MIGRATION MECHANISMS IN NATURALLY FRACTURED OIL-PRODUCING SILLS, NORTHERN NEUQUEN BASIN, ARGENTINA

Oil migration mechanisms associated with naturally fractured igneous reservoirs have been studied sparsely to date. We examined the geologic drivers controlling expulsion and migration around the Late Miocene oil producing andesitic sills of the Rio Grande Valley, Northern Neuquen Basin, Argentina. We integrated multi-scale data from 3D seismic, ~150 wells, 22 cores, outcrop and qualitative production data. The integration enables us to establish a fracture inventory model of the producing sill reservoirs, revealing consistent geometric patterns of faults, fractures and voids acting as migration conduits. We establish consistent macro- and micro-fracture networks inside and around the sills, that play different roles in migration. To understand their impact on migration, we tested three sill-fracture attributes for correlation with structural position and gross sill thickness; these are percentage of open versus closed fractures, percentage of oil stained fractures and average fracture aperture. Cores close to the sill source rock interface reveal micro-fracturing through brecciation. We confirm that lateral migration is of short distance nature and show evidence that the vertical migration component was large enough to charge sills located stratigraphically high above the source rock. Multiply interconnected macro-fracture networks and cavity zones confirm that the sills themselves could have acted as lateral migration conduits. We show evidence that the effects of alternating kinematic regimes, controlled by the seismic cycle, has enhanced oil migration into the sill reservoirs. Apparently transpressional overprinting of cooling fractures has been underestimated in its importance in charging commercial oil fields in fractured sill plays. The question of what role hydrothermal convection may have played in hydrocarbon transport, remains unclear and requires further research.