The Coupling Mechanism Of The Diagenetic Evolution And Oil Accumulation Process Of Chang-7Tight Sandstone Reservoir In Wubao Area, Ordos Basin

Based on core observation, conventional section, cast thin section, fluorescence thin section, SEM, particle size analysis, high-pressure mercury injection, inclusion temperature testing method, a systematic study was performed on tight oil reservoir characteristics, diagenesis and diagenetic evolution, the coupling relationship between the history of the tight sandstone reservoir and oil accumulation and the main control factors of tight oil distribution of Chang-7tight oil reservoir in Wubao area. We also pointed out where was the next favorable exploration area. The following results were obtained:The sedimentary period of Chang-7reservoir of Yanchang Formation in Wubao area was the deposition of the semi-deep lake to deep lake, at that time, the gravity flow sand bodies developed well, the reservoir quality was better so that it became one of the most favorable reservoir. The most favorable ones in Chang-7tight oil reservoir, that was Chang-73, was mainly in the deep lake facies mudstone within thin turbidite sandstone sedimentary, at Chang-72and Chang-71period, they gradually evolved into the semi-deep lake mudstone and gravity flow sandbody layers sedimentary, and have a good assemblage within source rocks, reservoir and caprock.The sandstone type of Chang-7reservoir was the feldspar sandstone and the lithic arkose sandstone, the fillings was mainly water mica and calcite. The average porosity was7.49%and the average permeability was0.173×10-3μm2, so it was a typical tight sandstone reservoir. The types of the reservoir pore are mainly all kinds of dissolved and remaining intergranular pores, that mainly refered to feldspar, the average rate of total pore amount was about3%. The connectivity of pore and throat is poor, mainly by the complex nano pore throat system, on the basis of mercury injection parameters, the pore structure is divided into3types, including I type and II type, as the main contribution of tight reservoir yield.The diagenesis of Chang-7reservoir is very strong, generally up to the stage A of the middle diagenetic stage. The original sedimentary fabric of biotite, plastic debris and fine grain size is the main foundation of tight sandstone reservoir. The reservoir further densification was caused by the strong compaction and carbonate cementation and so diagenetic production. However, dissolution of feldspar grains including those easy solution particles increased pore and easily make it an effective reservoir.On account of the porosity evolution curve in the Chang-7reservoir diagenetic evolution of sequences showed that the reservoir had been the densification in the early diagenetic stage B. According to the homogenization temperature of fluid, inclusion petrography, combining with the burial and thermal evolution history, the time of the petroleum migration and accumulation was determined in Early Cretaceous, the reservoir had a feature of densification and accumulation at the same time occuring.Through analyzing the stress of oil particles, without considering the abnormal pressure and hydrodynamic conditions, when the buoyancy and capillary resistance was equal, we could determine the upper porosity of the tight sandstone reservoir, that was12%; On the basis of the logging in oil occurrence and comprehensive geological analysis method, we also might determine the lower limit of Chang-7forming physical reservoir, that was5%.Study on inclusion showed that the Chang-7tight oil was the one stage charging continuous accumulation, dynamic migration of petroleum in the tight reservoir was abnormal high pressure formation from hydrocarbon generation, the type of migration was the mixing flow, with episodic reservoiring characteristics, that was mainly the vertical and near the source rock, their migration and accumulation pathway was a network and a system of micro cracks and pore throat, as the accumulation formed before the reservoir has been densified, oil-water was basically no difference.Chang-73of high quality oil source rocks in high intensity of hydrocarbon generation and expulsion, strong charging into the tight reservoir, priority to small resistance relatively low potential area migration, formed an quasi-continuous oil accumulation mode; the distribution of tight reservoir controlled the distribution of tight oil, the reservoir heterogeneity of tight oil controlled vertical enrichment; there were extensive development of semi-deep to deep lake facies mudstone and oil shale, they played an important role in tight oil accumulation.