Study On The Genesis And Densification Process Of The Lower Cretaceous Tengger Formation Pebbly Sandstone Reservoirs In The Lower Slope Of The Baiyinchagan Sag

The lower Cretaceous Tengger formation in the Tara fault structural zone of the northern Baiyinchagan Sag in the Erlian basin is a typical low permeability sandstone reservoir,which has the characteristics of changeable geological conditions,strong reservoir heterogeneity and complex reservoir spatial structure.It is the key of exploration and development of low permeability reservoir and the premise of breakthrough in the mining technology to deepen the understanding of diagenetic environment characteristics,reservoir development constraints,and the porosity recovery.In view of the above problems,this paper is studied the basis of the previous research results and guided by multi-disciplinary theory such as reservoir geology,sedimentology.With the reservoir genesis and densification process as the study key,using the casting thin sections,scanning electron microscope,XRD diffraction,fluid inclusions and other testing methods,to study on the reservoir petrology characteristics,reservoir diagenetic environment evolution and the recovery of physical properties.The achievements and understandings of this paper are as follows:(1)The genetic type of the Tengger formation reservoir sand body in the study area is fan delta.The Tengger formation sedimentary period corresponds to the fault depression stage of the lake basin,thus forming the sedimentary characteristics of near provenance,multi provenance,rapid phase transformation and narrow facies belt.The clastic rocks are complex in composition and low in structural maturity.The main types of reservoir space are residual intergranular pores and secondary pores.The physical parameters of the reservoir vary greatly,and the whole reservoir physical property is characterized by low porosity and low permeability.(2)The diagenetic environment evolution sequence of the Tengger formation reservoirs in the study area is constructed.The Tengger formation reservoirs in the study area is in the A-B stage of the middle diagenesis,during which three stages of oil and gas charge occurred,and experienced the diagenetic environment evolution process of early alkaline,acidic-alkaline alternation and late alkaline dynamic alternation.The sedimentary formation water was weakly alkaline due to the arid climate,and the diagenesis was mainly compaction drainage and cementation of carbonate cementation.The first stage of acidic-alkaline alternating diagenetic environment occurred,the organic matter gradually matured and quartz growth in the weakly acidic diagenetic environment increased.The diagenetic environment turned into the weakly alkaline environment and carbonate cementation developed.The first stage of oil and gas charge occurred.The carbonate and feldspar minerals dissolved under the influence of organic acid which controlled the form of the acid diagenetic environment.The formation of intergranular and intragranular dissolution pores improved the physical properties of the reservoirs.Meanwhile,quartz enlarged edge and self-shaped crystals developed.The second stage of oil and gas charge occurred.With the concentration of organic acid gradually decreasing,some quartz dissolution and the development of late carbonate cementation were caused by the alkaline diagenetic environment.The second acidic-alkaline alternation diagenetic environment occurred,the third stage of oil-gas charge occurred,and some quartz developed the third grade enlarged edge.Organic acid gradually consumed,and diagenesis is characterized by late carbonate cementation,pyrite,illite and chlorite assemblage,which proved that the diagenetic environment has been changed to weakly alkaline and continues to now.(3)The formation mechanism of physical properties of sand conglomerate reservoir in the Tengger formation clarified.The original sedimentary conditions of sedimentary fabric and sedimentary micro equality were the congenital factors controlling the development of reservoir physical properties of the Tengger formation in the study area.The difference of diagenesis is the key factor to determine the development of reservoir physical properties.Compaction and cementation lead to the deterioration of reservoir physical properties.The secondary pores formed by dissolution of feldspar and debris can improve reservoir physical properties.The diagenesis evolution model of feldspar reservoir and lithic reservoir was different.The former shows strong cementation,weak compaction and medium dissolution,while the latter shows strong compaction,weak cementation and weak dissolution.The structure is an important factor affecting the formation of oil and gas reservoirs in the study area.(4)The types and distribution characteristics of the Tengger component lithofacies in the study area investigated.The diagenetic facies of the Fan delta plain in Sanghe area is mainly characterized by moderate to strong compaction,weak cementation,weak to medium dissolution,and extremely poor reservoir physical properties.The fan delta plain distributary channel,fan delta front and the front fan delta subfacies in the Chalage area developed strong compaction diagenetic facies,which were the main reservoir types in the area with poor reservoir physical properties.The fan delta in the Sanghe and Gul areas developed strong compaction diagenetic facies.The thin sand layer between the front channel of fan delta and the main sand body of distributary channel developed strong cementation diagenetic facies,and the reservoir physical properties were poor.In the fan delta front of the Sanghe area,the distributary channel develops a small amount of weakly compacted weakly cemented diagenetic facies with good reservoir physical properties.In the fan delta front of the Sanghe area,the distributary channel of the fan delta front,the far source turbidite fan and other fine sandstones develop medium compaction and weak to medium cementation.The diagenetic facies with strong dissolution was the best physical properties.(5)The pore evolution process of the sand conglomerate reservoir of the Tengger formation in the study area quantitatively deduced.The burial depth is less than 1100 m.In the early weakly alkaline diagenetic environment,the increase of porosity contributed by quartz dissolution is 0.10%,and the decrease of porosity caused by cementation and compaction is 15.37%.The burial depth range is between 1100 m and 1300 m and the increase of porosity contributed by feldspar particles is 0.90%,and the decrease of porosity caused by quartz secondary enlargement is 1.20%.The burial depth is between 1300 m to 1500 m,and the increment of dissolution is 2.60%.The porosity loss caused by compaction and cementation is 2.78%,the burial depth is between 1500 m and 2000 m,the porosity loss caused by late carbonate cementation is 5.08%,the porosity loss caused by compaction is 3.34%,and the increment of dissolution of quartz is 0.45%.The burial depth is between 2000 m and 2500 m,and the porosity loss caused by compaction and cementation is 5.08%,the loss porosity is 1.21%,the secondary dissolution porosity and microfracture porosity increase is 1.74%.The dissolution porosity increase is 0.30%,and the cementation porosity decrease is 0.22%.