The Central Depression of the Wan'an Basin, South China Sea: A Giant Abundant Hydrocarbon-Generating Depression

The Wan'an basin, located in the southwestern part of the South China Sea, belongs to a large scale pull-apart basin. The central depression of this basin is an abundant hydrocarbon-generating sag, which includes two sets of source rocks, that are Oligocene lacustrine mudstones, Early-Middle Miocene marine mudstones. The Oligocene lacustrine source rocks in this depression contain dominantly type Ⅰ-Ⅱ kerogen with high organic carbon contents ranging from 0.50% to (2.26%) and increasing usually from margin to depression center. The Early Miocene marine source rocks comprise types (Ⅱ-) Ⅲ kerogen and have a relatively low organic carbon content of 0.69%-0.93%. Potential hydrocarbon source index in the most of the depression has ranges of 4-10 t/m² with a maximum value of 14 t/m² in the Oligocene source rocks and of 4-7 t/m² in Early Miocene source rocks. The oil-gas pools in the Wan'an basin are mainly distributed in the central depression and its adjacent areas including the NW fault-step belt, the northern and central lifts. But in the southern and northern depressions and nearby eastern lift, only a few hydrocarbon shows are observed. The results indicate that the oil-gas distribution in the Wan'an basin is obviously controlled by the abundant hydrocarbon-generating sag.