Mechanism Of Oil And Gas Accumulation In Tight Sandstone Reservoirs Controlled By Faults And Favorable Zone Prediction

Scientific questions: China is facing more and more serious shortage of oil and gas supply.Unconventional oil and gas have huge resources,and have become important replacement field of the conventional oil and gas in China.As one important part of unconventional oil and gas resources,tight sandstone oil and gas have huge resoureces potentail.While,many previous studies show that there are many developed fault systems in the tight sandstone reservoirs,which obviously control over the enrichment of oil and gas in tight sandstone reservoirs.Therefore,the studies on the mechanism of oil and gas enrichment in tight sandstone reservoirs controlled by faults are of great significance for tight oil and gas exploration.At present,previous researches mainly focus on the qualitative evaluation of the physical properties and static parameters of tight sandstone reservoirs.However,the characteristics of fault controlling oil and gas enrichment in tight sandstone reservoirs lack of systematic analysis.The main factors and mechanisms of fault controlling oil and gas enrichment in tight sandstone reservoirs are unclear.There are no effective methods to quantitatively evaluate the fault controlling oil and gas enrichment in tight sandstone reservoirs.These issues are of great significance for the evaluation and distribution prediction of oil and gas in the tight sandstone reservoir.Research procedure: The Nanpu Sag is an oil-rich sag with well-developed fault systems,and it is at the initial stage of deep unconventional oil and gas exploration at present.Many wells have very good oil and gas occurerence in tight sand resrevoir,revealing huge potential of tight oil and gas resources in the Nanpu Sag.Based on the anatomy of tight sandstone oil and gas reservoirs in the Nanpu Sag,this study analyzed the main controlling factors and mechanisms of fault controlling oil and gas enrichment in the Nanpu Sag,and established a quantitative evaluation model through physical experiments,which is well verified and applied in the Gaoliu area of the Nanpu Sag.Main research results: Based on the geological and experimental analysises,three main conclusions:(1)the faults control the development of fractures,and further affect the oil and gas enrichment in tight sandstone reservoirs.The oil-gas-water distribution in tight sandstone reservoir has obvious oil-water inversion characteristics.This feature reveals the role of non-buoyancy on the hydrocarbon enrichment in tight sand reservoir,which is mainly caused by the small pore throat radius.The capillary force difference between tight sand reservoir and surrounding rocks is the main driving force for oil and gas enrichment in tight sand reservoir,and the corresponding low interface potential energy region controls the degree of oil and gas accumulation.The large number of fractures induced by faults activities further increase the capillary force difference between tight sandstone reservoir and its surrounding rocks.It enhances the interface potential energy difference,and then controls the enrichment degree of oil and gas in tight sandstone reservoirs.(2)Based on the mechanical model of single orifice on flat plate,this study evaluated the stress distribution surrounding faults.Moreover,this study established the quantitative evaluation model of fracture development by experimental analysis.Furthermore,the quantitative evaluation method for faults controlling fractures development in tight sandstone reservoir is established,and the method has been proved to be reliable by production test.(3)Considering the driving force of the oil and gas migration in tight sandstone reservoir,as well as the hydrocarbon generation and expulsion characteristics of source rock,this study quantitatively evaluated the ability of faults transportating oil and gas in tight sandstone by using index SI.The larger the SI value is,the stronger the transportation ability of fault is.The fracture development degree controlled by faults was quantitatively evaluated by using index CI.The larger the CI value is,the more developed the fracture is.Based on the theory of low potential controlling oil and gas enrichment in reservoir,this study used index PI to quantitatively evaluate the oil and gas enriching degree controlled by the low interface potential energy in tight sandstone reservoir.The smaller the PI value is,the more oil and gas are enriched.The influence of faults on oil and gas enrichment in tight sandstone reservoir take effect by combinating these three independent and indispensable factors.Therefore,this study used index SCPI to quantitatively evaluate this controlling effect.The larger the SCPI value is,the more oil and gas will be enriched.Verification and application: well test and production data in the tight oil and gas reservoirs of the Gaoliu area demonstrated that this quantitative evaluation method is reliable.This method was used to predict the favorable areas for oil and gas accumulation in tight sandstone reservoir of the Gaoliu area of Nanpu Sag.Five favorable regions including the G4 well region,G7 well region,L1-12 well region,G11 well region,G83-10 well region and L21?5 well region,have high SCPI value and are sweet spots for oil and gas enrichment.It has significant implications for further exploring tight oil and gas in the Gaoliu area.