南海西北部莺琼陆坡重力流和等深流的时空关联

doi:10.13745/j.esf.sf.2022.10.20

摘要/Abstract

摘要：

南海西北部的莺琼陆坡深水区近些年获取了大量油气发现,展示了广阔的勘探前景。深水沉积往往受控于重力流、等深流等多种机制,它们在时空中如何分布、如何相互作用,又在多大程度上影响或控制着沉积格局,对水动力方面的认识的差异会导致沉积相展布方面的不同认识,进而影响对砂体类型和展布的分析,制约了石油勘探开发。论文基于地震数据所呈现的地震相和地层叠置样式,确认莺琼陆坡在中新世以来发育两类沉积体系:重力(流)和等深流沉积体系。二者在时空上存在毗邻而居、有序进退的关系,重力流沉积呈“台阶状”逐层上超于等深流沉积之上,呈进积形态,而等深流沉积体系则呈退积样式;二者之间构成了特殊成因的、跨时的“相变”面;进而相应恢复了两种不同方向的沉积作用(重力流和等深流)的时空关联。它们这种“台阶状”进退关系根源于中中新世以来莺琼陆坡所经历的多方向物源体系充沛的碎屑供应和由此导致的活跃的重力流活动;南海深水循环导致的等深流机制作用的强度也有相当的作用;总体上,等深流沉积体系的分布范围受重力流作用强弱的控制,形成了二者彼进此退的时空关系。

关键词: 南海, 莺琼陆坡, 深水, 重力流作用, 等深流作用

Abstract:

Large amounts of oil and gas are discovered in recent years in the Yingqiong slope deep-water area, northwestern South China Sea, demonstrating the region’s high potential for oil and gas exploration and production. Deep-water sedimentation is often controlled by various mechanisms including gravity flow and contour current. Where and when these mechanisms operate and interact, and to what extent they affect or control sedimentary patterns, can result in different understandings of the regional sedimentary facies distribution due to spatio-temporal variability of hydrodynamic properties. This in turn can complicate the determination of sand body type and distribution and thus hinder gas and oil exploration and development in the region. Based on the seismic facies and stratigraphic superimposed pattern presented by the seismic data, this paper confirms the existence of two types of Miocene sedimentary systems on the Yingqiong slope: gravity (flow) and contour current sedimentary systems. These two systems are adjacent to each other sequentially in time and space, with the former superimposing over the latter in a stepwise pattern prograding towards the slope while the latter exhibiting a retrograde pattern. The boundary between the two systems indicates a special, diachronic “facies transition” interface, and, accordingly, the spatio-temporal relationship between the two distinct sedimentary processes (gravity flow and contour current) is restored. The stepwise progradation-retrogradation relationship between the two sedimentary systems can be attributed to three factors: the abundant supply of clastic materials from a multi-directional Middle-Miocene source system; the resulting strong gravity flow activity; and contour currents induced by deep-water circulation in the South China Sea. The magnitude of gravity flow largely controls the distribution range of the contour current sedimentary system, leading to a spatio-temporal stepwise progradation-retrogradational pattern.

Key words: the South China Sea, Yingqiong slope, deep-water, gravity flow, contour current

中图分类号:

P736.21

王海荣, 余承谦, 樊太亮, 柴京超, 王宏语, 高红芳. 南海西北部莺琼陆坡重力流和等深流的时空关联[J]. 地学前缘, 2023, 30(4): 196-208.

WANG Hairong, YU Chengqian, FAN Tailiang, CHAI Jingchao, WANG Hongyu, GAO Hongfang. Spatio-temporal relationship between two kinds of deep-water sedimentation on the Ying-Qiong slope, South China Sea[J]. Earth Science Frontiers, 2023, 30(4): 196-208.

图/表 10

图1 南海北部莺琼陆坡研究概要图 A-南海周边大地构造单元组成(据文献[19]);B-南海地貌和研究区位置;C-研究区数据分布和关键构造-沉积要素;D-研究区地层格架。

Fig.1 Overview of the Yingqiong slope, northern South China Sea

表1 地震采集和处理参数

Table 1 Experimental parameters for seismic data acquisition and processing

震源	总容量	主频	炮间距	接收道数	道间距	采样率	记录长度	覆盖次数
G/G.I.气枪阵列	1 600 in³ (2 000 psi)	60~80 Hz	37.5 m	240	12.5 m	1 ms	9 s	40 fold

图2 莺琼陆坡区域深水沉积体系的格架--重力流沉积体系和等深流沉积体系右上图示意了研究区的地理位置。

Fig.2 Framework of the deep-water sedimentary system in the Yingqiong slope area. The sedimentary system can be divided into two types-gravity flow and contourite depositional systems. The inserted figure shows the geographical location of the study area.

图3 莺琼陆坡深水区域重力(流)沉积的地震响应特征剖面位置参见图2。

Fig.3 Seismic response characteristics of gravity flow deposits in the Yingqiong slope deep-water area. See Fig.2 for section location.

图4 莺琼陆坡深水区域等深流沉积的地震响应特征 A、B、C-3条不同测线,其中B和C彼此正交。剖面位置参见图2。

Fig.4 Seismic response characteristics of contourites in deep-water area of the Yingqiong slope. See Fig.1 for section locations.

图5 两种沉积体系时空分布图 A-两条相交测线所组成的复合测线,最显著的特征是等深流沉积逐渐被重力流沉积以台阶状所掩埋,直至最右侧的依然活跃的等深流沉积作用;B-A有关部分的清晰图示,重力流沉积体系中所识别的5个层位均截止于其和等深流沉积体系的交点,以表明这两类沉积体系的“台阶状”的、进退关系。剖面位置参见图8。

Fig.5 Seismic profiles. (A) Composite seismic profile along two intersecting survey lines. The most remarkable feature is that contourites are gradually buried in steps by gravity flow deposits, and this process ceases at the most active contourites on the right. (B) Enlarged profile. The five horizons identified in the gravity flow depositional system are cut off at the intersection with the contourite depositional system, which indicate a stepwise progradation-retrogradation relationship between the two sedimentary systems. See Fig.8 for section location.

图6 两种沉积体系时空关系图 A-研究区东西向的地震剖面,以剖面中部的等深流水道单元为界,左侧的等深流沉积已被来自西部和北部的重力流沉积所掩埋,而右侧不同，海床上依旧是活跃的等深流沉积作用;B、C-剖面有关部分的清晰图示。剖面位置参见图8。

Fig.6 Seismic profiles of the study area. (A) East-west seismic profile. Bounded by the channel units of contourites in the middle of the profile, contourites on the left are buried by gravity flow de-posits from the west and the north. Whereas on the right, there are still active contourites on the seabed. (B, C) Enlarged profiles. See Fig. 8 for section location.

图7 两类沉积体系有序的时空展布 A,B,C-自北而南依次排列的3条东西向地震测线,参考D所示的剖面关系,图中阴影区域表示重力流沉积,重力流沉积的厚度在不同剖面间的变化表明了重力流的有序向南推进,以及等深流机制有序的强迫性的向南退却;D-莺琼陆坡水深和剖面位置图。

Fig.7 Ordered spatio-temporal distribution of two kinds of sedimentary systems. A, B and C are three east-west seismic profiles arranged in sequence from north to south (refer to D for section locations). The shaded areas represent gravity flow deposits. The change of sediment thickness between different sections indicates the orderly southward progradation of gravity flow and the orderly forced southward retrogradation of contourites.

图8 两种沉积作用和两种沉积体系的时空关系

Fig.8 Temporo-spatial relationship between the two depositional processes or systems

图9 研究区内两种沉积作用之间的时空关联的模式 A、B和C代表演化时序。

Fig.9 Spatio-temporal correlation model between two types of deposition modes in the study area, showing geological changes through stages A, B and C in chronological order.

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