Sedimentary Evolution Research On Chang 6-8 In Yanchang Formation Of Triassic In Ordos Basin

In this paper, Chang 6-8 oil groups in Yanchang formation of Triassic in Chengchuan-Zhoujiawan area of Ordos basin were selected as the study area. After going through the research achievement of the former scholars, the author systematically studied the source directions, microfacies types and sedimentary system characters of the area, using basic theories of sedimentology, geophysics and geochemistry methods along with drilling and logging data. Taking each oil layer group as a mapping unit and adopting the single factor analysis and multifactor comprehensive mapping method, the author portrayed the individual well histogram, well-tie profiles contrast map and planar distribution map of the sedimentary facies. In the mean time, the author specifically modified the sedimentary evolution in various periods and constructed the sedimentary mode in this area.The general stratum thickness of Chang6-8 oil groups in Ordos basin is about 270-340m, which are relatively stable with a flat terrain.The study area was mainly affected by the northeast sedimentary source region whose source rocks are the Archaean high grade metamorphic rocks and volcanic rocks coming from Aaqing and Yin mountainsGenerally, Chang 6-8 oil groups in Yanchang formation of Triassic in Chengchuan-Zhoujiawan area were meandering river delta sedimentary system. The sedimentary evolution had an intense relationship with the evolution of the lake basin. The study area was located in the juncture of the delta plain and delta front in Chang8 period. In Chang7 period, the lake basin expanded to its almost range. All the study area were delta front sedimentary system. In Chang 6 period, the invasion of the lake had weakened, and the basin began to reduce. The delta along the basin margin began to advance to the basin center. With sedimentary velocity decreasing and the supplementary velocity increasing, the lake basin atrophied,and the propagation of delta in basin margin had strengthened.Controlled by the source direction,Chang 6-8 oil groups in Yanchang formation mainly evolved meandering fluial delta system in northeast direction,crossing Ansai and Dingbian delta system. The distribution of the sandbodies were controlled by the sedimentary facies belt which like a belt from northeast to southwest in Chang6 and 8 period. In Chang 7 period, most sand bodies were lentoid.