Saline Environment Source Rocks Evaluation of Continental Rift Basin: A Case Study in Lzw Depression, China

Bohai Bay Basin is a petroliferous basin located on the east coast of China. Generally, three organic rich layers (Es3, Es1 and Ed members) have been proved to be the main sources of oil accumulation. The fourth member of the Shahejie Formation (E2s4) was previously paid little attention as it is featured with deep burial depths of more than 3500m (11483ft) and few source rocks were discovered. However, in our recent investigation, regionally extensive evaporites were discovered in E2s4 Formation of LZW depression during the Early Eocene when the Bohai Bay Basin was developing as a rift basin. The E2s4 strata is featured with gypsum, halite, mudstone and carbonates deposits. Influence by Tan-Lu fault activities, a salt arch structure was formed. By means of a large amount of geochemical analysis, we studied the characteristics of source rocks in the saline lacustrine basin and analyzed the origin of source rocks in saline environment. Theory of transgression, deep brine and closed evaporation were proposed, of which closed evaporation theory is the most probable origin, we finally find evaporation is greater than the injection in the enclosed environment and deposits overlying evaporites are often of high quality.