Reservoir development mechanism and its influence on oil and gas under differential source and reservoir structure:A case study from Chang 71-2 tight sandstone of Qingcheng area,Ordos Basin
The strong reservoir heterogeneity and unclear understanding of formation mechanism of Chang71-2 member in Ordos Basin is a key issue that constrains hydrocarbon enrichment.Based on the logging,core observation,high pressure mercury injection,X diffraction analysis and other analysis tests,and further combined with sandstone percentage and sand-mud single layer thickness as the standard,the Chang 71-2 member of the study area was divided into three categories and corresponding nine sub-types of source-reservoir structures,including reservoir sandwiching source type,source sandwiching reservoir type and source-reservoir interbedding type.The results show that the reservoir in the reservoir sandwiching source type is generally better in physical property,showing more primary intergranular pores and secondary pores,good sorting and low displacement pressure.The physical properties of source sandwiching reservoir are poor,which are characterized by less pore con-tent and large displacement pressure.The reservoir quality of source-reservoir interbedding is between the two source-reservoir structures mentioned above.The formation of reservoir differences is con-trolled by flood gravity flow deposition,cementation and near-source organic acid dissolution.The source-reservoir structure reservoir with more thick layers(＞5 m)developed in the subracustrine fan,channel and sheet sand formed by strong hydrodynamic flood gravity flow has stronger anti-com-paction ability due to good sorting and more rigid particles,thus retaining more primary pores.On the contrary,the source-reservoir structure reservoirs with more thin layers(＜2 m)in semi-deep lakes and deep lakes,due to the large number of plastic particles in the rock composition and poor sorting,leads to a large reduction of primary pores under the action of compaction.The cementation in the study area is mainly composed of calcareous,siliceous and argillaceous cements.In the reservoir sand-wiching source type,the content of calcareous and argillaceous cements is less,and chlorite is more in the form of thin film covering the surface of rock particles,which inhibits siliceous cementation,re-sulting in a lower degree of cementation and better preservation of primary pores in the reservoir.However,the reservoir with more calcium and other cement content in the source sandwiching reser-voir type shows strong cementation,which leads to serious destruction of primary pores.Near-source organic acids play a constructive role in the development of pores,and the thick reservoir with large rock grain size can provide favorable dissolution channels and form more secondary pores.The thin layer reservoir with more development of very fine grained sandstone usually hinders dissolution and makes it difficult to form secondary pores.Finally,through the analysis of the oil-show well ratio and the oil saturation,it is shown that the oil content of the reservoir sandwiching source type,the source-reservoir interbedding type and the source sandwiching reservoir type decreases successively.Mean-while,the oil content of the reservoir in the corresponding sub-types shows that thick reservoir sand-wiching medium-thickness source,thick source thick reservoir interbeding,thick reservoir sandwiching thin source is the best.
source-reservoir structurereservoir differencegenetic mechanismOrdos BasinChang71-2 member of Yanchang Formation
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