Analysis on Sequence-tracts and Distribution of Depositional Systems of Paleocene Mingyuefeng Formation in West Lishui Sag, East China Sea
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摘要: 古新统明月峰组是丽水西次凹中重要的含油气层段,其浅海—三角洲相沉积构成了一个完整的三级层序。层序界面在地震剖面上表现为下超、削截、下切,界面底部的下切水道在测井曲线上具块状、箱型等特征性的反映。依据初始海泛面、最大海泛面和高位域晚期存在的明显海退界面,三级层序内可划分出低水位、海侵、高水位和下降体系域四部分。层序界面和主要的海侵、海退面均得到了古生物丰度和分异度的佐证。通过对上述4个体系域平面编图,揭示了不同时期沉积体系展布及其演化。低位—海侵期发育下切谷、滨岸碎屑—三角洲及扇三角洲前缘—远端浊积砂体,前者分布在盆地西缘斜坡带,物源来自西北和东南两个方向;后者发育于盆地中部的东缘陡坡带,物源由东向西推进。高位域和下降体系域以发育大型高角度进积的三角洲前缘砂为特征,高位域三角洲分布在盆地西侧,且南部比北部发育;下降域盆地沉积范围明显缩小,三角洲主要集中在盆地西侧中部。Abstract: The Paleocene Mingyuefeng Formation, an important oil/gas bearing segment of the west Lishui sag, filled shallow sea and deltaic deposit which has formed an integrated third order sequence. The sequence boundaries show downlap, truncation and incised characteristic on seismic profiles. The incised valley shows blocky and trunk configuration on well logs. According to the initial and maximal transgression surfaces, the sequence may be classified into lowstand tract, transgression tract and highstand tract. According to the minor regression surface, there has been recognized the falling sea level system tract in the late highstand tract. Sequence boundary, major transgression surfaces and the minor regression surface have been confirmed by the enrichment and differences of paleobiology fossils. Through the sequence-tract mapping of the above 4 tracts, the distribution and evolution of depositional systems in different periods have been revealed. There developed incised valley, shoreline delta classtic and prodelta turbidite in lowstand and transgression tract. The incised valley and shoreline delta distributed on the western slope margin of the basin, and the depositional systems mainly came from the northwest and the southeast of the basin. The sparse prodelta turbidite distributed on the eastern abrupt margin of the basin and the depositional systems came from the east. It is charactered by large-scale progradation delta front sandstone in the highstand tract and falling sea level tract. The highstand tract delta distributed on the west margin of the basin, and there developed more on the south than on the north. The depositional area of the basin has obviously shrunk in the falling sea level tract so that the deltaic sandbody centralized on the central section of the western of the basin.
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Key words:
- sequence-tract /
- depositional systems /
- Mingyuefeng Formation /
- west Lishui sag /
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