The Study Of Origin And Reservoir Controling Factors In Deep Water Sandstone In Chang7-chang6 Formation (Late Triassic),Southern Ordos Basin

The Chang6 and Chang7 formation in BinChang area,at the north of the Ordos basin,has developed the sand body which is close to the source rock and show a good hydrocarbon potential.The sandstone is characterized by quick variation in vertical and horizontal.While this sandstone is also belongs to the unconventional tight reservoir which has a low porosity and permeability.Due to the widely oiliness and partly enrichment in production,how to find sweet spots under the low-porosity and low-permeability background is the mainly problem.This essay use the geology,well logging and geochemistry methods to study the origin of the sandstone,the characteristic and controlling factors of reservoirs in Chang6 and Chang7 formation in Binchang area,southern Ordos basin.The results are showed as follows:In the aspect of the origin of deep-lacustrine sandstones,by observing cores and outcrops in study area,8 typically lithofacies and 6 representative lithofacies association types are concluded.Subsequently,on the one hand the character of transport fluid was analyzed by concluding sedimentary structure and microcosmic structural character in different lithofacies,on the other hand researching distribution patterns by utilizing abundant logging data and building' logging facies'.Lastly,on the basis of the previous research,we discussed the palaeogeographical pattern by studying the thickness distribution of Zhangjiatan shale which is at the bottom of study area.Basing the researches above,3 types of gravity flows'sandstone were recognised which are sandy debris flows,turbidity current and sandy slump.Among these types,the sandy debris flow is dominantly while turbidity current is secondary developed.The sandy slump is merely found in local area.The sandstone caused by sandy debris flow appears to be small in scales within single period,quickly thin out,poor continuity and multi-phases.The accompanying turbidity current has a widely distribution.However,it is hard to form thick sand bodies due to the small scale in single period.The thick sandstone in study is mianly form by the superposition of mutilple periods of gravity flows which sandy debris flow is predominate.Hence the thick sand bodies in the study area dominantly distribute in lump,show poor continuity and the sand bodies don't develop in the area which is close to provenance.After the work as above,the depositonal mode of deep-water sandstone is built.At the base of the conventional physical properties test and the high pressure Hg injection experiments,we utilize some reservoir characterization techniques such as the CT scan,constant-rate mercury injection experiment and nuclear magnetic resonance to build different reservoir microcosmic pore structures' discrepancy principles.Among these types,type I reservoir dominantly develop the residual intergranular pores and dissolved pores and has a high pore/throat ratio.This reservoir has great connectivity.The reservoir physical properties are always above 0.6mD and show a low overburden pressure sensibility while the irreducible water content is nearly 50%.Type ? reservoir mainly develop the residual intergranular pores,micropores and the permeability between 0.4-0.8mD which show a low pore/throat ratio and higher overburden pressure sensibility.Its irreducible water content is between 60%-70%.The pores in type ? reservoir is all nearly micropores.The permeability mianly under the 0.4mD and has no permeability under the pressure.The irreducible water content is above 70%?By researching reservoir diagenesis in the study area,compaction and clay cementation are mainly destructive diagenesis effects.The construction diagenesis is dissolution and tectonic disruption.The study area should develop two-stage dissolution.The early fresh water dissolution is weak in comprehensively and the pores are mainly destroyed by the cementation and compaction.It contributes less in construction diagenesis.Notwithstanding the late organic acids dissolution can only produce more pores in reservoir which has less cementation and compaction in the early stage.This dissolution effect contributes more than the previous one.But the late dissolution effects are due to the degree of the compaction and argilla cementation at the early stage.Hence the two destructive diagenesises have the most effective to the reservoir.On the other hand,the effect of compaction and cementation are controlled by the content of plastic rock and clay mineral.Based on the understanding,build two different diagenetic evolution paths,and explains the formation mechanism of different types of reservoir.The compaction and clay cement is weak in the reservoir with low plastic debris and clay matrix that result in the better preservation of erarly pores.Thus the organic acid is easy to enter the reservoir and form a strong dissolution character.As a result the relative reservoir is character by intergranular developed,throat clean,good connectivity and a low overburden pressure sensitive.Reservoir with above chareacteristics belongs to type ?.However,reservoir with high plastic debris or clay matrix often encounter a strong compaction or clay cement in early diagenesis which lead to the destruction of intergranular pores and early dissolution pores.Consequently,few intergranular pores and early dissolution pores were preserved and late orgaic acid is hard to enter and reform the reservoir.Thus this kind reservoir is character micro-pore developed,poor connectivity,and a high overburden pressure sensitive.Reservoir with these chareacteristics belongs to type ? or ?.Finally,based on the controlling factor analysis of sandstone in different origin,the favourable area is more likely to develope in the sandstone formed by sandy debris flow which is thick in thickness and has a character of strong dissolution.