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Sequence Stratigraphy and Depositional Systems Analysis of Paleocene in Lishui Sag, East China Sea Shelf Basin

Abstract

The East China Sea Shelf Basin (ECSSB) located on the continental shelf is a typical back-arc rift basin, which is divided further by a series of sag and basement ridge. Lishui sag lied along the southwest margin of the ECSSB is underlain by a productive, gas-prone, marine Paleocene section. In combination with tectono-stratigraphic analysis, sequence stratigraphy and seismic geomorphology, an integrated approach was performed to map high-frequency sequence and systems tracts by using well and three-dimensional seismic data. The approach consists of (1) the indexes of the growth faults were calculated to analyze differential activity on major faults. A relative eustatic level was reconstructed by employing accurate measurements of a series of sedimentary indicators that obtained from seismic facies analysis. According to the combination of eustatic and tectonic forces, different sequence stratigraphic models including depositional sequences I (containing lowstand, transgressive and highstand systems tract), depositional sequences IV (containing falling stage, lowstand, transgressive and highstand systems tract) and a transgressive–regressive sequence (containing transgressive and regressive systems tract) were used to fit the field observations from a particular tectonic setting since the tectonic setting and eustatic level changed in different sequence. Five third-order sequences and thirteen system tracts were recognized in the Paleocene. (2) Due to the episodic rifting and differential activity on faults, various types of transfer zones were recognized in different sedimentary period. Transfer zones controlled the direction of sediment transportation and thus have great influence on the distribution depositional systems. (3) The depositional facies in the Lower E1m formation and Upper E1l formation have been imaged on a series of strata slices. Four types of depositional systems have been recognized: a. incised valley fills; b. shelf edge slop fan; c. prograding tidal delta systems composed of elongate belts sand bodies perpendicular to the shoreline; d. onshelf barrier bar and offshelf tidal sand ridges. The distribution of sand bodies is controlled by the transfer zones indicating that the interpretation results have high reliability. Therefore, this integrated analysis improves the accuracy of petroleum plays prediction.