Sequence stratigraphy of Paleocene system in Weizhou Formation of Beibuwan basin can be divided into 3 extra sequences and 9 Ⅲ-order sequences

a composite fan is largely developed in lowstand system tracts of extra sequence I

Ⅱ and Ⅲ in the low part of Weizhou Formation. Rocks with coarse lithology are found namely

massive miscellaneous sandstone

conglomerate bearing sandstone with graded bedding

moderate coarse sandstone

massive bedding sandstone

sandstone with cross bedding

siltstone with wave bedding or deformed bedding

mudstone with horizontal lamination bedding and typical turbidite etc. The composite fan

as for its distribution and form

was formed due to cross-superimposition of leafy body which chiefly originated from braided channel and gravity flow waterway during lowstand sedimentation and has double original features of traction current and gravity flow. The fan body was formed by gradually prograding of clastic sediments from the slope of lake edge to the lacustrine basin

produced from downwards to upwards a succession of sediment namely

distal fan

medium-part fan and near-end fan. Curves of well-logging show as fingerlike or tooth-like

toothed box-like and bell-like with high and medium amplitude and seismic reflection is a hill-like protruding. The composite fan surrounded by hydrocarbon- producing lacustrine mudstones is a favorable assembly for hydrocarbon origin

reserving and capping

with quite good physical properties

where stratigraphic traps or structure-stratigraphic composite trap hydrocarbon reservoirs are commonly formed.