The transgressive systems tract of the continental sequences in the Biyang fault depression: oil and gas geological significance

The transgressive systems tract of the continental sepquences in the Biyang fault depression as a lacustrine basin is dominated by a retrogradational fan delta. The fluvial fan facies area becomes narrower and narrower at the edges of the slope zone close to the structural hinges. There are gradations from a littoral-shallow lake basinwards into a bathyal lake.The hydrocarbon reservoirs in the transgressive systems tract consist mainly of braided channel sandstone bodies in the alluvial fan facies,distributary channel,subaqueous distributary channel,chnnel mouth bar,and frontal sheet sandstone bodies in the fan delta facies. Since all of them were laid down in different depositional environments and physical and chemical conditions,the types and contents of the clay minerals from the sandstones and conglomerates may differ greatly and have exerted an important effect on the reservoir quality of the sandstones in the transgressive systems tract.