Research On High Resolution Sequence Stratigraphy Of Shanxi Formation In Northeastern Part Of Ordos Basin

Ordos basin, which located at the west part of North China platform, is a large-scale superimposed Craton basin. Upper Paleozoic gas-bearing system is coal origin gas-bearing system which have characteristics of self-generation, self-reservoir and self-coverage. Shanxi Formation is important gas-bearing strata, and it was formed on a gentle slope high in north and low-lying in south setting after overall regressive of Taiyuan Formation. Fluvial-delta build-up of terrigenous origin detrital material with steady thickness and distinct characteristics was formed.Based on the principle of high resolution sequence stratigraphy, as well taken into account synthetically base level cycle and volumetric partitioning and facies differentiation controlled by tectonics movement and sediment supply and sea-level change and climate, high resolution sequence stratigraphy division was applied in Shanxi Formation. Shanxi Formation belongs to transitional phase of later Paleozoic long-term base level falling. Shanxi Formation itself makes up mid-term base level cycle, and its ascending half cycle is made up of lower Member of Shanxi Formation, which has the characteristics of several coal-bearing bed and sediment retrogress. The falling half cycle, which has the characteristics of strong transgress towards lake, is composed of upper Member of Shanxi Formation. According to vertical association and cycle characteristics, such as weathering and denudation surface, scour surface, sudden change surface of lithologic character and lithofacies, channel incise, transfer surface between ascending and falling, mid-term cycle of Shanxi Formation can be divided into 5 short-term cycles, namely SSC1, SSC2, SSC3, SSC4, SSC5 in ascending order.From palaeo-structure pattern, distribution of sedimentary system, depositional structure and texture, composition characteristics of debris, cathodoluminescence of quartz, characteristics of heavy minerals, characteristics of rare earth element and trace element, sediment source of study area was discussed. It indicated that debris material of Shanxi Formation mainly derived from pre-cambrian metamorphic rock from the north part of Ordos basin, there is also a little portion of debris come from volcanic rock from the north and northeast part of the basin, while debris derived from sedimentary rock is even less.Based on outcrop observation, core description, single well-log and sequence analysis, connecting-well section, combined with lithological and mineralogical analysis and geochemical method, delta system and fluvial system were identified. Delta system is mainly composed of delta plain and delta front, while pro-delta is not well developed. On delta plain, distributary channel forms framework sand bodies. Between distributary channel, peat swamp and pit, as well as natural levee and crevasse splay are developed. Framework sand bodies of front delta is sub-aqueous distributary channel, while mouth bar and distal bar and front sand sheet are not well developed or preserved. Fine grain sediment of distributary bay is spread widely between sub-aqueous distributary channels. Fluvial system is distributed at the north part of the study area, developed during early and late period of Shanxi formation. Fluvial system is dominated by meandering river, while braided river is not only confined within small area, but short time as well.Appling the method of cartography with single factor and dominance aspect, we have drawn contour line map of sand thickness, contour line map of ratio between sand thickness and stratum thickness, contour line map of coal thickness in each short-term cycle. We discussed distribution characteristics of reservoir sand body and evolution of sedimentary facies and microfacies.Distribution of depositional facies zone and vertical association is controlled by tectonic movement, climate change, sediment supply and base level variation. Shanxi Formation is strongly affected by low amplitude episode tectonic movement. The rate of sediment supply has the feature of intermittence.