Basement-hosted sand injectites: use of field examples to advance understanding of hydrocarbon reservoirs in fractured crystalline basement rocks

Sedimentary injectites hosted within basement rocks, where preserved on land, offer a means to investigate the geometry, extent, dimensions and spacing of fractures that form an interconnected network within faulted/fractured crystalline host rock. Hydrocarbon occurrences within fractured basement are of high interest following the success of basement-targeted exploration in the North Sea and UK continental shelf. In some cases, these settings lack direct access to the fractured basement that constitute the crystalline-rock reservoir, due to the presence of a thick sedimentary cover. For this reason, we investigated two regional-scale crystalline-rock-hosted systems of sedimentary injectites and sediment-filled fractures that occupy basement fracture arrays within granitoids. Localities are in the Serre massif of Calabria, Italy and the Front Range of Colorado (USA). The injected sediment within fractures acts as a natural ‘proppant’ that maintained open pathways for fluid migration or accumulation. Study of the arrays of sediment-filled fractures and faults advances our understanding of unconventional fluid migration pathways, controls upon the porosity and permeability, and the potential of crystalline basement rock to act as a hydrocarbon reservoir