西湖凹陷平湖构造带平湖组沉积体系及层序地层研究

doi:10.19657/j.geoscience.1000-8527.2020.002

摘要/Abstract

摘要：

根据钻井岩心、测井及地震等资料对西湖凹陷平湖构造带平湖组沉积体系及层序地层进行研究,并结合泥质岩地球化学特征对古盐度与古水深进行分析。研究结果表明,以平湖组中部P7砂层组底面为界,其下为断陷期,断裂系统对沉积相分布具有控制作用;其上则为拗陷期,向上断裂系统对沉积相分布控制作用逐渐减弱,直至消失。研究区平湖组古盐度与古水深整体反映了振荡式水进、水退的沉积背景,该沉积时期主要以淡水沉积为特征,间歇发育半咸水沉积特征,水体深度一般为0~8 m,在平湖组三段沉积时期达到最大,近14 m。平湖组自下而上可以划分为3个三级层序(Ⅰ—Ⅲ),分别对应平湖组五段、平湖组三—四段、平湖组一—二段,发育三角洲-潮坪沉积体系。层序Ⅰ沉积时期,受北东-南西向断裂影响,以三角洲沉积体系为特征,且断裂对沉积相展布具有控制作用;层序Ⅱ沉积时期,断裂活动减弱,地形逐渐被填平补齐,潮汐作用影响增大,潮坪沉积环境占主导地位,发育少量潮道和潮汐砂坝;层序Ⅲ沉积时期,海水逐渐退去,在宽缓斜坡背景下发育受潮汐影响的三角洲和潮坪沉积。

关键词: 西湖凹陷, 平湖构造带, 古盐度, 古水深, 沉积体系, 层序地层

Abstract:

In this paper, depositional system and stratigraphy of Pinghu Formation in Pinghu tectonic belt of Xihu Sag were discussed in detail based on the drilling, core logging and seismic data. The paleo-salinity and paleo-water depths were also discussed based on mudstone geochemical compositions. The results show that the bottom of P7 sand group in the middle part of Pinghu Formation is bounded by the fault-depression transition. The fault system controls the distributions of sedimentary facies before the P7 sand group deposited. Above the P7 sand group, however, the influence of fault system on the distribution of sedimentary facies reduced gradually until it disappeared. Paleo-salinity analysis shows that Pinghu Formation is mainly characterized by fresh water deposition, and semi-salt water deposition developed intermittently. Paleo-water depth analysis indicates that the water depth was generally 0-8 m, and reached a maximum in the 3rd member of the Formation (nearly 14 m). Pinghu Formation can be divided into three 3rd order depositional sequences from bottom to top, corresponding to the 5th, 4th to 3rd, and 2nd to 1st member, respectively. The overall depositional setting of tidal flat to deltaic system was developed when Pinghu Formation deposited. During the deposition of sequence I, the sedimentary environment was largely deltaic, and was controlled by the NE-SW-trending fault system. During the deposition of sequence Ⅱ, the faulting influence decreased but the tidal influence increased, and the environment was dominated by tidal flat with local tidal channels. During the deposition of sequence Ⅲ, marine regression occurred gradually, forming tidal flat and tidal-influenced deltaic systems under the background of broad and gentle slope.

Key words: Pinghu tectonic belt, Pinghu Formation, paleo-salinity, paleo-water depth, sedimentary system, sequence stratigraphy

中图分类号:

P736

蒋一鸣, 邵龙义, 李帅, 赵洪, 康世龙, 沈文超, 易琦. 西湖凹陷平湖构造带平湖组沉积体系及层序地层研究[J]. 现代地质, 2020, 34(01): 141-153.

JIANG Yiming, SHAO Longyi, LI Shuai, ZHAO Hong, KANG Shilong, SHEN Wenchao, YI Qi. Deposition System and Stratigraphy of Pinghu Formation in Pinghu Tectonic Belt, Xihu Sag[J]. Geoscience, 2020, 34(01): 141-153.

图/表 12

图1 西湖凹陷平湖构造带构造纲要图(据中海石油(中国)上海分公司)

Fig.1 Regional map of Pinghu tectonic belt in Xihu sag (modified after Shanghai Company of CNOOC)

图2 西湖凹陷综合柱状图(据武法东等[15]修改)

Fig.2 Integrated stratigraphic column of Xihu sag (modified after Wu et al.[15])

图3 西湖凹陷平湖构造带不同时期地层厚度 (a)平湖组五段;(b)平湖组三—四段;(c)平湖组一—二段

Fig.3 Maps showing the stratigraphic thickness of different periods in Pinghu tectonic belt, Xihu sag

图4 西湖凹陷平湖构造带地震剖面(剖面位置见图1)

Fig.4 Seismic sections in Pinghu tectonic belt, Xihu sag (profile location seeing in Fig.1)

图5 西湖凹陷平湖构造带A3井INPEFA曲线层序划分应用

Fig.5 Application of INPEFA curve to sequence division of Pinghu Formation in well A3 of Pinghu tectonic belt, Xihu sag

图6 西湖凹陷平湖构造带平湖组辫状河三角洲平原沉积典型照片 (a)含砾砂岩,块状构造,砾石分选、磨圆较好,定向排列,A4井,4 194 m;(b)中砂岩,块状构造,A4井,3 805.3 m;(c)含泥砾细砂岩,泥砾呈撕裂尖棱状,A3井,4 008.9 m;(d)暗灰色泥岩,4 470 m

Fig. 6 Representative photos showing characteristics of braided river deltaic plain deposits in the Pinghu Formation of the Pinghu tectonic belt, Xihu sag

图7 西湖凹陷平湖构造带平湖组三角洲沉积体系沉积特征 (a)A2井;(b)A3井

Fig.7 Depositional characteristics of the tidal-influenced deltaic system in Pinghu Formation of Pinghu tectonic belt, Xihu sag

图8 西湖凹陷平湖构造带平湖组受潮汐影响的三角洲前缘沉积典型照片 (a)细砂岩,平行层理,A3井,P7,4 012.9 m;(b)灰色细砂岩,低角度交错层理,A4井,4 192 m;(c)粉砂岩,沙纹层理,A4井,P5,3 594.8 m

Fig. 8 Representative photos showing characteristics of the tidal-influenced deltaic deposits in Pinghu Formation of Pinghu tectonic belt, Xihu sag

图9 西湖凹陷平湖构造带平湖组潮坪沉积典型照片 (a)含砾细砂岩,砾石含量较少,无明显层理,A1井,4 196 m;(b)双向交错层理细砂岩,A3井,4 015.2 m;(c)泥岩,发育透镜状层理,A1井,4 349.5 m

Fig.9 Representative photos showing characteristics of the tidal flat deposits in Pinghu Formation of Pinghu tectonic belt, Xihu sag

图10 西湖凹陷平湖构造带平湖组上段潮坪沉积体系沉积特征(A2井)

Fig.10 Columnar section showing depositional characteristics of tidal flat in the Upper Member of Pinghu Formation in the Pinghu tectonic belt, Xihu sag (well A2)

图11 西湖凹陷平湖构造带南北向平湖组连井层序地层对比剖面

Fig.11 N-S trending stratigraphic correlation of Pinghu Formation in Pinghu tectonic belt, Xihu sag

图12 西湖凹陷平湖构造带平湖组沉积相展布图 (a)层序Ⅰ沉积相分布;(b)层序Ⅱ沉积相分布;(c)层序Ⅲ沉积相分布

Fig.12 Diagrams showing the paleogeography of Pinghu Formation in Pinghu tectonic belt, Xihu sag

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