| Block B is located in the northwest of Arabia's Rub al Khali Basin in south-central of Saudi Arabia. The Unayzah formation in the study area is one of the main layers for exploration and development of Sinopec, which is typically of low porosity and low permeability reservoirs, the lithology changes quickly in the vertical and the reservoir type varies, the distribution and controlling factors for favorable reservoir is uncertain, all of these difficulties has directly affected the exploration and development of oil and gas. Therefore under these circumstances, it is necessary to make good study of the reservoir characteristics and begin the evaluation of reservoirs.On the basis of previous research, this paper takes full use of drilling, mudding, well logging and lots of experimental data to deeply study the lithology, physical properties, pore structure, classification and evaluation of the reservoir and distribution properties of Unayzah formation. The lithology of study area is mainly with Quartz sandstone, medium and fine grained; low porosity and permeability with large differences between wells, with some medium porosity and permeability; the main pore types are intergranular pore, remnant primary intergranular pore and dissolution pores, pore structure is mainly with coarse-narrow pore, medium-little throat. According to physical properties, pore structures and relative permeability curves, the criterion of reservoir classification is established and defined 4 types of reservoirs in the study area. Classâ… andâ…¡are with good flow capacity, classâ…¢is poor and classâ…£is non-reservoir.The paper makes use of well-logging data to interpret the reservoir physical parameters and study the physical characteristics of sand in Unayzah formation; study the effects of reservoir lithology, the sedimentary facies and reservoir diagenesis on the reservoir property. The result shows: sandstone with above medium grain and the rock clay content less than 10% are better; the favorable sedimentary facies for reservoir development is the delta distributary channel and fluvial channel deposits; quartz overgrowth and clay mineral filling are the main diagenesis to reduce reservoir porosity and permeability, the second is compaction. Based on the study of sedimentary facies, reservoir parameters, evaluations are mainly conducted on the core interval of single well. In combination with sand thickness, sand-mudstone ratio and effective thickness of reservoirs, the author has comprehensively evaluated the character of Unayzah and pointed out the beneficial zones.Generally the reservoir of segment C in the middle depression of research area is better, the segment B is better near the southern Well MKSR-02, on the eastern uplift for segments A and C are good, classâ… andâ…¡reservoir are well developed. The reservoir has regularities of distribution vertically, the thickness of sand layer A varies not too much which is mainly classâ… andâ…¡reservoir, but wells in the central depression areas are relatively poor. The thickness of sand in segment B thickens from west to east; the reservoir of only two wells are good, which are MKSR-02 in the south and HDDH-02 in the east; reservoirs of HDDH-02 are mainly classâ… andâ…¡reservoirs which are very thick separately. Reservoirs of segment C varies, and the reservoirs of well SHRH-02 in the middle depression and wells FRAS-01 and HDDH-02 in the east uplift are good, while reservoirs of well ATNB-02 and MKSR-02 are poor. Currently the relatively favorable area for exploration the are FRAS structure and HDDH structure which are developed along the western border of Hadidah rise, followed by ATNB and MKSR structures, and the favorable exploration area shifts to the northwest in the MKSR structure.
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