et al 2007). Fresh mineral surfaces exposed by grain-boundary sliding and/or grain crushing provide preferential sites for cementation, creating cementation bands (Fossen et al 2007).
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