现代缓坡三角洲沉积模式--以鄱阳湖赣江三角洲为例

在我国，缓坡三角洲形成了不少大油田。关于缓坡三角洲的沉积模式，前人做了大量研究，但大都针对古代地层。由于古代地层精细对比和沉积环境解释常常因人而异，因此总结出来的沉积模式五花八门，可靠性较低。鉴于此，本文从现代实例入手，选择了鄱阳湖赣江三角洲，采用野外实地测量、探槽、浅钻孔、探地雷达、粒度分析等方法，对其沉积特征进行系统研究，建立了缓坡三角洲沉积模式。本次研究主要取得了如下重要认识:①在三角洲平原上发现了两种新的砂体类型，即汊口滩和并口滩，分别位于分流河道分汊处和合并处，这意味着分流河道的分汊、合并部位也是有利的油气聚集部位；②在分流河道砂体中存在连片披覆型、孤立充填型、孤立残留型三种类型的落淤层，并提出了其形成机制；③统计了分流河道的定量参数，提出了分流河道宽度的变化规律，即从上三角洲平原到下三角洲平原，分流河道逐渐变窄，分一次分汊其宽度平均降低1/2，而且分汊频率增高，数量变多，弯度降低，沉积物变细变薄，砂体间连通性变差；④三角洲前缘不发育水下分流河道，而是发育河口坝和席状滩；⑤由于坡度平缓，湖平面小幅度变化会引起湖岸线大规模进退，可导致水下与水上沉积频繁交互，因此缓坡三角洲沉积在地震上不会有叠瓦状或反S形前积反射，而是呈平行或亚平行反射。

Depositional Model of Modern Gentle-slope Delta: A case study from Ganjiang Delta in Poyang Lake

In China, gentle-slope deltas have formed lots of  large oilfields. As to the depositional model of gentle-slope deltas, numerous  studies have been made, but most of them were on ancient strata. As different researchers may give out different interpretation about stratigraphic correlation and sedimentary environments, the depositional models summaried are various, and their reliability is low. Therefore, in this paper a modern gentle-slope delta-Gangjiang Delta in Poyang Lake-was studied systematically, through methods such as field observation and measurement,  troughs, shallow boreholes, radar, grainsize analysis, etc. And the depositional model of gentle-slope delta was established. Following important conclusions were drawn through this study:① Two new types of sandbodies were found on the delta plain, i.e. branching-point bar and combining-point bar, the former occurs at the branching point of a distributary channel, and the latter occurs at the combining point of two distributary channels. This implies that branching points and combining points are also favorable place for oil and gas accumulation. ② Within channel sandbodies, three types of mud drapes are found, i.e. sheet drape, isolated drape and residual drape, and their origin was interpreted. ③ Statistics of quantitative parameters of distributary channels were made, and it was found that in downstream direction the channels become narrower as branching increases, and the width decrease about 1/2 after each branching; channel branching frequency increases; channel sinuosity decreases; channel sediment becomes finer and thinner, and sandbody interconnection degree decreases. ④ In delta front, there are no underwater distributary channels, and only mouth bars and sand sheet are developed. ⑤ Due to gentle slope, small fluctuation of lake level may cause lagre scale shifting of coastline, and may result in frequent alternation of subaerial and underwater deposits. In seismic reflection profile, gentle-slope delta deposits will not show imbricated reflection, and only show parallel or subparallel reflection.