Oil and gas is only a part of it. You also have at least 1) the desire to monopolize security over the sea lines of communication passing through the SCS, and 2

Oil and gas is only a part of it. You also have at least 1) the desire to monopolize security over the sea lines of communication passing through the SCS, and 2) the huge threat to projecting its maritime power posed by the chain of islands from Japan through the Philippines to Indonesia