深水沉积层序特点及构成要素

Sequence Stratigraphy Characteristics and Sedimentary Elements in Deepwater

本文在回顾当前国际上深水沉积研究热点的基础上,结合在墨西哥湾深水研究的成果系统描述了深水沉积的定义、形成机理、深水沉积层序及深水沉积构成要素的特点.深水沉积主要是在重力流作用下深水环境的沉积,主要形成于相对水平面下降和早期上升的时期,主要分布在低位体系域中.深水层序以凝缩段为边界,块状搬运沉积最早形成并直接位于层序界面上,其上被河道-天然堤沉积所覆盖.典型深水沉积的要素主要由河道、天然堤及越岸沉积、板状砂、块状搬运沉积等构成,这些沉积要素时空上有序地分布.深水河道是物源的主要通道和沉积的重要场所,从上游至下游河道弯曲度增加,能量逐渐减弱.侧向迁移明显,垂向上由富砂的顺直河道演化为相对富泥的弯曲河道.天然堤及越岸沉积以泥质为主,天然堤沿河道呈楔状分布,其近端砂岩含量高,地层厚且倾角较陡;远端砂岩含量低,地层薄且平缓,侧向连续性好但垂向连续性差.板状砂主要为深水扇前缘非限制性沉积,可分为块型和层型.块型侧向连续性好,同时垂向连通性高.层型侧向连续性好,垂向连通性差.块状搬运沉积主要是低水位期坡上沉积物失稳形成的各类滑塌体及碎屑流,其对下伏地层侵蚀明显,分布广泛,变形构造常见,可作为油气良好的封盖层.

This paper mainly presents the definition and forming process of deepwater sediment, as well as the characteristics of deepwater sequence stratigraphy and sedimentary elements. The deepwater sediment is deposited in deepwater environment by the gravity flow, forms during the drop and earlier rise of relative water level, and mainly distributes in lowstand systems tract. The deepwater sequence is bounded by condensed section. Mass transport deposits form the beginning of a sequence formation and sit on the sequence boundary. Then they are overlaid by channel-levee deposits. The typical deepwater sedimentary elements include channel, levee and overbank, sheet sand, and mass transport deposits. These elements are systematically deposited in time and space. Channel is the main sediment conduit and deposition area. From updip to downdip area, the sinuosity of channel increases and the flow energy decreases. The channel migrates laterally and evolves from lower low sinuous sand-＇rich to upper high sinuous mud-rich channel. Levee and overbank are dominated by muddy deposits, and levee distributes along the channel and exhibits wedge shape. The proximal levee is characterized by high content of sandstone, thicker deposit and big dip angle, while the distal levee is characterized by low content of sandstone, thinner deposit, small dip angle, good lateral continuity and poor vertical communication. Sheet sand is the distal unconfined deposits of the deepwater fan and can be divided into conglomerate and layered types, and conglomerate sheet sand has a good lateral continuity and vertical communication. The layered sheet sand has a good lateral continuity and poor vertical communication. Mass transport deposits are the slumps, slides and debris flow due to the slope failure during lowstand period. It can erode the underlying strata, and the deformation structure is very common in mass transport deposit, and it can also serve as a good seal for oil and gas.