Fractured basement reservoirs, often igneous or metamorphic rocks like granite or gneiss, contrast with conventional sedimentary reservoirs. Without primary porosity, they depend on fracture networks for hydrocarbon storage and flow. Effective oil and gas production requires understanding the fracture network and its controls, lithological and mineralogical influences, and the hydrocarbon system.
Fracture networks primarily govern reservoir performance, shaped by tectonic stress and varying in scale, density, and orientation. High-density fracture zones near faults boost permeability and connectivity, aiding hydrocarbon migration and accumulation. The structural setting is vital, with basement reservoirs often in uplifted or faulted blocks, where the tectonic history dictates fracture patterns. Pre-existing weaknesses, such as ancient shear zones, may reactivate, increasing fracture density. Multiple tectonic events may enhance fracturing, particularly along reactivated faults. Present-day stress fields affect fracture aperture, with open fractures aligned to maximum horizontal stress, improving flow rates. Essential datasets, which include borehole imaging, core samples, and 3D seismic, help distinguish open versus sealed fractures, as mineral infills like quartz or calcite can impair permeability.
Lithology and mineralogy significantly impact fracture development and reservoir quality. Hard, brittle rocks like granite fracture easily under stress, forming extensive networks, whereas ductile rocks deform plastically, reducing fracture formation. Mineral composition affects fracture toughness – quartz-rich rocks fracture more readily than those dominated by feldspar. Heterogeneities like dikes or veins can compartmentalize reservoirs or locally enhance fracturing. Petrographic analyses aid in identifying productive zones by clarifying these lithological controls. Additionally, alteration processes, such as weathering in granite, can increase brittleness and fracture density, making the study of erosional un-conformities crucial.
Fractured basement reservoirs follow petroleum system principles, needing a trap, seal, source, migration, and reservoir. The trap is typically an uplift or fault block geometry. The source ideally lies directly above or beside the fractured basement for easy migration into fractures. Key factors also include the diagenetic history—burial and uplift – which impacts fracture preservation. Lastly, geochemical analyses of hydrocarbons and fluid inclusions may help reconstruct this history, verifying an effective petroleum system.
Successful production from fractured basement reservoirs depends on understanding their geological characteristics. Though relatively uncommon, these reservoirs achieve commercial success globally. Notable examples include Bach Ho Field (Vietnam), Renqiu Oil Field (China), Suban Gas Field (Indonesia), Mumbai High Field (India), La Paz Field (Venezuela), Edvard Grieg (Norway), Lancaster (UK), Wilmington and Edison fields in California (USA), and fields in the central Kansas uplift (USA).
