Study On The Microcharacteristics Of Fluvial And Delta Reservoirs

Su77area and Yu10area, located in the east Ⅱ block of Sulige Gas Field and the northern part of Zizhou Gas Field separately, are both the new exploration aeras. The primary oil-bearing beds of the two areas are Permian Shanxi Formation and Xiashihezi Formation, but the productivities of the two areas varies markedly. In order to find out the reason leading to their differences, many systemic research includimg sedimentary facies, reservoir petrology, characteristics of micro-pore throat, diagenesis, reservoir quality and heterogeneity have been carried out comparatively, drawing support from multi-disciplinary and testing measures. Based on all the studies, the controlling factors of reservoir quality are analyzed and comprehensive reservoir evaluation is studied.The sediments’ source rocks of Upper Paleozoic Shanxi and Xiashihezi Formations in northeastern Ordos Basin are mainly derived from the YinShan ancient lands in the north, the detrital source rocks are dominated by Archean mesometamorphic to hypometamorphic rocks, accompanied by abundant acidic intrusive rocks. Through the comprehensive analysis, it is showed that the provenance directions and source rocks of Su77and Yu10area are the same.During the period of Shan23, the climate was warm and humid, Su77area, relative close to the provenance, was dominated by meandering river depositional system. At the same time, Yu10area, relative to the south, was dominated by meandering river deltaic front depositional system. Affected by the transgression, the underwater distributary deposits accepted the alteration of seawater, making the compositional and textural maturity higher. During the period of lower He8, the climate was hot and dry, braided river system with strong hydrodynamics was developed in Su77area. Meanwhile, braided river deltaic plain was developed in northern and deltaic front was developed in southern Yu10area.Affected by the transgression, the main rock type of Shan23reservoir in YulO area is quartz sandstone, plenty of primary intergranular pores are reserved in the reservoir. The content of matrix is high and the type of cements observed is mainly silica and kaolinite, causing the reservoir low in porosity and high in permeability. The characters of Shan23reservoir in Su77area are quite different, the content of quartz sandstone, lithic quartzose sandstone and lithic sandstone are nearly the same. Various types of multi-pores including dissolved pores, intercrystal pores, intergranular pores and their combinations can all be found in the reservoir. The content of matrix is lower, and kaolinite and silica are the most usual cements. The reservoir is high in porosity and a little lower in permeability. The lower He8reservoirs in the two areas showed similar characters, the main rock type was lithic quartzose sandstone with dissolved pores and intercrystal pores. The content of matrix is high, mainly illite and various kinds of carbonate cements. However the lower Hes reservoir in Su77area is high in porosity and low in permeability while the reservoir in Yu10area is both low in porosity and permeability. The result of constant-speed mercury penetration shows the main pore-throat type is mesopore-megalospore and fine throat. Contribution value of permeability is affected by the size and manifold distribution of throat radius.It is proved that the reservoirs of Shan23and lower Hes submembers in Su77and Yu10areas are mostly in the middle diagenesis phase B. The diagenesis strength of different lithologies differs from each other. The diagenesis characters of quartz sandstone proves to be weak compaction, mid-strong cementation and weak dissolution, but lithic sandstone is strong compaction, mid-strong cementation and strong dissolution, and the diagenesis character of lithic quartzose sandstone is between the other two sandstones. Considering the discrepancies of diagenetic minerals and pore structures, seven diagenesis facies can be divided, providing references for comprehensive evaluation of reservoirs.The type and compositional maturity of sand stone reservoirs are determined by the characters of source rocks, which also affect the spacial distribution partterns of the depositional system. Depositional facies, controlling the grain size, detrital sand composition and contacting types of sandbodies, influencing the strength of early diagenesis, is the chief factor affecting reservoir quality. Furthermore, the later diagenesis enlarged the differences. So deposition and diagenesis were the two main factors causing the differences of the reservoirs in the two study areas. Taking all the factors including lithology, quality, pore structure, capillary pressure curve, strength of diagenesis and diagenesis facies into consideration, the reservoirs of Shan23and lower He8submembers in Su77and Yu10area are comprehensively evaluated. The results show that the Shan23reservoir in Yu10area is the best one, Shan23and lower He8reservoir in Su77area follow next, and lower Hes reservoir in Yu10area is the worst one.