4. Potential High-Quality Reservoir Sediments in the Gas Hydrate Stability Zone

In addition to sandy turbidite exploration targets that are common along many continental margins, sediments having greater reservoir potential may also exist. These terrigenous sediments would have been deposited in continental rifts that evolved into oceanic zones. Terrigenous continental margin sediments, known for their high-quality reservoir characteristics, may be subsided along continental flanks where they are now masked by thick marine sediment sequences. Following this subsidence and as part of a slow subsidence with the shoreline moving progressively up slope, paralic deposition took place along temporary shorelines, often interacting with fluviatile sediment supply or reworking older terrigenous sediment depositing upon the flooding terrigenous sediments. The majority of the subsided terrigenous and paralic sediments are likely to be deeply buried and be below the gas hydrate stability zone (GHSZ) and are exploration targets for conventional hydrocarbons. Younger high-quality reservoir sediments have a greater likelihood of occurring in the GHSZ because they will not be so deeply buried by marine sediments. In addition, margins starved of marine sediment may contain these high-quality terrigenous and paralic sediments. Older high-quality reservoir sediments may also be lifted by faulting so that they are present within GHSZs. Although high-quality reservoirs not formed in marine turbidite systems are liable to be rare, they could be extremely rich.