| The exploration of beach-bar sandy reservoirs in Jiyang Depression, which is regarded as an effective way to increase reserve and production in oil field, was dated from the1960s, so far the accumulative proved reserves have reached1.01923×108t. However, we lack deep understanding of the controlling factors and the dynamics of hydrocarbon accumulation in the beach-bar sand bodies, especially the effects of the overpressure, the driving forces of hydrocarbon migration for different accumulation phases are not well constrained, and the evolution of the pressure, the mode of hydrocarbon charging and accumulation and the spaces of hydrocarbon accumulation are also uncertain. This thesis will focus on the upper fourth sub-Member of Shahejie Formation in the western Dongying Depression. Based on the basic data, such as seismic data, logging data, analysis and testing data, the palaeopressure of different accumulation phases will be studied by comprehensive application thermodynamic modeling of fluid inclusions and equivalent depth method. The reservoir property parameters of different accumulation phases by the previous restoration model will be used to correlate the relationship between formation pressure evolution and the reservoir property parameters evolution of beach-bar sand bodies by the combining the comprehensive geological analysis with sedimentary and diagenetic evolution. Finally, the control of the overpressure evolution to hydrocarbon accumulation in beach-bar sandy reservoirs has be discussed here.Therefore, some research fruits have be obtained as following:1. The overpressured systems are characterized by zones in vertical and blocks in plane in the upper fourth sub-Member of Shahejie Formation in the western Dongying Depression. Three zones can be divided in vertical:the normal pressured zone, the transitional pressured zone and the overpressured zone; two overpressured systems developed in plane:Lijin sag and Boxing sag.2. The acoustic logging and density logging display obvious response to overpressures, which can be used to determine that the disequilibrium compaction is the main genetic mechanism of the overpressure in the western Dongying Depression, and its contribution to overpressure is more than90%.3. Both the crude oil properties (include relative density, sulfur content and dynamic viscosity) and the formation water characteristics (total dissolved solids and concentration of different ions) show regular variation in different pressured environments, and can be used as the indicators for different pressured systems, such as the normal pressured system (the pressure coefficient in the range of0.8to1.2), the transitional pressured system (the pressure coefficient in the range of1.2to1.5) and overpressured system (the pressure coefficient is greater than1.5) according to this regular variation.4. The reservoir characteristics in the different pressured esystems are sharply different as well. In the normal pressured system, parameters of crude oil properties have a wide range; reservoir porosities obey normal compaction law; there are four hydrological environment types in vertica, the formation water is mainly chloride calcium type which the ratio of sodium and chlorine is approximately1, the total dissolved solids and the concentration of each ions are relatively low; there have structural, stratigraphic and structural-stratigraphic traps. In the transitional pressured system, crude oil properties become relatively better, the parameter ranges are relatively narrow; the secondary porosity zone developed; two hydrological environment types in vertical occurred, the formation water of chloride calcium type which the ratio of sodium and chlorine is less than1begins to appear, and the formation water of magnesium chloride type increases conspicuously, emergencing of the phenomenon of sodium loss and calcium excess; reservoir traps are mainly structural and structural-lithologic. In the overpressured system, crude oil properties are the best, the parameters range is very narrow and the oil type is single; the secondary porosity zone is very developed; there is only one hydrological environment types in vertical, the fonnation water is mainly chloride calcium type which the ratio of sodium and chlorine is less than1and magnesium chloride type, the phenomenon of sodium loss and calcium excess are serious; reservoir traps are mainly lithologic and structural-lithologic.5. The systematically analysis of the fluid inclusions shows that there are at least four episodes of hydrocarbon charging and five episodes of thermal fluid activity (including four episodes of hydrocarbon-bearing fluid activity) in the reservoir of the upper fourth sub-Member of Shahejie Formation. The study area experienced three phases of hydrocarbon charging:the first phase occurred during the time of34.8～25.1Ma, i.e. the late period of the first member of Shahejie Formation deposition to the late period of Dongying Formation deposition; the second phase during the time of12.5-4.8Ma, i.e. the period of Guantao Formation deposition to the early period of Minghuazhen Formation deposition; the third phase during the time of4.3～0Ma, i.e. the middle period of Minghuazhen Formation deposition to the present.6. The restoration of the fluid pressure suggests that the overpressure of the upper fourth sub-Member of Shahejie Formation in the studied area started to develop from lOMa to the present. And the fluid pressure evolution experienced three cycles, corresponding to the three phases of the hydrocarbon charging in respectively:the first cycle corresponding to the first phase of hydrocarbon charging at a normal pressured system; the second corresponding to the second phase of hydrocarbon charging at the beginning of overpressure forming, i.e. form a normal pressured to a lower overpressured system; the third cycle corresponding to the third phase of hydrocarbon charging at a overpressured system.7. By coupling the fluid pressure and capillary force of each accumulation phases, the driven forces of the primary and secondary oil migration of the beach-bar sandy reservoirs will be obtained. And the driven forces distribution indicate that under the affect of the formation differential pressure the hydrocarbon in the source rocks vertically migrated downwards to the sand and mud interface, then migrated laterally along the sand and mud interface under the combined force of the formation differential pressure and buoyancy. Clearly the accumulation dynamic gradient determined the migration trend and distance. When the buoyancy and accumulation resistance were given, the rate of the formation differential pressure changed with the distance would determine the accumulation dynamic gradient. On the pressure contour map, hydrocarbon would migrate along the crowd contours. While the rate of the formation differential pressure changed with the distance gradually decreased, the buoyancy began to play a mian effect on the hydrocarbon migration, dominating the hydrocarbon accumulation and the distribution of the hydrocarbon reservoirs. |
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