Characteristics and Mechanisms of Shallow Igneous Intrusions and Their Implications on Hydrocarbon Geology in the Baiyun Sag
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摘要: 珠江口盆地白云凹陷内发育大量早中新世（约15.5 Ma）浅成岩浆侵入体（侵入深度 < 3 km）.目前对其形态特征、相互关系以及岩浆侵入诱发的强制褶皱等尚缺乏系统研究.通过利用高精度三维地震资料，细致刻画了侵入体的形态、岩浆运移通道以及岩床复合体内各岩床的空间关系，定量分析了强制褶皱与侵入体的几何关系.白云凹陷内岩浆侵入体形态多样，主要包括碟状、板状、舌状、透镜状等.岩床复合体内各岩床的形态和空间相对位置可以判断岩浆来源和流动方向.岩浆侵入体主要通过临近岩床侧向供给或下部岩墙垂向供给这两种方式侵入至目前所在层位.侵入体形态主要受控于岩浆的粘度、围岩内聚力以及侵位深度3个方面.塑性变形使褶皱幅度通常小于侵入体的厚度，强制褶皱可以形成圈闭，圈闭的规模主要由下伏侵入体的范围和厚度决定.侵入体上方由差异压实产生的断层为油气运输提供了有利通道.Abstract: A large number of Early Miocene (ca. 15.5Ma) shallow igneous intrusions(intrusion depths < 3 000 m) have been identified in the Baiyun Sag of Pearl River Mouth Basin. Their morphological characteristics, relationships and forced folds above them have not been systematically studied. This paper detailedly describes the shapes, vertical migration pathways and spatial relationships (sill complexes) of igneous intrusions, using high-resolution 3D seismic data. Moreover, the geomorphological relationship between the forced folds and the intrusions are quantitatively analyzed. The igneous intrusions are characterized by saucer, strata-concordant, tongue orlentoid shapes. The source and flow direction of magma could be indicated from the geomorphological characteristics and spatial relationship of sills. Magma emplacement is transported from adjacent sills(lateral supply) or from dykes(vertical supply).The geomorphological characteristics of igneous intrusion are mainly controlled by the viscosity of magma, the cohesion of host rock and the emplacement depth of magma. Moreover, some igneous intrusions could trigger sediment deformations of overlying strata and form forced folds. The amplitudes of forced folds are usually less than the thickness of the igneous intrusion, because of the plastic deformation. These folds could serve as hydrocarbon traps whose sizes are mainly influenced by the scale and thickness of underlying igneous intrusion. Furthermore, normal faults that are located within the folded strata and generated by differential compaction provide pathways for vertical hydrocarbon migration.
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Key words:
- Igneous intrusion /
- Igneous feeding relationship /
- Sill /
- Forced fold /
- Baiyun Sag /
- petroleum geology
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图 1 研究区地质概况
a. 珠江口盆地地质概况及研究区位置（修改自Sun et al., 2020）；b. 地震剖面及侵入体位置：背景为T40界面地形，倾向为NW-SE方向；白色部分超出资料范围；灰色部分为侵入体，黑色虚线为侵入体被遮挡部分；红色实线为地震剖面位置；X-X'指示剖面方向
Fig. 1. Geological setting of the study area
图 2 岩浆侵入体地震剖面特征
岩浆侵入体表现强振幅现象，呈碟状、板状、舌状、复合体等形态
Fig. 2. Seismic characteristics of igneous intrusions
图 3 板状岩床、碟状岩床空间形态及地震剖面特征
a，b. 分别为板状岩床的空间形态和剖面特征；c. 碟状岩床空间形态；d，e. 碟状岩床剖面特征
Fig. 3. Spatial morphology and seismic characteristics of strata-concordant sill and saucer-shaped sill
图 4 舌状岩床、透镜状复合体特征
a，b. 舌状岩床剖面及空间特征；c，e. 透镜状侵入体剖面、平面及空间特征；d. 透镜状侵入体厚度图
Fig. 4. Characteristics of tongue-shaped sill and lentoid-shaped igneous complex
图 5 岩浆供给通道特征
a~c. 侧向供给：由附近岩床提供岩浆，黄色箭头曲线为侧向运移路径；d~e. 垂向供给：由下部岩墙提供岩浆，红色虚线为通道轮廓，黄色虚线指示同相轴偏移现象
Fig. 5. Characteristics of magma feeding pathways
图 6 岩床复合体Ⅰ、Ⅱ的空间关系和岩浆供给关系
a~e. 岩床复合体Ⅰ：包含3个岩床，其中岩床A6通过侧向供给为A7和A8提供岩浆；f~i. 岩床复合体Ⅱ：包含4个岩床，其中A3、A9由下部岩墙垂向供给岩浆，A9通过侧向供给为A10和A11提供岩浆；j. 供给模式简图
Fig. 6. Spatial relationship and magma supply relationship of sills complex Ⅰ and Ⅱ
图 7 侵入体厚度(T)与褶皱幅度(R)之间的关系
灰色虚线分别为斜率为1和0.33的边界线；部分数据引自Hansen and Cartwright（2006b）；Jackson et al.（2013）；Magee et al.（2013b）；Montanari et al.（2017）；Geng et al.（2020）
Fig. 7. Relationship between thickness (T) of igneous intrusion and amplitude (R) of forced fold
图 8 岩浆侵入体相关的油气聚集模式
a. 岩浆侵入造成上覆地层隆起，形成穹窿状圈闭. 绿色部分为油气，橙色箭头曲线指示油气运移方向；b. 透镜状岩浆复合体上部发育一系列断层，为油气提供垂向运移通道，绿色虚线为圈闭边界
Fig. 8. Hydrocarbon accumulation patterns associated with igneous intrusion
表 1 研究区内浅成岩浆侵入体形态特征
Table 1. Morphological characteristics of shallow igneous intrusions in the study area
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