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Chicken-wire fractures

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Why chicken-wire ?

These fractures are often called chicken-wire fractures (Fig. 1) as form a meshwork pattern like that of chickenwire (Fig 1). They are common in carbonates such as chalk and in evaporites such as anhydrite (Fig. 1).

Figure 1. (a) Chickenwire fractures in anhydrite core from a North Sea well, (b) chickenwire itself. The core photo is from North Sea Core Ltd - see ref [2].

Chicken-wire fractures in anhydrite

Anhydrite forms in sabkha environments where the water is shallow and salinity is quite high. Gypsum can precipitate from the water which then dehydrates to form anhydrite (CaS04) often with a distinctive chickenwire texture. Anhydrite that looks like this is often referred to as nodular (Fig. 1a, Fig. 2). Chicken-wire fractures are directly observed on rock core and can also be indirectly observed on image logs [1]

Figure 2. Chickenwire fractures in anhydrite interbedded with carbonate reservoir (brown lithology) in a Middle East oilfield.

Relevance to production

These fractures are generally closed and therefore do not provide any additional permeability. Anhydrites are often pressure barriers and seals to reservoirs in the N Sea (Fig. 3).

Figure 3. Nodular anhydrite/chicken-wire fractures in southern North sea well 47/14a-8. The BGS section is 9028 – 9034 ft.

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