Abstract: Relationship between faults and fractures in tight reservoirs

WILLEMSE, E.J.M., D.C.P. PEACOCK, A. AYDIN*, D. D. POLLARD

Outcrop and well data suggest that faults in low porosity rocks are commonly associated with fractures. Sometimes fractures form swarms in the immediate fault vicinity. In other cases, the fracture distribution is more even, and there is no distinct increase in fracture density towards faults. Understanding these contrasting spatial fracture distributions is important for assessing reservoir and well performance.

In limestones along the Bristol Channel (UK), strike-slip faults initiate as en echelon opening-mode fractures. The fractures occur in a narrow zone and link end-to-end by stylolites that cut the bridges between overstepping veins. As fault displacement increases, slip occurs along the solution seams, causing centimeter-scale calcite-filled pull-aparts. Multiple generations of solution seams and opening-mode tail cracks form at the tip of sliding surfaces, progressively fragmenting the material until a through-going fault zone develops. Meter-scale oversteps between such fault zones show pressure solution cleavage. Slip along this cleavage causes end-to-end linkage of fault zones, leading to faults several hundred meters long. Similar structures also occur in carbonaceous shales and siltstones and along dip-slip faults.

In granites of the Sierra Nevada (USA), faulting is preceded by a separate phase of opening-mode fracturing. Faults nucleate along some of the pre-existing veins. Veins then link end-to-end along tail cracks that propagate at a high angle from the ends of the faulted veins. With increasing shear, pairs of faulted veins link up side-to-side to form densely fractured fault zones. Finally, faults hundreds of meters long form by end-to-end linkage of non-coplanar fault zones. In this case, faulted veins and fault zones occur throughout the rock mass, and there is no dramatic increase in fracture density towards the major fault zones. The initial phases of this faulting process have been described in granites, schists and in porous eolian sandstones.

Without cement precipitation, the in-plane permeability of fault zones in both the limestone and granite would be high. However, the structure of the fault zones and the spatial distribution of fractures around them depend partly on the pre-faulting history. If faulting occurs in intact rock, narrow, well-developed fracture swarms develop, separated by unfractured regions. Such reservoirs may be characterized by poor recovery and locally high watercut risk. If faulting occurs in previously jointed rock, faults are embedded within a system of conductive fractures and minor fault zones. In such reservoirs, it may be possible to obtain higher recovery factors with moderate watercut risk.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria