The Responding Mechanism Between Diagenetic Evolution-hydrocarbon Filling And Microscopic Pore-throat Structures Of Tight Sandstone Reservoir

The southwest area of the Ordos Basin is one of the main favorable oil-bearing area in China.As typical tight sandstone reservoirs,Chang 6 and Chang 8 segments of Yanchang Formation have received extensive attention.Nowadays,the research of diagenetic evolution and hydrocarbon filling in their own independent stages basically.Burial-thermal evolution history-hydrocarbon filling has not been set up effective bridge between pore structure.Regular and single test is very limited in the evaluation of microscopic pore structure characteristics of tight sandstone reservoir.These phenomena have seriously restricted the improvement of theory and exploration-development of tight sandstone.Microscopic identification and physical property tests have been conducted to analysis the petrology,physical property and diagenesis.Fluorescence,components and other geochemical analysis have also been conducted to determine the hydrocarbon filling sequence.A new sandstone classification criterion has been used based on the diagenetic environment and components.Besides,pressure-controlled mercury intrusion(PCMI),rate-controlled mercury intrusion(RCMI),nuclear magnetic resonance(NMR)and micro-CT tests have been conducted for the purpose of characterization of reservoir microscopic pore structures comprehensively.Finally,the dynamic responding relationships between burial,diagenesis,hydrocarbon filling and pore-throat evolution Chang 6 and Chang 8 tight sandstones in the southwest region of Ordos Basin have established,and the response characteristics of different types of sandstone are discussed based on the production data.This study mainly obtains the following understanding:(1)The lithology of Chang 6 segments in the research area is dominated by arkose,while that of Chang 8 segments is dominated by lithic arkose and feldspathic litharenite.The mean value of air porosity and air permeability in the Chang 6 segments is 9.6 % and 0.25 mD,respectively,while those values in Chang 8 segments is 7.2 % and 0.29 mD,respectively.Those results indicate that the target segments are belong to typical tight sandstone reservoir.(2)Chang 8 tight sandstone was more sensitive to the compaction,while the clay minerals were the main types of cements in the research area.Abundant moldic pores leads to the high dissolution rates in Chang 8 tight sandstone.(3)Based on the diagenetic environment and component characteristics,the tight sandstone of the target layers in the research area can be divided into four categories,and the reservoir and seepage capacity are successively weakened: low-soft sandstone with continuous acid filling,low-soft sandstone with intermittent acid filling,high-soft sandstone and calcareous sandstone.(4)The free hydrocarbons are the most mature and widespread one.Both Chang 6 and Chang 8 tight sandstones have experienced 4 periods of hydrocarbon charging,and the most important one was happened in the middle to late Cretaceous,during which more than half of the oil was injected into the reservoir.(5)The development of pore-throats of calcareous sandstone,high-soft component sandstone,low-soft component sandstone with acid intermittent filling and low-soft component sandstone with acid continuous filling gradually became worse,and the pore space changed from micro-fine pores to medium-large pores.The radius of mainstream throats is the most important mercury injection derived parameter of reservoir quality prediction.(6)The total pore-throat size distribution obtained by the combination of PCMI,RCMI and NMR methods is highly consistent with the results obtained under the microscope.(7)The diagenetic evolution path of calcareous sandstone,high-soft component sandstone,low-soft component sandstone with acid intermittent filling and low-soft component sandstone with acid continuous filling in the research area is gradually complex,the pore preservation degree is gradually increased,and the productivity is gradually improved.The response results of production dynamic data show that the sandstone types based on diagenetic environment and component characteristics can guide actual production to a certain extent,and the low-soft component sandstone reservoir with continuous acid filling is the preferred target for oil field exploration and development.