Study Of Reservoir Geology Of Chang 4+5 Oil-bearing Formation In Majiashan Area, Ordos Basin

Majiashan area is affiliated with the Jiyuan Oilfield, three neighbouring wellblocks have been exploited, including Geng63 wellblock, Geng116 wellblock and Geng117 wellblock, with the characteristics of low-productivity of single-wells, high compositive moisture content, early breakthrough in the commissioning and so on. The exploitative differences in efficacy among these three neighbouring wellblocks are rising sharply, however, the controlling factors of different productivity in these oil reservoirs is currently an open question, causing difficulties in further production adjustment of the oil field. Based on this key question, the paper makes a profoud analysis of reservoir geology characteristics, and verifys the influence of sequence stratigraphy, Sedimentation and heterogeneity over reservoir productivity, and finally studied the controlling factors of differences in exploitative efficacy from macroscopic and microscopic perspective respectively.By analyzing the cores and the well-logs of study area, the interface characteristics at all levels, such as erosion surface, flooding surface and lithofacies-transform surface, are distinguished. According to the research of high-resolution sequence stratigraphy, Chang4+5 Member is divided into 1 middle base-level cycle, 2 short base-level cycles and 4 super short base-level cycles. Based on the principles of base-level cycle having control over development of sandstone reservoir, isochronic contrast of sandbody is carried out in the sequence stratigraphic framework, and the distributing feature of sandstone is further analyzed. The paper points out that the accommodation and A/S ratio is lower at the bottom of uplifted hemicycle of middle base-level cycle, and Geng117 wellblock in the direction of lake, as the preponderant unloading zone, developed the best sandstone reservoir of study area.Combined with the regional geological background, the source direction of Majiashan area is the northeast, the research area mainly developed meandering river delta front sub-facies, and its preponderant sandbody is of the underwater distributary channel while its mouth bar is not well developed. By means of fine observation and description of cores and analyzing characteristics of well-logs, using sedimentary principles, the vertical evolution and planar distribution of microfacies are studied. It shows that the underwater distributary channel of Chang4+522 is obviously better developed than the other overlying three sublayers, and channels changed and moved more frequently in Geng63 and Geng116 wellblocks, while river energy relatively decreased and channels developed relatively stably in Geng117 wellblock.Combined with the data, the paper analyzes reservoir heterogeneity according to parameters of reservoir properties, interlayers, and parameters of permeability anisotropy. It shows that the reservoir heterogeneity of Geng117 wellblock is obviously better than that of Geng63 and Geng116 wellblocks in Chang4+522. In conclusion, the reservoir heterogeneity influences differences in exploitative effectiveness directly, while stratigraphic frameworks and development characteristics of sedimentary microfacies control oil-gas distribution and accumulation radically.