Depositional System And Favorable Reservoir Of Shanxi Formation-He 8 Member In Ordos Basin

The Ordos Basin is rich in natural gas resources in the Upper Paleozoic.The Permian Shanxi Formation and the eighth member of the Lower Shihezi Formation are the main gas-bearing formations in the basin.The development and distribution characteristics of the basin sedimentary system and the high-quality reservoir developmental and distribution law of the Permian Shanxi Formation and He 8 Member during the sedimentary period are not very clear,which is the key issue restricting the exploration and deployment of this area.This article takes modern sedimentology principles,sequence stratigraphy and reservoir geology as theoretical guidance,comprehensively uses data such as outcrop,core,logging,thin slice and laboratory analysis and test,combined with sedimentary tank simulation experiment and thin slice identification,scanning electron microscope and other microscopic Analytical methods focus on the main contents of sedimentary system characteristics,formation and distribution of relatively high-quality reservoirs.Based on the sedimentary environment identification of sedimentary facies,five major types of depositional systems,such as alluvial fans,rivers,braided river deltas,meandering river deltas,and lakes,and corresponding types of sedimentary facies are identified.It is believed that there is transgression in the southern part of the basin in the early deposition of Shanxi Formation.On the basis of comprehensive source analysis,paleomorphology and ancient water system restoration,and sequence stratigraphic division and comparison,the depositional system and sedimentary facies distribution of each sequence are clarified,two types of Basin sedimentary fill patterns are established,which are "Alluvial-Shallow Lake" of SQ1?SQ4(Shanxi Formation)and "Alluvial-flood lake" of SQ5(Box 8 section).It is believed that a large-scale shallow-water lake basin gentle-slope source-to-sink system developed in the depositional period of Shanxi Formation-He 8 Member in Ordos Basin.The two main provenance areas are the Alashan ancient land in the north-Yinshan mountain range and the Qinling-Longxi ancient land in the south.During the SQ1-SQ4 deposition period,five source-to-sink systems of "Wetland alluvial fan-braided river-meanderingriver-shallow water delta-coastal shallow lake" source-sink system are developed in the northwest,north,northeast,southwest and south;During the SQ5 deposition period,five source-to-sink systems of "Semi-arid alluvial fan-braided river-meandering river-flood lake" source-sink system are developed in the northwest,north,northeast,southwest and south.With the help of sedimentary physics simulation experiments,the controlling factors for the distribution of large-area sand bodies in shallow lake basins in large depressions include gentle paleo-geomorphology,lake level rise and fall,multi-source supply,and the role of staged � relay transportation �.Three types of sedimentary patterns of large-scale sandstone continuous distribution,such as frequent alternation with sand dams,"upstream channel diversion of sand bodies into lakes",and "lateral channel body lateral migration".Through research and analysis of the petrological characteristics,reservoir characteristics and diagenesis of the reservoirs in the study area,it is believed that the development of relatively high-quality reservoirs is jointly controlled by lithology,sedimentary facies and diagenetic facies,and relatively high-quality reservoirs are distributed in the parent rock with a single lithology Provenance system,braided river high-energy sedimentary facies belt and favorable diagenetic facies belt.