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.

Keywords

overpressure / thermal gradient and heat flow / high-wax oil / expulsion efficiency / Damintun depression / Bohai Bay basin

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