Geochemical Evidences of Hydrocarbon Expulsion Threshold and Its Application

The concept of Hydrocarbon expulsion is taken as an important theoretical base in the identification of effective source rock, and therefore, plays a significant role in petroleum exploration and resource assessment. Occurrence of hydrocarbon expulsion threshold is discussed by means of geochemical approaches. The results show that the parameters of (S_1+S_2)/TOC, HI showing source rock potential and the abundance of resident hydrocarbons-A decrease obviously with increasing depth once source rocks reach the expulsion threshold, and obvious variation of chemical composition as well as (abundance) and relative distribution of biomarkers can also be observed. Similar chemical compositions between crude oils and soluble organic matters are observed after the source rocks entered the expulsion threshold, however, the discrepancies are present within the source rocks below the expulsion threshold. Taking as an example of the Dongying depression of the Bohaiwan basin, the expulsion thresholds of Es~4 mudstone and shale in Bamianhe area are about 2 200 and 2 000 m respectively, with expulsion intensity of 0-1 100 and 0-150 kg/m~2 and expulsed accumulative hydrocarbons of 7.365×10~8 and 1.435×10~8 t.