Geological conditions favourable for high-wax oil enrichment in Damintun depression, Bohai Bay basin

Abstract

The Damintun (大民屯) depression, a small (about 800 km2 in area) subunit in the Bohai (渤海) Bay basin, hosts nearly 2×108 t of high-wax oils with wax contents up to 60%. The high-wax oils have high consolidation temperatures and viscosities. The high-wax oils were generated from the fourth member of the Shahejie Formation (Es 4), which is also important source rocks for oils in other subunits of the Bohai Bay basin. Yet high-wax oils have not been found in significant volumes elsewhere in the Bohai Bay basin. Geological conditions favourable for high-wax oil enrichment were studied. This study shows that the unusual concentrations of high-wax oils in the depression seem to result from at least three different factors: (1) the presence of organic-matter rich source rocks which were prone to generate wax-rich hydrocarbons; (2) the formation of early overpressures which increased the expulsion efficiency of waxy hydrocarbons; and (3) reductions in subsidence rate and basal heat flows, which minimized the thermal cracking of high molecular-weight (waxy) hydrocarbons, and therefore prevented the high-wax oils from being transformed into less waxy equivalents.