Research On Accumulation Mechanisms And Richment Conditions For Terrestrial Tight Sand Oil

To accelerate the exploration and exploitation process of tight oil reservoirs is crucial for guaranteeing the domestic hydrocarbon production and national energy security,since plenty of terrestrial sedimentation in China is always evaluated as being rich in the terrestrial tight sand oil resources,for instance,the typical Huahai Depression in the Jiuquan Basin.However,the Jiuquan Basin is characterized as complex structure revolution,variable sedimentary environment and unstable layer thickness,which lead to extremely complicated geological and geochemistry features,i.e.,hydrocarbon accumulation and expulsion history,reservoir properties,structure types of source rocks and reserved spaces as well as hydrocarbon migration and accumulation processes.Because the hydrocarbon accumulation conditions and mechanism in that place has not been convincingly discussed so far,the distribution of favorable area is hard to be predicted.In this paper,geochemistry analysis and a lot of measuring or observing methods,including basin modeling,physical modeling,thin sections and scanning electron microscope(SEM)observations,unclear magnetic resonance(NMR)and fluid inclusions analysis,are integrally employed in describing the tight sand oil reservoir in the Xiagou and Zhonggou formations,Huahai Depression.The static geological characters including structures,sedimentations,source rocks and reservoir spaces in this area are ascertained,while the dynamic accumulation processes such as the hydrocarbon generation and expulsion,reservoir porosity and hydrocarbon migration are also recovered.Therefore,the accumulation mechanism is clarified and relevant favorable area is predicted.The hydrocarbon in the Xiagou and Zhonggou formation,Huahai Depression is often generated,expulsed and accumulated at an early period;the oil could be expulsed even during the low maturated stage;the expulsion amount of source rock is relatively small,so abundant hydrocarbons are left;as a whole,the expulsion process is not efficient and the average amount of residual hydrocarbon is about 3.3 times of that of expulsed.The tight reservoirs in the Xiagou and Zhonggou formation have suffered the severe compaction and cementation,sustained cementation and slightly dissolution at the early,middle and later sedimentary period,respectively.These reservoirs are compacted to be substantially tight under 120Ma-100 Ma.The structure types of source rocks and reserved spaces in the Huahai Depression can be classified into four species,including the structure that reserved spaces are located on the top of source rocks,the structure that reserved spaces beneath the source rocks,the sandwich structure formed by the stratiform combination of reserved spaces and source rocks,and the structure that the hydrocarbons are generated and preserved in the same place,according to their distribution areas.In particular,as the most popular category,the last type has the largest thickness and is often distributed in the depression center.The hydrocarbon in the Huahai Depression is accumulated at an early stage,and the charging power for tight oil is primarily provided by the fluid expansion and overpressure transference from hydrocarbon generating pressurization.Besides,the dual structure including micro-faults and pores is the main channel for oil migration and accumulation.The lateral migration distance for hydrocarbon is often larger than that of vertical.The hydrocarbon accumulation efficiency is not high.The lower charging limit of tight sand oil aperture always varies from 22.4nm to 90.4nm.The accumulation of tight oil is suggested to be integrally effected by many elements,i.e.,TOC content,maturity of organic matter,sedimentary facies,reservoir properties,structure types of source rocks and reserved spaces,fracture and formation pressure.Thus,the potential tight oil areas in Xiagou formation and upper Zhonggou formation can be divided into three levels,respectively.The Class I corresponds to the best and was mainly distributed in the central fault zone,while the second best area(Class II)was developed around the Class I periphery.And the third best one(Class ?)that has a relatively poor oiliness was further stretched from the periphery of Class II to the western slope zone and the southern fault zone.