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Sedimentological Signatures of Paleogene in Lishui Sag, East China Sea Shelf Basin

Abstract

Sedimentological signatures of Paleogene in Lishui Sag, East China Sea Shelf Basin

Zhongqiang Sun1, Jinliang Zhang1, Jinshui Liu2, Wenlong Shen2, Yang Li1

1 Faculty of Geographical Science, Beijing Normal University,

2 Shanghai Branch of CNOOC, Shanghai, China.

The East China Sea Shelf Basin (ECSSB) is a typical back-arc rift basin and is divided further by a series of sags and basement ridges. The Lishui Sag is on the southwest margin of the ECSSB. Analysis of sedimentological signatures is based on the study of 712 feet of Paleogene cores derived from seven wells displaying a wide variety of lithofacies, sedimentary structures and ichnological data in combination with 2D and 3D seismic data of 5000km2 area and well logging curves of 18 exploratory wells in the Lishui-Jiaojiang Field. Thirteen facies are defined and grouped into three categories (wave-influenced delta, fan delta and basin facies). Wave-dominated deltaic deposits occur mostly in the west of Lishui sag, fluvial discharge in the delta front and prodelta was repeatedly reworked by wave and formed some isolated bars and these deposits are unbioturbated to sparsely bioturbated and contain some ichnotaxa that typically do not occur in brackish water. Fan deltaic deposits are present in the southwest and northeast of Lishui sag, it is divided further into fan delta front and distal fan delta. The subaqueous fan delta front includes distributary channels and wave-influenced interdistributary bays. Further seaward, successions are characterized by terminal distributary-channel and sand sheet deposits. These deposits are the intensely bioturbated. Basin facies not only record high-energy wave ravinement and transition to deeper-water deposits with highly variable bioturbation but also record low-energy, suspension sediment fall-out deposition in the absence of waves and currents, bioturbation is typically absent. From sedimentological signatures perspective, four main depositional settings are identified: wave-dominated delta, fan delta front, distal fan delta and basin facies. Thirteen facies are defined further: foreshore, upper shoreface, wave-dominated lower to middle shoreface, weakly wave-affected middle to lower shoreface, offshore transition, upper offshore, lower offshore, wave-dominated proximal delta front, wave-dominated distal delta front, storm-influenced proximal prodelta, storm-influenced distal prodelta, transgressive deposits and shelf facies.