| Ordos Basin is a large-scale hydrocarbon-bearing basin in China,which is found that the natural gas reserves are abundant.The Upper Paleozoic gas exploration in the Yanchang exploration area in the southeastern part of the basin has also yielded remarkable results.Although there is a certain understanding of the characteristics of sedimentary reservoirs and the accumulation of gas reservoirs,research on the main controlling factors of hydrocarbon accumulation,regularity of forming reservoir and the exploration potential of natural gas is still relatively weak due to the wide distribution of the Upper Paleozoic gas reservoirs,stratigraphic dispersion,density of reservoir and complex geological history.Based on the previous research results and the new exploration data,this paper studies the Upper Paleozoic gas reservoirs' sedimentary facies,lithology,physical property,heterogeneity and caprocks by comprehensive using microscope observation,well logging interpretation,analysis and testing,etc.A pattern of regularity of forming natural gas reservoir in the Upper Paleozoic is established,and the natural gas enrichment regularity in the study area is further defined.The main understanding includes:Two major provenance and delta depositional systems in the southeastern Ordos Basin are identified.The Delta system was extended and gathered to Yichuan-Fuxian area in the central and southern part of Ordos Basin.The distribution of delta front sand body gradually increased,reaching the peak in he 8,covering the whole area.The diagenesis and diagenetic facies distribution of the Upper Palaeozoic sandstones in the study area are analyzed.It is considered that the dissolution pore clay mineral together with siliceous cemented strong dissolution facies is the most favorable diagenetic facies,the dissolution is strong,pore development and high gas production.High acid content of cement,poorly developed pore,basic gas free.The function relation between displacement pressure and acoustic time is established.The displacement pressure of mudstone caprock is calculated,and the relationship between discharge pressure and gas content is quantitatively analyzed.Through research,the range of displacement pressure in the study area is mainly from 9 to 11 MPa,while the displacement pressure from low to high transition part is range from 9.5 to 10 MPa.The natural gas enrichment is better,caprock sealing performance as well as the oil and gas production is good.The mudstone compaction study shows that the maximum value of excess pressure generally appears in the upper Shihezi Formation,and the pressure distribution of the overpressure body at the top of the storage box is consistent with the degree of gas enrichment in the regional distribution stability and continuity.The overpressure in the top of the pressure storage box in western part is stable and continuous,and the enrichment of natural gas shows relatively high abundance and yield.The geological features and enrichment regularities of natural gas accumulation are summarized.It is believed that the multilayered sand bodies are vertically stacked and the lateral contiguous sheets provide a huge reservoir space.The thick mudstone caprock at the top of the Upper Paleozoic provides a good Storage conditions,the stability of the pressure storage box affect the degree of natural gas enrichment. |
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