乌里雅斯太凹陷斜坡带湖底扇相砾岩体沉积特征与隐蔽油藏勘探

The Depositional Characteristics of Conglomerate Bodies and Exploration in Subtle Oil Pools of Sublacustrine Fan Facies in Slope Zone, Wuliyasitai Sag

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。

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 Formationin Wuliyasitai Sag, Erlian Basin. However, There is still much controversy about the exact origin of the conglomerate bodies and bewilderment about its size, space distribution, reservoir characteristics and formational model 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, depositional model, 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 gray mudstone of deeper lacustrine facies are the deposits of sublacustrine fan facies with feeder channel. The sedimentary structures that formed by sediment gravity flow, such as massive bedding, normalgraded bedding, superpositiongraded bedding and partialreverse graded bedding are main and recognized. Massive and fine upwards rhythmic layeringsimilar to 'AAA' sedimentary sequenceare dominantand representfor the sedimentation of gravity flow channels. In the seismic profiles, their seismic facies with clear moundyand channel filling configurationare 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 lithofaciesof the main channel in the inner fan subfacies are composed of massive muddy conglomerate facies and superpositiongraded 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 transporting ways, consist of massive sandstone facies, conglomerate facies with pebbleand glutenitefacies with boulder. The outer fan subfacies are composed of thin turbiditebeds which 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 multiple-source 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 rapidlywith 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 uplift and forms graduallythe slope geomorphywith biggradient 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 break-slope zones controlformation and development of sublacustrine fan. In the base of tectonic break-slope zones, a great deal of clastic fragmenttransported by floodwatergravity flows passes the feeder channel and downloads in turnto form the sublacustrine fan with feeder channel. The conglomerate bodies of sublacustrine fan facies have favorable associationof source rocks, reservoirsand seal bedsand perfect formational conditions in subtle oil pools. The braid channel microfacies areas in the middle fan su