| 断陷湖盆浅水三角洲沉积特征:以Muglad盆地Unity凹陷Aradeiba组为例 |
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| 引用本文： | 范乐元,吴嘉鹏,刁宛,李洋.断陷湖盆浅水三角洲沉积特征:以Muglad盆地Unity凹陷Aradeiba组为例[J].地学前缘,2021,28(1):155-166. |
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| 作者姓名： | 范乐元 吴嘉鹏 刁宛 李洋 |
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| 作者单位： | 中国石油集团长城钻探工程有限公司国际测井公司,北京100101;中国石油东方地球物理公司研究院,河北涿州072750 |
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| 基金项目： | 中国石油天然气集团公司海外重大科技项目“海外天然气复杂储层精细评价与预测技术”(2018D-4305) |
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| 摘 要： | 浅水三角洲是沉积学和油气勘探开发领域的热点,目前研究主要集中在大型坳陷型盆地内,部分学者研究证明在断陷湖盆萎缩期或裂陷初期也存在浅水三角洲沉积,但研究较为薄弱。本文利用岩心、测井、地震以及分析化验资料,对Muglad盆地研究区内Aradeiba组浅水三角洲的沉积有利条件、沉积特征以及垂向演化特征进行了深入剖析,结合湖平面变化、物源供应等情况,建立其沉积演化模式。研究区在Aradeiba期构造相对简单,基底起伏不大,地势宽缓;古气候温暖潮湿,利于母岩区风化作用的进行,物源充足;古水体水浅动荡,具备形成浅水三角洲的有利沉积条件。综合组分、沉积构造、岩心旋回及粒度、录井等多方面资料,分析研究区内发育的浅水三角洲。取心段由多个小型冲刷面或沉积间断面分隔的正韵律叠加而成,每个正韵律下部为中-粗砂岩沉积,发育强水动力沉积构造,顶部则多为杂色泥岩。石英含量很高,分选好,砂岩粒度概率曲线以两段式为主,跳跃总体含量较高,也有一定量的悬浮总体,反映了浅水牵引流的沉积特点。根据Aradeiba组岩心相及测井相特征,研究区内主要为浅水三角洲前缘沉积。延伸远并且多分叉的水下分支河道发育,表现出高幅箱型-高幅钟型-中高幅薄箱型等多种测井相类型;河口坝在测井曲线上呈中高幅漏斗型。Aradeiba组自上而下划分为5个小层,结合测井解释成果、岩心相与测井相特征以及砂岩含量图,对不同小层的垂向沉积特征分析,发现不同小层主体沉积微相发生明显演化。综合湖平面、构造演化等因素,提出Unity凹陷Aradeiba组各小层沉积演化模式。裂陷初期,湖平面上升,第5小层主要为远砂坝、席状砂沉积;随着断裂活动加强,物源充足,浅水三角洲规模增大,第4、第3小层分别主要为水下分支河道、河口坝沉积;第2小层沉积时,湖泊水动力增强,分支河道发生明显席状化;伴随湖平面快速上升,第1小层主要为浅湖相泥岩披覆沉积。以上分析,对于加深Muglad盆地的沉积学认识并拓展油气勘探的领域和范围具有一定的指导意义。
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| 关 键 词： | 浅水三角洲 沉积特征 Aradeiba组 Unity凹陷 Muglad盆地 |
| 收稿时间： | 2019-12-05 |
Sedimentary characteristics of the shallow water delta in rifted lacustrine basin:A case study in the Aradeiba Formation,Unity Sag,Muglad Basin |
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| FAN Leyuan,WU Jiapeng,DIAO Wan,LI Yang.Sedimentary characteristics of the shallow water delta in rifted lacustrine basin:A case study in the Aradeiba Formation,Unity Sag,Muglad Basin[J].Earth Science Frontiers,2021,28(1):155-166. |
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| Authors: | FAN Leyuan WU Jiapeng DIAO Wan LI Yang |
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| Institution: | 1. International Logging Company of CNPC Greatwall Drilling Company, Beijing 100101, China2. Geophysics Research Institute, BGP, CNPC, Zhuozhou 072750, China |
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| Abstract: | Shallow water delta is a hot topic in sedimentology and hydrocarbon exploration with much attention given to depression lacustrine basins. Some scholars have studied the sedimentary evolution of shallow water delta in rifted lacustrine basins and proved that it can also develop in the basins’ recess or early rift stage; however, as a whole, the sedimentology of shallow water delta in rifted lacustrine basins was poorly introduced. Here, based on the core, well logging, seismic and laboratory analysis data, the favorable conditions of sedimentation, sedimentary characteristics and vertical evolution of the shallow water delta in Aradeiba Formation are studied in this paper, and the sedimentary evolution model of the shallow water delta in the early stage of rifting period is proposed in combination with the changes of lake level and the sediment supply. Relatively simple structure with a wide gentle terrain inherited from little basement fluctuation, warm humid paleoclimate conducive to source area weathering and sufficient source materials, and frequently fluctuating shallow paleolake level (inferred by core mudstone data) are the favorable structural and sedimentary conditions for shallow water delta development during the early 2nd rifting period in the Cretaceous Aradeiba Formation. Comprehensive analysis results of core depositional cycles, sedimentary structures and components, grain size features and mud logging characteristics show that shallow water delta developed in the study area. As it is shown, the core is segmented by several small scour surfaces or sedimentary sections in an upward rhythm. The lower part of each segment contains medium-coarse sandstone with strong hydrodynamic sedimentary structures, such as trough and wedge cross-beddings, while the upper part deposits mostly variegated mudstone. The content of quartz, well sorted, is very high, and the grain size probability curve of sandstone mainly has the two-segment shape, showing high saltation and a certain amount of suspended population consistent with the sedimentary characteristics of shallow water traction flow. The development of distributary channels is a significant feature of the shallow delta front, featuring a variety of logging facies types, such as high amplitude box, high amplitude bell and medium/high amplitude thin box type. Another important type of reservoir sandbody, mouth-bar, shows medium/high amplitude funnel type on the logging curve. The Aradeiba Formation in the study area is divided vertically into 5 sublayers based on drilling and logging data of several oil fields in the Unity Sag. Each sublayer exhibits obviously different main sedimentary microfacies revealed by logging interpretation results, core facies, logging facies analysis and sandstone percentage maps. Taking into account of factors such as lake level and tectonic revolution, we propose a sedimentary evolution model for the Aradeiba Formation as follows. In the 2nd early rifting stage, mainly distal bars and sandsheets deposited in the 5th sublayer. As rift activity strengthened, with enriching sediment supplies, distributary channels and mouth bar developed in the 4th and 3rd sublayers, respectively, enlarging the shallow-water delta. Strengthening lake hydrodynamics at the 2nd sublayer deposition resulted in sheetlike distributary channel dominance, while rapidly rising lake level led to deposition of mainly shallow lake drape mudstone in the 1stsublayer. This study deepens the understanding of the sedimentology of the Muglad Basin and provides guidance for expanding hydrocarbon exploration in this basin. |
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| Keywords: | shallow water delta sedimentary characteristics Aradeiba Formation Unity Sag Muglad Basin |
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