Sequence Stratigraphy of Lower Cretaceous in the Northern Wuerxun Sag

Key words: 
• Wuerxun sag /
• down-faulted basin /
• sequence stratigraphy /
• oil accumulation /

Abstract: The formation and evolution of the sequences in continental down-faulted basin are mainly controlled by eposodic tectonism.The strength of boundary fault activity and the velocity of basement depression are the main reason that lead to the formation of different sequences.Principal marks to identify sequence types are the sequence boundary characters.The mechanisms of formation of sequence boundary and sequence surface are the uplift of whole lacustrine basin,the cease of the movement of boundary fault and the depression of basement.Based on these studies,four types of sequences have been identified from the lower Cretaceous strata in the northern Wuerxun sag,i.e.,simply faulted sequence,quickly faulting-depressed sequence,syngenetic faulting-depressed sequence and simply depressed sequence.The development of simply faulted sequence and simply depressed sequence are mainly controlled by sag boundary fault and basement depression respectively,whereas the formation of quickly faulting-depressed sequence and syngenetic faulting-depressed sequence are dependent on both boundary fault and basement depression.Due to different tectonic background,these sequences are different in scale and composition. Different kinds of sequences have different system tracts and petroleum potential,and the most favorable sequence types are the quickly faulting-depressed sequence and syngenetic faulting-depressed sequence,for they both contain best source,reservoir and seal rocks.