Deposition System and Stratigraphy of Pinghu Formation in Pinghu Tectonic Belt, Xihu Sag

doi:10.19657/j.geoscience.1000-8527.2020.002

Abstract

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

CLC Number:

P736

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.

Figures/Tables 12

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

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

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

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

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

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

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

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

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

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)

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

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

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