SOME DEPOSITIONAL CHARACTERISTICS AND MICROFACIES SUBDIVISION OF COARSE CLASTIC ALLUVIAL FANS

Four depositional characteristics of coarse clastic alluvial fans have been recognized in the investigations performed on recent and ancient fans in Xinjiang.Such features have not ,been paid attention to,or even never been described in previous literatures.These depositional characteristics arc of significance for identification of coarse clastic fans,especially the ancient one and that duried in subground sediments. Therefore, such uniQue characteristics can be used as depositional facies indicators. 1 ) Alluvial sidiment bedding: Stratified structure formed from thin to moderate thick, aggradated and overlapped alluvial sediments.Vertically, such stratified structure appears as a number of sand and gravel or pebble strata occur alternatively, while no distinct, regular bedding plane can be observed. 2 ) Propping conglomerate; It is a loose packed gravel or pebble bed occurring in alluvial fan sediments and characterized by the mutual prop of pebbles with rather good sorting, it can not be consolideted by cementation, due to rare or absent sandy/ argillaceous infilling matericals in pore spaces. 3 ) Funnel-like electrolog response. 4 ) Normal probability cumulative curves of grain size characterized by wide range of grain size distribution and gentle slope of curve. The origin, property and distribution of these characteristics mentioned above have been stated in this paper. Furthermore,the differences between the propping conglomerate and sieve sediments, alluvial bedding and parallel stratification, as well as that between the type "A"electrolog curve of coarce clastic alluvial fan and that of the frontal fringe facies of delta have been described. Due to the different conditions of topography and hydrokinetic, the sediments located in different parts of the coarse clastic alluvial fan are different, unique features, which will affect the development of hydrocarbon reservoir and aquifer of the alluvial fan. The second part of this paper presents a microfacies subdivision system of alluvial fan,based on investigation made on recent alluial fan and that of the Middle Triassic in Karamay oilfield. The alluvial fan may be divided into three subfacies and subdivided into several microfacies as follows: 1 ) Fan top subfacies zone including four microfacies, namely the main ravine, lateral margin ravine, flood zone and ravine bank. 2 ) Mid-fan subfacies zone including three microfacies, namely the braided stream, braided sand island and flood flow zone. 3 ) Fan margin subfacies zone. Again, the inter-fan zone between two adjacent fans may be subdivided into two microfacies, namely the inter-fan bank and the inter-fan depression. All the involved depositional features have been described in this paper respectively. The investigations have proved that the application of such subdivision and delineation system of coarse clastic alluvial fan may be to development studies on such reservoirs h籿e obtained better results.