| Niujuanhu oilfield is located in the northwest of Malang depression in Santanghu Basin. The reservoir of Xishanyao Formation is a broad smooth east-west spread irregular anticline and characterized by simple structure. Based on problems during field production, through analyzing geologic character of Xishanyao Formation and meticulous correlation of strata, microfacies and favorable sandy body and reservoir characters are studied. All recognition above provides reliable geological basis for development strategy and well pattern adjustment.On the basis of results of previous studies, combined with latest data from field production and experiments, microfacies of Xishanyao Formation, which is divided in to four sublayers including X21、X22、X23、X24 , are studied. The water area of the lake had experienced the course of shrinkage and the lake level was shoaling. Braided Delta Front and shore-shallow lacustrine have been recognized, including microfacies such as subaqueous distributary channel, subaqueous interdistributary bay, mouth bar and frontal sand sheet. Coupled with data of well logging and core data, based on microfacies maps above, meticulous description of relationship between lithology and electrical property in the cored wells has been conducted. According to general relationship between lithology and electrical property from cored wells, the lithology of target stratum including pebbled sandstone and glutinite, gritrock, medium sandstone, fine sandstone, siltstone and mudstone are recognized. Based on studies of microfacies, interwell lithology correlation has been done and gritrock and medium sandstone are viewed as preferred sand body and mainly distributed in X22 and X23 sublayers. The preferred sand body in the south of Niujuanhu Oilfield is north north-west spread, and the preferred sand body in the north is mainly south-north spread, which match provenance. The monitoring result of tracers has good relevance with the distributary character of preferred sand body.Based on the physical properties of Xishanyao Formation, the comparison of typical low-permeability reservoirs including Fuyang Formation in Xinli Oilfield and 0.3mD Reservoir in Changqing oilfield is conducted. The results of the comparison show that high percentage of clastic debris and thin throat are the two dominated reasons for the low permeability of Xishanyao Formation. According to study of microfacies of Xishanyao Formation, the comparison of physical properties among subaqueous distributary channel, mouth bar, frontal sand sheet and subaqueous interdistributary bay and shore-shallow lacustrine has been done and shows that average values of porosity and permeability from sandstone of subaqueous distributary channel to that of shore-shallow lacustrine decrease. The comparison of heterogeneity of microfacies above shows that the average value of coefficient of variation is the highest one among the five microfacies, and that of frontal sand sheet is the lowest one. The homogeneity of sandstone in mouth bar is better than subaqueous distributary channel, mainly because strong flushing action occurs in mouth bar and sandstone is characterized by well sorting and roundness; however, sandstone in the subaqueous distributary channel is close to provenance and high percentage of clastic debris and poor roundness and cemented interbed adjacent to mudstone are attributed to the degree of heterogeneity. The development of cemented interbed in the target bed greatly influences heterogeneity of reservoir. In the Xishanyao Formation reservoir, the cemented interbed is mainly found in both the top and bottom of sand body, which is adjacent to mudstone. The cemented interbed in the middle of sandstone is closely related with interdistributary bay and mainly found in the east and north of Niujuanhu oilfield. The cemented interbed in the bottom of sandstone is distributed in the whole oilfield. |
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