Fluid Analysis And Hydrocarbon Migration And Accumulation Of Majiagou Formation, Yanchang Area, Ordos Basin

Yanchang area is located in the southeast of Yi-shaan slope, Ordos Basin. Majiagou Formation is the key exploration layer of lower Paleozoic. At present, there is a lot of natural gas show in well drilling and gas testing gets high-yield gas production. Gas reservoir shows a bright exploration prospect. Multi-phase tectonic movements and complex diagenesis improved carbonate reservoir property significantly. Reservoir paleo-fluid included the information such as fluid composition, formation paleo-temperature, paleo-salinity, paleo-density, and paleo-pressure et al, in the process of sedimentary, diagenesis and hydrocarbon accumulation. Therefore, the study of reservoir organic fluid has certain theoretical meaning in terms of hydrocarbon accumulation.On the basis of outcrop observation, core observation, thin section observation, casting thin sections observation and all kinds of analysis test data, reservoir characteristics, diagenesis types and diagenetic sequence of Majiagou Formation were studied systematically.The hydrocarbon source rock is upper Paleozoic coal bed. By fluid inclusion analysis technology, fluid property in different period was discussed. Carbon and oxygen stable isotopic of carbonate cement revealed its formation environment and origin. Strontium isotope of carbonate were used to preliminary analyze the sedimentary environment and karstification.Combing homogeneous temperature and time-temperature burial history, different periods of hydrocarbon filling and accumulation were ascertained. At the same time, fluid property of lower and upper Paleozoic was compared. Finally, paleo-fulid flow pattern was reconstructed.The major conclusions are summarized as follows:1) Diagenesis types were various.The main diagenetic types of reservoir in Majiagou Formation of Yanchang area included compaction, metasomatism, dissolution, cementation and fracturing. Compaction,dissolution and cementation of epidiagenesis and late diagenesis were essential to reservoir property;2) Three events of hydrocarbon charging had occurred in Shanxi Formation.The first hydrocarbon charging happened in early stage of Triassic (from222Ma to212Ma).The second happened in the early of early stage of Cretaceous (from140Ma to136Ma).The third happened from late of early stage of Cretaceous(from125Ma to120Ma).3) Two events of hydrocarbon charging had occurred in Majiagou Formation. The first hydrocarbon charging happened in the early of early stage of Cretaceous, while the homogeneous temperature of fluid inclusions were predominantly130～140℃.The second happened in late of early stage of Cretaceous,the range of homogeneous temperature fluid inclusions was160～170℃.Early carbonate cements formed in Caledonian and was related to fresh water leaching. Late carbonate cements formed in the condition of deep buried depth, related to organic acid decarboxylation. Calculated paleo-temperature by oxygen stable isotopic proved its formation environment above. Organic acid karstification during diagenesis stages at burial time was the main type karstification of Majiagou reservoir.Upper and lower Paleozoic paleo-fluid flow pattern indicated that hydrocarbon charging was continuous.