The Central Depression of the Wan'an Basin, South China Sea: A Giant Abundant Hydrocarbon-Generating Depression
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摘要: 万安盆地位于万安断裂西侧, 是一个大型的走滑拉张盆地.其中部坳陷为富生烃坳陷, 发育有渐新统湖沼相、海湾相泥岩和下中新统浅海相泥岩2套主要烃源岩, 渐新统烃源岩具有良好的生烃潜力, 下中新统烃源岩次之.万安盆地的油气藏主要分布于中部坳陷及其相邻的西北断阶带、北部隆起和中部隆起, 而在北部坳陷、南部坳陷和东部隆起上仅见油气显示.研究表明, 万安盆地这种油气分布特征明显地受中部富生烃坳陷所控制.Abstract: 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.
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图 5 万安盆地下中新统炭质泥岩甾萜烷生物标志化合物分布特征(Todd et al., 1997)
Fig. 5. Distribution features of biological compound of terpane and sterane in Lower Miocene carbargillite, Wan'an basin
表 1 万安盆地油气藏纵向分布特征
Table 1. Characteristics of vertical oil-gas reservoirs distribution in Wan'an basin
表 2 万安盆地油气横向分布
Table 2. Characteristics of horizontal oil-gas reservoirs distribution in Wan'an basin
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