Multiple episodes of sand injection leading to top seal failure: An outcrop study of the Santa Cruz Injection Complex, California

The presence of sand injections has been shown 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 some 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. The first event, datable to the Late Miocene, resulted in large volumes of sand being emplaced within the top-seal units, and was followed by accumulation of hydrocarbons within the newly injected sandstones. Later, a series of brittle tectonic events, associated with the San Andreas/San Gregorio Fault System, caused remobilization and accumulation of sand along newly formed fault planes. Our case study documents this combination of pervasive brittle deformation and sandstone injection along fault structures, which can ultimately disrupt the integrity of a host unit leading top seal failure and leakage of hydrocarbons.