 The basement complex has a weathered surface, from long exposure to the elements tens of millions of years ago, prior to its subsiding into the sea and being covered with the sediments that now contain oil.In places the overlying sedimentary layers buttress against steeper portions of the basement schist, which when fractured may itself be an oil reservoir for hydrocarbons seeping upward from those buttressing units.The overlying sedimentary layers are a mix of Holocene, Pleistocene, and Pliocene continental deposits and Miocene marine deposits.