Rules Of Chang4+5 Reservior's Oil Accumulation And Distribution In Hujianshan Oilfield, Ordos Basin

On the basis of the former research achievements and acquired information, with the new theories and methods, including sedimentology, structure geology, logging geology, reservoir geology, petroleum geology, this article systematically analyzed the sedimentary facies, reservoir characteristics and petroleum accumulation rules of the Chang 4+5 oil-bearing formation in Hujianshan oilfield, Ordos basin.Hujianshan oilfield is located in the center of the Shanbei slope. The Chang 4+5 oil-bearing formation is the major oil-produce layer. According to the lithologic features, electrical property, sedimentary rhythm, and the distribution of regional marked beds, the Chang 4+5 oil-bearing formation can be divided into two oil layers, i.e. Chang 4+51 and Chang 4+52. The Chang 4+52 oil layer can be further divided into Chang 4+521and Chang 4+522 sublayers. Based on the results of fine stratigraphic correlation, the top structures of each oil layer were analyzed. The results show that each oil layer has the resemble structure property which is similar to the regional structure. They all display the feature of monocline, with some nose-shaped uplifts.According to a series of facies symbols, including rock types and their association, sedimentary rhythm, grainsize distribution of sandstone, primary sedimentary structures, fossils and sandbody shape, combined with regional depositional background and logging curve, the author discussed the types of sedimentary facies, microfacies and their characteristics of the Chang 4+5 oil-bearing formation in the studied area. It is considered that the formation in Hujianshan oilfield is a set of lacustrine delta deposition, in which the Chang 4+51 belongs to delta plain subfacies including distributary channel, inter-channel depression, levee and crevasse-splay microfacies. The Chang 4+521and Chang 4+522 sublayers belong to prodelta subfacies developing subaqueous distributary channel, interdistributary bay, channel-mouth bar and sheet sand microfacies.Based on core observation and a lot of samples analysis datum, the reservoir sandstone of the Chang 4+5 oil-bearing formation are mainly fine arkose and lithic arkose. Generally, the reservoir's physical properties are low porosity and low permeability with some relative high porosity and high permeability area. The types of pore are mainly intergranular pore and intergranular dissolution pore. The types of pore structure are mainly fine pore-mini throat, medium pore-mini throat. The Chang 4+5 oil-layer reservoir can be divided in to 4 types, i.e. typeâ… (good), typeâ…¡(fair), typeâ…¢(poor), typeâ…£(very poor), according to evalation parameters such as physical properties, pore structure etc. Among which the type III reservoir is the most common.The research result shows that the petroleum accumulation of the Chang 4+5 oil layer is mainly controlled by sedimentary facies and the reservoir's physical properties. The reservoirs are typical lithologic reservoirs which mainly developed in main stream with thicker sandbody and relative good physical properites.