Eruption cycles and stages of Cenozoic volcanic rocks and their significance to hydrocarbon accumulations in Liaohe Basin

The volcanic cycles and stages in the eastern sag of Liaohe Basin were studied to identify interfaces inside the Cenozoic volcanic formations using volcano-stratigraphic methods and seismic volcano-stratigraphy theories. Drilling cores and thin sections from 300 wells were analyzed and interpreted by combining well loggings and seismic profiles. The spatial distribution of volcanic activities was revealed by mapping the volcanic formations of each cycle and stage. The relationship between oil and gas bearing layers and volcanic formations was analyzed to discuss the significance of volcanic cycles and stages to hydrocarbon accumulations. The results suggest that the Cenozoic volcanic formation in the Liaohe Basin is divided into 4 cycles and 15 stages. The 1st cycle at the bottom and the 4th cycle at the top are composed of basalts,which indicates that the Cenozoic volcanic activities began and ended with mafic magmas. The Cenozoic volcanic formation is dominated in the 2nd and 3rd cycle,and these two cycles show the same lithologic sequence from basalt to trachyte to basalt. The distribution of volcanic rocks is controlled by major faults of northeast orientations,and the maximum thickness of volcanic rocks is mainly along the fault zone.Volcanic rocks in the early stage 1 and stage 2 were widely spread in the whole sag,while the basalts in the final stage 15 are limited,and the distribution center of volcanic rocks from stage 3 to stage 14 migrated from north to south. By means of division and correlation of the volcanic sequences,the spatial distribution of the favorable volcanic stages is revealed in order to discover new hydrocarbon bearing volcanic formations for the future exploration. Previous targets are primarily on the trachytes of stage 5in the 3rd Member of Shahejie Formation,however significant hydrocarbon accumulations have been also found recently in the basalts of the adjacent stage 4,stage 6 and stage 7,suggesting a promising future exploration in this area.