Chang 8 low permeability reservoirs genesis of Zhenting area in Ordos basin

Chang 8 reservoirs of Zhenting area in Ordos basin are mainly of feldspar lithic sandstone,with average porosity of 11.7% and permeability of 0.56 ×10-3μm2,and therefore it belongs to a typical low porosity low permeability reservoir.Through the casting thin sections observation of Chang 8 reservoir,the diagenesis characteristics of the reservoir has been made clear and the quantitative analysis of the impact of various diagenesis on reservoirs has been carried out.And then the causes of the permeability reservoir formation have been discussed.The main reason of the low permeability reservoir is the compaction,while the late cementation,especially the calcite filling is the direct cause of the low permeability reservoir.Results show that before the large-scale hydrocarbon generation the reservoir porosity should be a round 15% ~ 25%,belonging to high porosity reservoir While a massive cementation can lead to reservoir porosity quickly dropped to 8% ~ 18%,forming the low porosity low permeability reservoir at present.