Structure-Sedimentary Response Relationship of Wenchang Formation in Baiyun Sag, Pearl River Mouth Basin
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摘要: 为揭示白云凹陷文昌组构造演化与沉积充填间的耦合关系,基于三维地震数据和钻井、测井资料,采用层序原形结构剖面恢复、增强地震相分析、砂体地震扫描解释等新方法,对白云凹陷文昌组构造演化及其控制下的沉积充填过程进行了详细研究. 研究表明,响应于控凹断层“弱‒极强‒较强‒弱”的活动过程,文昌组沉积期白云凹陷经历了初始断陷期(WCSQ1)、强断陷早期(WCSQ2)、强断陷晚期(WCSQ3)和弱断陷期(WCSQ4、WCSQ5)4个构造演化阶段. 相应地,白云主洼经历了河流‒湖泊、超深湖发生、超深湖充填、深湖‒浅湖发生等沉积演化过程. 其中旋转断块翘倾侧的北部缓坡带主要为河流‒浅湖过渡环境,发育大‒中型辫状河三角洲沉积体系;断块倾没侧的南部陡坡带主要为半深湖‒深湖环境,发育近岸水下扇‒扇三角洲沉积体系;湖盆中心为深湖‒超深湖环境,以泥岩沉积为主,深湖区周缘发育湖底扇沉积;同时期白云东洼为陡坡+岩浆底侵形成的多隆洼地貌,以小型近岸水下扇‒扇三角洲沉积体系为主,局部发育火山碎屑沉积. 文昌组沉积砂体的孔隙性受白云凹陷控凹断裂和岩浆的活动强度、物源体系及次级洼陷古地理特征等因素共同控制.Abstract: Based on 3D seismic data, drilling and logging data, the tectonic evolution and its controlled sedimentary filling process of Wenchang Formation in Baiyun Sag were studied in detail by using new methods such as the restoration of original form structure section, enhanced seismic facies analysis and seismic scanning interpretation of sand body in order to explore the structure-sedimentary response relationship of faulted lake basin. The study shows that in response to the activity process of "weak-very strong-relatively strong-weak" of sag-controlling faults, Baiyun Sag experienced four tectonic evolution stages during the Wenchang Formation: initial fault depression stage (WCSQ1), early stage of strong fault depression (WCSQ2), late stage of strong fault depression (WCSQ3) and weak fault depression stage (WCSQ4, WCSQ5). Correspondingly, Baiyun Main Sag underwent the sedimentary evolution processes of fluvial-lacustrine, ultra-deep lake occurrence, ultra-deep lake filling, deep lake-shallow lake occurrence. The northern gentle slope zone located on the tilting side of rotating fault block mainly developed a fluvial-shallow lake transitional environment and a braided river delta depositional system of large-medium scale. The southern steep slope zone located on the plunging side of rotating fault block mainly developed a semi-deep lacustrine to deep lacustrine environment and a nearshore subaqueous fan-fan delta depositional system. The center of lake basin mainly developed a deep lake-ultra-deep lake environment and argillaceous deposits. The deposits of turbidite fan were developed around the deep lake. However, the Baiyun East Sag appeared as a landform of multiple uplift-depression formed by steep slope and magmatic underplating, and mainly developed small-scale inshore subaqueous fan-fan delta sedimentary system with volcaniclastic sediments. The porosity of sand bodies of Wenchang Formation was jointly controlled by the activity intensity of sag-controlling faults and magma, provenance system and paleogeographic characteristics of secondary depressions in Baiyun Sag.
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图 3 白云凹陷主裂陷期地壳拆离薄化与断陷湖盆结构演化模式(据庞雄等, 2018修改)
Fig. 3. Evolution model of crustal detachment thinning and structure of rifted lacustrine basin during the main rifting period in Baiyun Sag (modified from Pang et al., 2018)
图 10 AA’测线砂岩地震解释剖面(测线位置见图 1b)
WCSQ1:1~4均为河道砂体,呈底面下凹、右下倾斜对称波形、短轴反射,表明WCSQ1从河流向湖盆转变,5为水下分流河道(近岸水下扇);WCSQ2:1~3均为河道砂体,4为河口坝砂体(叠瓦状构型),5为前缘席状砂,6为斜坡水道砂,7为水下分流河道(扇三角洲),8为河口坝;WCSQ3:1~2为河道砂,3为河口坝,4为沿岸砂坝,5为前缘席状砂,6~7为斜坡扇砂,8为河口坝砂;WCSQ4:1~2为河道砂,3为河口坝砂,4为席状砂,5为斜坡扇砂,6为湖底扇砂,7为斜坡扇砂,8为河口坝砂;WCSQ5:1为河道砂,2为沿岸砂坝、河口坝,3为斜坡扇砂(短轴点状反射),4为湖底扇砂,5为斜坡扇砂,6为水下分流河道
Fig. 10. Sandstone seismic interpretation profile of Line AA' (line location is shown in Fig.1b)
图 13 白云凹陷文昌组砂岩发育特征
a.W9‒1井古近系砂岩类型三角图(井位置见图 1b),文昌组砂岩的岩屑含量较高,且岩屑类型为火山喷出岩,表现出成分成熟度低、近物源沉积的特征;b.白云凹陷南缘云开、云荔隆起区文昌组砂岩孔隙度分布图,其中云开隆起区文昌组砂岩主要表现为低孔特征,云荔隆起区文昌组砂岩则表现为中低孔特征
Fig. 13. Sandstone development characteristics of Wenchang Formation in Baiyun Sag
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