北部湾盆地涠洲组复合扇体特征及油气勘探意义

北部湾盆地下第三系涠洲组的层序地层可分为 3个超层序和 9个三级层序,复合扇主要发育于涠洲组下部超层序 1的层序 2和层序 3低水位体系域。岩性较粗,主要有块状混杂砂砾岩,具递变层理的含砾砂岩及中粗砂岩、块状层理砂岩、具交错层理的砂岩、具波状或变形层理的粉砂岩及水平纹理的泥质岩、典型浊积岩等岩相。在分布上和形态上主要由低水位沉积期重力流水道、辫状河道成因形成的扇朵叶体互相叠置而成,具有牵引流及重力流成因的双重特点。扇体由碎屑沉积物自湖边缘斜坡向湖盆逐步进积,自下而上形成远端扇、中部扇和近端扇的沉积序列。测井曲线为高、中幅指形或齿形及齿化箱形和钟形,地震反射呈丘状凸起。复合扇处于湖相生烃泥质岩的包围之中,具有非常有利的生、储、盖组合,且物性较好,常形成地层圈闭或构造地层复合圈闭油气藏。

CHARACTERISTICS OF COMPOSITE FAN BODIES IN WEIZHOU FORMATION OF BEIBUWAN BASIN AND OIL & GAS PROSPECTING SIGNIFICANCE

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.