Abstract:

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.

Key words: Beibuwan basin, composite fan body, sequence stratigraphy, facies, significance