Fracture swarms are concentrated zones of open fractures (Figure 1) with significant aperture but lacking any major offsets of marker horizons. It may also be a single fracture with extreme aperture e.g., due to extensive dissolution of a carbonate matrix rock. They are usually more through going than diffuse fractures, and not confined to individual beds (Figure 2, Figure 3). These intervals of high fracture density are unlikely to be preserved in rock core - they may appear as rubble zones or are not recovered at all.
Figure 1. Fracture swarms running obliquely into the cliff. Zechstein Carbonate platforms, Marsden Bay, NE England. There is also pervasive background or diffuse fracturing (Figure 3) which is often bed-bound .
Figure 2. Strata-bound fractures and swarms in the Bibi Hakimeh Field; an ideal shallowing up cycle of the Asmari Formation with a typical fracture pattern in a forelimb of the anticline (Source: Wennberg et al 2005).
Figure 3. Bed bound fractures in dolomite (brown coloured rocks, the limestones are grey coloured). The laminated unit above the limestone (at bottom) is unfractured. This is overlain by unfractured turbidites. the thinner turbidite layer above is heavily fractured. Same coastal section as Figure 1.
In the subsurface, these may be difficult to interpret from static data (seismic, image logs etc) data but it’s well worth checking out dynamic data. High influxes of flow over short intervals on a production log test (PLT) can indicate fracture swarms (Figure 4). It may be that only a few large fractures or swarms control production - obvious, if for example, the PLT log shows that an interval of less than 1 m contributes to > 80 % of the production at a flow of 18 000 bbl/day. I’ve come across seismic scale faults (e.g., Devonian fault example in blog “fault zone terminology”) that contribute little/not at all to flow; it is the swarms (that are detectable on image logs and PLTs for example) that control the flow.
Figure 3. Result from a production log test (PLT) - there are 2 test results (yellow and blue data) for the same well showing that most of the flow occurs over a thin interval.
Mud loss data is cheap (free !) and a very good fracture indicator. Continuous, spurt (small) losses can indicate background or diffuse fractures (joints), whereas more infrequent, larger losses can be related to fracture swarms.
Its also worth checking out well test data - in particular the boundary phase, as linear flow behaviour may be evident.
Reference
O. P. Wennberg, M. Azizzadeh, E. Blanc, P. Brockbank, K.B. Lyslo, S. Ogilvie, L.D. Salem & T. Svånå. 2005. Use of outcrop analogues in fractured reservoir characterization – an example from the Dezful Embayment, SW-Iran. Extended abstract for EAGE 67th Conference & Exhibition — Madrid, Spain, 13 - 16 June 2005.