The Pearl River Mouth Basin, a Tertiary depocenter with a multiphase history, is composed of a series of subbasins. The westernmost of these is the Wenchang B Depression, currently being explored by a partnership of Esso China Limited and Shell Exploration (China) Ltd., in cooperation with Nanhai West Oil Corporation of the People's Republic of China. From extensive seismic and well data in the depression, four distinct stages of tectonic development can be recognized. The first stage is crustal accretion that provides a relatively young and weak substrate for later basin evolution. Prerift sag basin development in the early Tertiary represents the second stage. Distribution of fluvial and lacustrine sediments of this period appears to be largely independent of later grab n formation. The third stage of development was the rifting that created the present basin form. Initial low-angle normal faults were quickly overprinted by widespread east-west, high-angle normal faults that show evidence of right-lateral motion. Marine influences first appeared in the deeper parts of the basin midway through the graben infill sequences and gradually became more widespread. The fourth and final stage began in the upper Oligocene, when active rifting ended and margin sag toward the South China Sea began. Weak tensional and wrench faulting, coupled with rare compressional events, continues to the present.