Characteristics Of Lacustrine Carbonate Reservoirs Of The Upper Es4 In Western Zhanhua Sag

With the development of petroleum exploration, considerable oil-gas reserves have been found in lacustrine carbonate rocks, which have drawed a great many geologists'attentions. This paper presents a study on the lacustrine carbonates of the upper submember of the 4th member of the Eogene Shahejie Formation (Es4) in the western Zhanhua sag, including Yidong, Luojia and Shaojia area. Based on the physical properties as well as lithologic and geometric characteristics, the basic characteristics of carbonate reservoirs formation were studied. According to the research of geochemistry and diagenesis, the formation of carbonates and the evolution of pore space were respectively discussed. At last, different kinds of carbonate reservoir formations were evaluated and the favorable places for prospecting were discussed.By the means of drill core observation and microscopic identify, the major rock types were examined, including calcarenite, bioaccumulated limestone, sandy limestone, micritic limestone, algal dolomite and micritic dolomite. Calcarenite and bioaccumulated limestone are the most common rocks among them. Pores observed in the study consist of algal skeleton pores, fenester pores, intergranular and intragranular pores, while fractures include tectoclase, corroded fissure and stylolite. Algal skeleton pores and intergranular pores are the best developed ones. The studied secion can be divided into 3 subfacies, inculing littoral lake, shallow lake and semi-deep lake subfacies. The littoral lake subfacies are composed of mud flat, sand flat and shoal microfacies, while the shallow lake subfacies consist of bank, interbank and algal reef microfacies.According to the analysis of chemical composition, this study presents that the carbonate rocks are mainly composed of pure limestones, indicating the relatively stable sedimentary environment. Trace element study shows that Mn concentrates in the shoal environment, which is characterized by enrichment of algae and organic material. The sedimentary environment of carbonates can be characterized by shallow water, medium or medium-low energy, brackish-salt water and light amount terrigenous influx. The total REE contents (∑REE) of lacustrine carbonates are low and present few fluctuation. The normalized REE patterns display light REE enrichments, which proves the carbonates were indeed deposited in the lacustrine environment, while the analysis of positive Eu anomalies indicated the hydrothermal venting occurrence in paleolake systems. The pore structure of carbonates were divided into 4 types, including medium pore with coarse throat, small pore with medium throat, small pore with fine throat, micro pore with micro throat. Major factors controlling reservoir formation characteristics include lithologic characteristics, sedimentary facies and diagenesis. The carbonates reservoir formations in studied area were divided into 4 types. TypeⅠis the best reservoir formation and formed in the high energy environments including bank and algal reef. TypeⅡis the preferable one and developed in the medium energy shoal environment, while typeⅢis the poor reservoir formation and developed in the low energy inter-bank environment. TypeⅣ, which was formed in the extremely low energy mud flat, is the non-reservoir formation.