The Depositional Character istics of Conglomerate Bodies and Exploration in Subtle Oil Pools of Sublacustrine Fan Facies in Slope Zone, Wuliyasitai Sag
摘要: 乌里雅斯太凹陷南洼槽斜坡带发育一套厚度大、粒级粗和相变快的砾岩体,根据岩心观察、岩电组合特征和地震相分析等手段,结合岩矿等分析化验资料,认为该套砾岩体属具补给水道的湖底扇相沉积。砾岩体夹于较深湖亚相的深灰色泥岩中,发育沉积物重力流成因的沉积构造,内扇、中扇和外扇等相带发育齐全,并具有多期发育的特点。物源区具有的线物源或多物源特征,以及极粗的粒级造成补给水道的稳定性较差、规模较小、迁移性较强,沿斜坡带形成了纵向上由南向北依次叠置、横向上叠加连片的多个湖底扇相砾岩体。坡度很陡的斜坡,特别是构造坡折带的存在控制了湖底扇的形成和发育,使湖底扇相砾岩体主要沿构造坡折带下部分布,具有很好的隐蔽油藏成藏条件,以中扇辫状沟道微相区最为有利,已有多口井发现厚油层并获工业油流,实现了隐蔽油藏勘探的重大突破。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.
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