Evaluation on Source Rocks in Miaoxi Middle-Southern Sag and Its Aspiration to Exploration

The exploration degree on marginal sags in Bohai Sea is low,and the exploration key is to study the hydrocarbon generation and expulsion capability of source rocks.In order to make clear about the hydrocarbon accumulation regularity in Miaoxi middle-southern sag,analysis of geochemical characteristic of source rock and oil-source correlation have been conducted,and the advantage hydrocarbon migration pathways for low mature oils have been made clear.In addition,the hydrocarbon rich target series and zones have been forecasted.The study results indicated that Sha-3 Member is thick and well developed in Miaoxi middle-southern sag,the organic matter matrix type of source rock are mainly mixed type,deposited in brackish water and salty water environments,being main oil sources for Penglai 25-6 oilfield with low mature oil.It has been proved that Sha-3 Member is the main hydrocarbon generation series for the study area.The analyzing results about thermal evolution of source rock and hydrocarbon expulsion intensity indicated that the thermal evolution degree of source rock is low in Miaoxi middle-southern sag,and the driving force for primary hydrocarbon migration is low.Therefore,the amount of hydrocarbon migrated through single type of secondary pathway is higher than that through compound pathway,and the chance of hydrocarbon accumulation in Guantao formation through short distance migration by faults was higher.Faults activity was strong in middle strip-slip zone,and cut deeply the source rocks.The main strip-slip faults and the derivated faults cut anticlines,and the faulted anticline traps have great exploration potential.