Beach subsidence from oil and gas extraction

Removal of natural gas and oil from porous sands causes the overlying rock layers to subside or compact in the area directly above the hydrocarbon reserve. In off-shore oil and gas extraction the sea floor lowers. The extent of the subsidence depends on many factors inherent to the operation, including the nature of sediments and rock and rate of extraction. Examples are provided using historical data from existing hydrocarbon reserves in Louisiana, USA and Venice, Italy, where the rate of subsidence of the land surface was directly related to the rate of hydrocarbon and water extraction. Sandy Islet, Scott Reef, on the Northern Australian continental shelf is the only known sea turtle nesting beach in Australia positioned directly above a hydrocarbon reserve. Being a low sand cay, Sandy Islet is threatened by rising sea levels due to global warming and thermal expansion of tropical waters and increased cyclone activity and intensity. Subsidence of the island by compaction and compression of the underlying strata pose another threat during hydrocarbon extraction. Extraction of hydrocarbons onshore will likely prduce similar compaction that could lower the shore in the immediate area. Such subsidence of the narrow beaches often backed by cliffs could drastically alter existing nesting areas that are used by sea turtles. Examples of the narrow cliff-backed beaches of the Northern Territory will be used to highlight this threat to the northern Australian coastline.