Giuseppe Palladino, Roberto Emanuele Rizzo, Gustavo Zvirtes, Antonio Grippa, Andrew Hurst, Ruy Paulo Philipp, David Healy, Ian Alsop

The presence of sand injections has proven to enhance the likelihood of hydrocarbon traps within siliciclastic successions. Through the development of large interconnected networks of sills and dykes, sand injection complexes provide a volume of porous and permeable rocks within the low permeability host units. Overall, the formation of sand injection complexes requires extensive fracturing and hydrofracturing, which can be particularly pronounced when sand injections are coupled with brittle tectonic deformation. In exceptional circumstances, this process may threaten the integrity of the reservoir top seal thereby preventing further hydrocarbon accumulation. Studying exceptional exposures along the coastal area of Santa Cruz in California, we report evidence for top seal failure associated with injection episodes. Two distinct sand injection episodes are proposed. A first event, datable to Late Miocene, allowed for large volumes of sand to be emplaced within the top-seal units followed by accumulation of hydrocarbons within newly injected sandstones. Later, a series of brittle tectonic events, associated with San Andreas/San Gregorio Fault System, caused remobilisation and accumulation of sand along newly formed fault planes. This combination of pervasive brittle deformation and sandstone injection along fault structures, as this detailed case study documents, can disrupt the integrity of a host unit, leading to the overall failure of a top seal and leakage of hydrocarbons.