Research On Sedimentary Of The Middle Jurassic Qianfoya Formation In The Yuanba Area Of Northeast Sichuan Basin

Yuanba area is the main exploration and development blocks of Sichuan basin,exploration result demonstrated that the Middle Jurassic has a good potentiality of oil,but there are less studies of Qianfoya Formation. Accordingly, based on the sedimentology, the analysis of the fieldwork, thin section and core observation is used for the studying of sedimentary facies. Then, the depositional model and evolution can be evaluated. This research is benefite for the exploration of the Middle Jurassic hydrocarbon reserves. As a major cross-section view of the present study is cross-sectional Wangcang, Guangyuan, Nanjiang and Quxian field sections.Based on the measured sections and cross-sectional observation wells core, From older to younger, Qianfoya Formation can be divided into three stages. Some variegated mudstone deposited in first stage, which is composed of interbeds of gray fine sandstone- siltstone, argillaceous siltstone and shale, logging trace is serration;the second stage is mainly consist of gray-black shale, with interbeds of thin-bedded fine sandstone-siltstone and argillaceous siltstone, logging trace is box-section;variegated mudstone is mainly deposited in the third stage, and fine sandstone-siltstone is deposited with interbeds of gray-black shale, logging trace is serration and infundibulate.Qianfoya Formation can be divided into 3 subfacies of delta-plain, delta-front and shallow-lake. The delta plain subface includes underwater distributary channel,mouth bar, distal bar, sand shee and subaqueous bay microfacies. Delta-front subface contains the microfacies of distributary channel and overbank. Shallow lake has two microfacies of shallow lake mud and shallow lake bar.The sediment source of Qianfoya Formation is mainly from the Longmenshan and Micangshan-Dabashan. Affected by the Yanshanian tectonic movement, The gradual accumulation of sediments in the study area from east to west The Delta deposition is decreasing from Qian1 to Qian3 stages, and the lacustrine sediment is gradually spread.Qianfoya Formation developed the delta- lacustrine depositional model in the Middle Jurassic., delta-front and shallow lake developed best. Qian 1 stage, the mainstreamway developed in the middle and west area, delta push the lake to the southern edge of the study area; Qian 2 stage, the main streamway developed steadily, delta in east area had begun to shrink, Qian 3 stage, western and middle streamway pushed to the lake,eastern area shrinked to northeast.