Study On Heavy Oil Accumulation Conditions In The Orinoco Oil Belt,Venezuela

Orinoco Oil Belt is the world's largest heavy oil and extra heavy oil reservoir,and has great develop potential.However,the research on the accumulation characteristics of the heavy oil belt,especially the cause of the belt,is relatively weak.Based on the related theories and methods of petroleum geology,this paper systematically studies the conditions for the accumulation of heavy oil through the analysis of source rocks,reservoirs,traps,and hydrocarbon accumulation models of heavy oil,and discusses the origin of heavy oil further.The main conclusions are as follows:The Orinoco oil belt is located in the southern slope zone of the Eastern Venezuela foreland basin.The heavy oil belt is accumulated outside the source.All of the crude oil comes from the Maturin subbasin and the Guarico subbasin depression in the northern of the heavy oil belt.The main source rocks are the Querecual formation and San Antonio formation carbonates and shales of the Guayuta Group sediments deposited at the basin's passive continental margin phase.But it cannot be ruled out that some of the crude oil may come from the Upper Oligocene-Lower Miocene Naricual formation's mudstones,and at least two different types are found in the Boyaca and Carabobo blocks.So that the oil source in different blocks of the heavy oil belt is not exactly the same.The Orinoco oil belt has a total of 5 reservoirs of the Cretaceous,Oligocene,Intramiocene,Lower Miocene and Middle Miocene.The Intramiocene sequence is the main oil-bearing stratum,and basal sand of Oficina formation is the most important.The structural location of Orinoco oil belt and the overriding style of layers determine that the type of trap is dominated by litho-stratigraphic traps,with relatively few structural traps.Because of the unique physical properties of heavy oil,the main plugging layer of the heavy oil belt is asphaltic seals and tar pads.Heavy oil accumulation is a multi-phase blending of crude oils and migration from long distances from north to south(150-300Km).In the process of migration,it has undergone various densification,making heavy oil properties(chemical composition,density,viscosity,etc.)change regularity from north to south.According to the study,crude oil comes from mature source rocks,and is not generated directly from the source rock in the low mature stage,but mainly due to secondary effects.Biodegradation is the main mechanism of formation of the heavy oil.The degradation of crude oil is between 4 and 8.In addition to the biodegradation,the water washing also has an important influence on the crude oil densification.Together with the mixing of crude oils of different stages,it eventually leads to the complexity and heterogeneity of the nature of the entire heavy oil belt crude oil.It can also be confirmed by crude oil geochemical data and actual survey conditions.