Overpressure By Hydrocarbon Generation As The Dynamic For Tight Oil Migration

The study on tight oil accumulation drawing more and more attention as the tight oil has gradually been a highlight of unconventional exploration in recent years,especially the dynamic of tight oil migration,which is one of the most important research aspects.Because of the poor reservoir property,the dynamic condition of tight oil for migration and accumulation is much rigorous.The migration dynamics not only dominates tight oil charging or not,but also affects the accumulation range and enrichment degree.Most of previous studies hold the point that overpressure by hydrocarbon generation is the dominating content of tight oil reservoiring dynamic,and based on this understanding,our research on the fluid pressure variation in source rock during hydrocarbon generation and expulsion by geology information analyzation,physical analog experiments and numerical simulation,is aimed at finding an available measure to calculate the tight oil charging dynamic,and evaluating the tight oil accumulation characteristics under this dynamic condition.This study is carried out based on examples of the tight oil in Yanchang formation and the source rocks(shale and dark mudstones)of Chang-7 member from Ordos Basin through following four main contents:geologic features and overpressure mechanism of Chang-7 source rocks,overpressure caused by hydrocarbon generation,hydrocarbon expulsion through microfractures in source rocks,dynamic of tight oil migration and accumulation.The study on Chang-7 source rocks reveals that the shale and dark mudstones have a lot of differences,such as log response,distribution,geochemistry,rock mechanics,and especially the spatial distribution of kerogen,which shows up as a state of layer morphology in shale but scatter in dark mudstones.The study on overpressure mechanism by logging data analysis draws a conclusion that the development of overpressure in Chang-7 source rocks was caused by compaction disequilibrium combined with hydrocarbon generation,which indicates the common overpressure for the source rocks in a maturated stage.On the basis of geological characteristics and kerogen distribution study of Chang-7 source rocks,put forward "Pressure units during hydrocarbon generation" as the basic spatial place for pressure variation caused by hydrocarbon generation.Besides,establish and test two fluid pressure calculating model of pie and scatter during hydrocarbon generation in shale and dark mudstones respectively through three groups of physical analog experiments,and the result shows that overpressure by hydrocatbon generation can reach more than 100Mpa without consideration of fluid expulsion.Thermal simulation experiment of shale reveals that,hydracarbon generation can give rise to a host of microcracks in sourcerocks,which play an significant role in oil and gas migration.Based on this fact and theory of fracture mechanics,calculate the fracture pressure of shale and dark mudstones and the hydrocarbon conversion rate when cracking associated with fluid pressure calculating model established above.Farther,it is proved that the fracture propagation caused by hydrocarbon generation is instable in preliminary stage and it may propagate to 1.5?2 times and 2?3.5 times longer in shale and dark mudstones respectively than the initial size,which forms a fracture network for hydrocarbon migration in source rocks.Further more,take source rock evolution and fluid pressure droop into consideration and reckon the geologic time every fracture occoured,as well as the primarily migration amount of hydrocarbon and oil charging dynamic.The result revealed that hydrocarbon expulsion in shale has a feature of episodic while that in dark mudstones is continuous;oil charging dynamic is at the peak up to 50Mpa in shale and 30Mpa in dark during first fracture and then tends to become stable constants of 8Mpa and 5Mpa respectively.Finally,according to studies above,make researches on the differences between oil charging dynamic in tight sandstones(Van der Waals' force)and normal sandstones(buoyancy)from dynamical characteristics and effects to discuss the tight oil migration distance and accumulation mechanism.Moreover,analyze the effect of mudstone barriers and fissures on oil accumulation in tight sandtones by cases and come to conclusions that:mudstone barriers in tight sandstones can block the oil migration and will lead to a blank-zone beside the barriers;and tight sandstones with good property and low displacement pressure contacting the fractures are the most possible target for oil accumulation.