Cored sediments from the gas hydrate prospect area offshore southwestern Taiwan exhibit porosities, water contents, and bulk densities comparable to sediments in other gas-hydrate potential areas. Short-distance transport and rapid deposition of the sediments are implied by their poor sorting and detrital and clay mineralogy. These features and the organicrich nature of the sediments present an environment suitable for formation of gas hydrates. High methane concentrations of porewaters and gas-escape structures of the sediments further imply possible occurrence of gas hydrates in the region. Prominent overgrowth microstructures on authigenic pyrite framboids at shallow depths are consistent with sulfate reduction and pyrite precipitation related to migration and oxidation of methane possibly released from gas hydrate zones at later stages of early diagenesis.