Keywords: 
• Wuliyasitai Sag /
• slope zone /
• sublacustrine fan /
• depositional characteristics /
• subtle oil pools

Abstract: Some conglomerate bodies that have heavy thickness, extreme coarse grains size and fast facies change occur in the slope zone of the First Member of Tenger Formation in Wuliyasitai Sag, Erlian Basin. However, There is stillmuch controversy about the exact origin of the conglomerate bodies and bewilderment about its size, space distribution, reservoir characteristics and formationalmodel in subtle oil pools. Based on analysis of its tectonic setting and depositional conditions in detail, accorded to the result of cores observation, combination characteristics of borehole lithology and log and seismic facies analysis, etc., and combined with analytic data of petrology and mineralogy and physical property, etc., it is studied in detail that the origin, sedimentary facies zones distribution characteristics,depositionalmodel, reservoir property and formational conditions in subtle oil pools of conglomerate bodies involved in this area. The result indicates that the conglomerate bodies that surrounded by dark graymudstone of deeper lacustrine facies are the deposits of sublacustrine fan facieswith feeder channel. The sedimentary structures that formed by sediment gravity flow, such as massive bedding, normal graded bedding, superposition graded bedding and partial reverse graded bedding are main and recognized. Massive and fine upwards rhythmic layering similar to "AAA" sedimentary sequence are dominant and rep resent for the sedimentation of gravity flow channels. In the seismic profiles,their seismic facieswith clearmoundy and channel filling configuration are typical. The subfacies zones of sublacustrine fan facies develop perfectly and can be classified to inner fan, middle fan and outer fan subfacies. In general, the inner fan subfacies has characteristics of a single channel that described as main channel. The lithofacies of the main channel in the inner fan subfacies are composed ofmassive muddy conglomerate facies and superposition graded bedding conglomerate facies, which deposited alternately by high-density debris flows and grain flows. The middle fan subfacies show the depositional characters of braid channel and its lithofacies,which grain flows and high-density turbidity current are the main transportingways, consist ofmassive sandstone facies, conglomerate facies with pebble and glutenite facies with boulder. The outer fan subfacies are composed of thin turbidite bedswhich can be described by typical Bouma sequence. The sublacustrine fan facies of above have more than two depositional stages. The source area with the characteristics of multip le2source or linear source and extreme coarse grains size results in poor stability, small size and strong tendency to migration of the feeder channel and two or more conglomerate bodies of sublacustrine fan fancies may be formed along the slope zone. Vertically and laterally,these conglomerate bodies overlap each other and distribute in a large area. In the early period of Tenger, the lacustrine basin expands rapidly with deep subsidence in Wuliyasitai Sag. The environment of deep water that fits the sublacustrine fan facies is dominant. Meanwhile, because the tectonic activity of the northwestern margin fault enhances sharply, the southeastern slope zone begins to up lift and forms gradually the slope geomorphy with big gradient and high angle. At the same time, some tectonic break-slope zones occur on the slope with the differential activity of some secondary consequent faults such as Tai 21 eastern fault, etc. The steep grade of the slope, especially tectonic break2slope zones control formation and development of sublacustrine fan. In the base of tectonic break-2slope zones, a great deal of clastic fragment transported by floodwater gravity flows passes the feeder channel and downloads in turn to form the sublacustrine fan with feeder channel. The conglomerate bodies of sublacustrine fan facies have favorable association of source rocks, reservoirs and seal beds and perfect formational conditions in subtle oil pools. The braid channelmicrofacies areas in the middle fan subfaciesmay be the most favorable reservoir zones and have been found thick oil beds in some wells of Tai 43, Tai 47, etc. The effect of producing test in these wells is very good. Although the reservoir property of the main channelmicrofacies areas in the inner fan subfacies is poorer than that of the braid channelmicrofacies areas in the middle fan subfacies, they are still favorable reservoir zones and have obtained commercial oil stream in Tai 21 well, etc. In recent years, the conglomerate bodies of sublacustrine fan facies in the slope zone have been achieved great breakthrough of exp loration in the subtle oil pools.