Effects Of The Lower Paleozoic Diagenetic Fluid Evolution On Reservoirs In Southwest Dongying Depression

The carbonate reservoir of the lower Paleozoic in Dongying sag is controlled by multiple types and stages of diagenetic fluid after experiencing multiple stages of tectonic activity and post sedimenta,it leads to the enhancement of reservoir heterogeneity,which seriously restricts the further exploration and development of the study area.Taking the Lower Paleozoic carbonate reservoir in the southwest of the Dongying Depression as the study object,through the mineralogy and electron probe analysis technology,identified the types of fillings in the fractures（holes）in this study area,and combining geochemical data and pore evolution characteristics,the diagenetic fluid properties,evolution model and its influence on the reservoir of the fillings are explored.The results show that:（1）limestones,dolomites,igneous rocks and a small amount of sand（shale）rocks are mainly developed in the area.The main filling minerals in the fractures and holes include 8 kinds of calcite fillings,3 kinds of dolomite fillings and a small amount of hydrothermal filling minerals such as pyrites and celestites;（2）The Lower Paleozoic strata experienced multiple types and period of diagenestic fluid action,including:（pene）contemporaneous period-meteoric water dissolution;（pene）contemporaneous period-cementation of fibrous calcite in seawater environment;Basin subsidence period-the recrystallization of the Mg-rich formation water;Early Hercynian movement period-meteoric water dissolution;Early and middle Hercynian movement period-cementation of mixed water grape-like calcite;Middle and late Hercynian movement period-cementation of meteoric water in vertical dissolved fractures;Yanshan movement period-the hydrothermal fluids dissolved the original rocks with different properties,forming the Mg-rich,Ca-rich and Sr-rich hydrothermal fluids,and precipitated the ferroan dolomites,saddle dolomites,granular ferroan calcites and celestite.Under the influence of terrigenous clastic materials,the ferroan dolomites and granular ferroan calcites exhibit high ⁸⁷Sr/⁸⁶Sr values and extremely positive Eu anomalies,respectively;Himalayan movement period-the hydrothermal fluid dissolves the gypsum and salt layers in Sha-3 Sha-4 formation to form S-rich hydrothermal fluid,and dissolves carbonate formation to obtain Ca²⁺,and precipitates anhydrite in fractures;（3）It determined the reservoir space types of the Lower Paleozoic carbonate reservoirs in the study area,and revealed the reasons for the differences of reservoir physical properties under different diagenetic fluids.The reservoir spaces of the Lower Paleozoic carbonate reservoir are divided into fractures and vugs（holes）.The influence of diagenetic fluid on reservoir physical property is controlled by lithology,the texture of the fillings,tectonic activities,magma and other factors.High quality reservoirs are often formed in areas affected by supergene meteroic water karst,and hydrothermal,organic acid dissolution;（4）It established two defferent types of high-quality reservoir evolution models.The Mg-rich and Sr-rich hydrothermal fluids intruded faults and reservoirs with high original porosity,forming hydrothermal dissolution,ferroan dolomite and celestite fillings that can improve the reservoir physical propertues,forming fault-deep fluid controlled reservoirs.The two periods of meteoric water dissolution in the Caledonian and Hercynian periods are superimposed on each other,forming the karst highland reservoir.