COMPARATIVE SEDIMENTOLOGY OF ALLUVIAL FANS—The Rule of Enrichment of Oil-Gas and Groundwater

The study of sedimentary facies and distribution of groundwater-bearing beds of modern alluvial fans may be applied to reconstruct the palaeogeographic environment and to estimate the rule of enrichment of oil-gas in a fanglomerate oil field.In elongated fault basins,the types of faults along the tR o sides of the basin often differ; one is a single deep-penetrating fault, the other is a series of step-faults.As a result ,the types of alluvial fans which are related to the activity of the faults also differ.They differ distinctively in the thickness and width of the accumulated beds, and in the texture and heterogeneity of the deposits.The alluvial fans related to single deep-penetrating fault are characterized by greater thickness which decre-ases from fanhead to the rim,and by smaller width,coarser pebbles and less heterogeneity of granulo m etry.In such alluvial fans,the difference in lithological cha- racters are mainly present a m ong the subfacies of fanhead,midfan and fanrim.The groundwater is well-distributed in the form of plane. The alluvial fans related to step faults are characterized by minor thickness,w hick increases from fanhead to the rim; by greater width, finer grain size and greater heterogeneity of granulo me- try. The lithology varies across the fan as along its axis.There are channel-flood subfacies and sheetflood subCacies.T he former consists of coarse-grains w hick con-tain little mud,thus presenting greater porosity and being more pervious,forming a belt of ground water-bearing beds. The latter is fine-grained, less pervious and groundwater-poor.It is present as wide slieets on both sides of the channelflood sub facies Through the trend-surface anaysis of the thickness,the clustered analysis of lithology,the size analysis of pebbles and the co m parison of different electro-lithologic profiles. The Third Member of the Upper Wuerhe Formation(Permian)of the No. 5 District of Klamaii Oil Field, }injiang, are considered as the type of alluvi-al fan deposits related to step-faults. The concentration of oil-gas in these fang-lomerates conforms quite well to the distribution of groundwater in modern alluvial fans of the same type.