Fractured basement — an overlooked play type with significant potential from a global seismic database

Abstract

Fractured basement reservoirs are most commonly defined as metamorphic and igneous rocks unconformably overlain by a sedimentary sequence, where faulting has led to the creation of a natural fracture network where hydrocarbons can accumulate. Here, we also consider basement rocks to include those of sedimentary origin with little or no matrix porosity (North, 1990), such as the Cambro-Ordovician quartzitic sandstones of the Table Mountain group in South Africa. Fractured quartzites and granites are generally considered to be the optimum reservoirs (Koning, 2013). Basement reservoirs have been recognized for decades but are still often disregarded – with wells barely penetrating them – as many oil companies stop drilling as soon as basement rocks are intersected. Where basement has been penetrated to a sufficient depth (approximately 300 m), significant volumes of undiscovered hydrocarbons may still have been missed by a failure to intersect the fracture systems (Aguilera, 1996). Despite inadequate exploration, fractured basement rocks are important oil and gas reservoirs around the globe (Figure 1). Though many were originally found by chance, the large discoveries made in Vietnam and more recently in the UK are now paving the way for an exploration strategy of this unconventional highly prospective play type.