Differential Diagenesis And Complex Gas And Water Distribution In Tight Sandstone Reservoirs In The North Tianhuan Depression,Ordos Basin

The north Tianhuan depression is located in the Ordos Basin,and there is a strong reservoir heterogeneity of the Upper Paleozoic reservoir in the north Tianhuan depression.The gas and water distribution of the reservoir is very complex,with ambiguous controlling factors.The complex gas and water distribution may indicate the differences in the gas and water occurrence space and pore space of the reservoir.All these may be caused by differential diagenesis.Three typical well areas were selected for analysis: the S176 well area,L57 well area,and the S307 well area to study the differential diagenesis,using the methods of microcosmic and the macroscopic analysis,on the base of the complex gas and water distribution and the strong heterogeneity of the reservoir in the north Tianhuan depression.The microscopic observation was used as the core for diagenesis research,with the combination of scanning electron microscopy,cathodoluminescence,carbon and oxygen isotope,nuclear magnetic resonance,gas and water permeability,and other testing methods.Based on the comparison of the diagenesis of the three well areas,and the diagenesis of gas layers and water layers,the differential diagenesis with the characteristics of complex gas-water distribution was studied,to clarify the relationship between difference diagenesis and complicated gas-water distribution.The main understandings obtained from the research are as follows:(1)There are significant diagenetic differences in the gas layers and water layers of the three well areas in the north Tianhuan depression.There developed strong compaction,strong kaolinite cementation,and weak dissolution in the S176 well area.While there develop strong compaction,strong calcite and clay mineral cementation,and weak dissolution in the L57 well area.There developed weak compaction,strong siliceous cementation,stronger clay mineral cementation,weak calcite cementation and strong dissolution in the S307 well area.The difference in the reservoirs inside the S307 well area was determined by the strength of dissolution and anti-compaction ability.The pores in the gas layer are well developed,dominated by the dissolution pores,resulting in weak compaction and strong dissolution diagenesis.There almost no pore in the dry layer,which developed strong compaction,weak dissolution and strong cementation diagenesis.There were few pores in the poor gas layer,which dominated by the intercrystalline pores,developing middle compaction,strong cementation and weak dissolution diagenesis.(2)The reservoir diagenesis in the north Tianhuan depression can be divided into two idealized models.Type ? was characterized by weak compaction(strong anti-compaction ability),early strong siliceous cementation and late strong dissolution diagenesis;Type ? was characterized by strong compaction(Weak compaction resistance),early strong siliceous,carbonate cementation,late strong clay mineral cementation and late strong compaction,weak dissolution diagenesis.(3)The differential diagenesis in the north Tianhuan depression was mainly manifested as follows: a.The different contents of the miscellaneous bases,plastic minerals,and rigid debris;b.The type of the cementation and the degree of the cementation;The intensity of the early compaction and the degree of the late dissolution;d.Type of the diagenetic sequence;e.Characteristic types of gas-water seepages;e.the difference in original water saturation.The different configurations of these six aspects led to strong heterogeneity of the reservoirs and determined the development degree of the reservoir pore.Ultimately,these factors led to the current space and status of gas-water distribution.