Research On Sedimentary Facies And Sequence Stratigraphy In The Chang6-8 Oil-beds Of Triassic Yanchang Formation In Longdong Area

Based on the synthetic research of field outcrops, drilling section, well log and office compilation, combined with analyses of geological background, lithological association, sedimentary structure and palaeobiological character, the researched area in the Chang6-8 oil-beds of Triassic Yanchang formation can be divided into deltas, braided deltas, fan deltas and lakes etc. four kinds of sedimentary facies. Furthermore, eleven sedimentary subfacies are identified. Respectively, the sedimentary subfacies are delta plain, delta front, prodelta, braided delta plain, braided delta front, braided prodelta, fan delta plain, fan delta front, fan prodelta, saucer lake and deep lake.On the basis of the types of sand stone, light and heavy mineral associations, planimetric map of sedimentary facies and directions of palaeoflow, it is considered that detrital material of terrigenous deposit of Chang6-8 oil-beds in this area come from southwest provenance, northeast provenance, west provenance and south provenance, respectively, which are the denudation area of Huating-longxian county in southwestern, Yinshan ancient land in the northern, the denudation area of Yantong mountain and Tongxin to the west of Yangjiajuzi - Yinjiacheng county and Qinling ancient land in the southern.There are different main sedimentary facies in different periods of Triassic Yanchang formation in Longdong area and the distribution of petroleum deposit was obviously controlled by sedimentary facies belts. Deltas, fan deltas, and braided deltas developed in this area during Chang8 stage and the fronts were the most importent sedimentary subfacies. The lake basin range was widen and the deep lake subfacies developed in the researched area during Chang7 stage. The lake basin range contracted when it got to Chang6 stage. Consequently, the fan deltas, braided deltas, deltas, saucer lake and local deep lake facies developed in the area. The oil shale in the lower Chang7 oil-bed was the main source rock of oil in the researched area. Major reservoir included sand bodies of subaquatic distributary channels in deltas, braided deltas , fan deltas, and sandbodies of turbidtte in deep lake.The evolutionary process in the late Triassic period of Ordos basin was complex. During ChanglO stage, the Ordos basin was in preliminary subsidence period. The subsidence was enlarged during Chang9 and Chang8 stage and it reached the maximum during Chang7 stage. After this hereafter, the basin began to shrivel and was died out at last . The Ordos basin had gone through the whole process that included the birth, development and wither of the lake basin in the Yanchang formatoin.Regarded . paralled unconformity and discontinuous surface of sedimentary facies as boundary of sequence, the Yanchang formation in the researched area can be divided into four sequence which are belong to model I sequence. It include three systems tracts. Respectively, the systems tracts are Lowstand systems tract, Transgressive systems tract and Highstand systems tract. It is considered that there is a very closed relationship between sedimentary sequence and oil gas accumulation. Sequence controlled the association of source beds , reservoirs and cap rocks, controlled the distribution of reservoirs and affected the stockpile function of reservoirs. Better reservoirs are developed in Lowstand systems tracts while better source beds and cap rocks are developed in Transgressive and Highstand systems tracts.