北部湾盆地涠西南凹陷WZ油田古近系流沙港组一段沉积相

Sedimentary facies of the Member 1 of Paleogene Liushagang Formation of WZ Oilfield in Weixinan Sag,Beibuwan Basin

北部湾盆地涠西南凹陷WZ油田古近系流沙港组一段是主要含油层系,实钻表明储集层成因复杂、厚度变化大、非均质性强,致使砂体分布预测困难。通过精细的岩心观察,结合各类测井、录井及岩心分析化验资料综合研究,认为流一段属陡坡型扇三角洲沉积,主要发育扇三角洲前缘亚相,其中夹杂有丰富的重力流沉积。扇三角洲前缘亚相主要由水下分流河道、河口坝、远砂坝和河道间沉积微相组成;重力流沉积包括重力滑塌沉积、碎屑流沉积、浊流沉积及液化流沉积,规模均较小。在深入的单井沉积微相分析和多井对比基础上,编制了流一段主力Ⅳ油组和Ⅱ油组的沉积微相平面图,展示出研究区总体沉积格局为稳定的扇三角洲前缘环境,物源充分,主要来自北部和西北部,因斜坡带较陡,在扇三角洲前缘至前扇三角洲环境发育丰富的各类重力流沉积。根据各沉积微相的特征与展布,建立了本区较深水陡坡型扇三角洲沉积模式。分析了不同微相砂体的物性与含油性,认为扇三角洲前缘水下分流河道砂体最好,其次是河口坝砂体,远砂坝和远端浊积砂体相对较差。提出了有利的挖潜区域和调整井位,实钻遇油层数十米,为油田增储上产发挥了重要作用。

Abstract The Member 1 of Paleogene Liushagang Formation is the major oil-bearing formation of WZ Oilfield in Weixinan Sag of Beibuwan Basin. The actual drilling results demonstrated its complex ori- gin types, obvious change of sandbody thickness and strong heterogeneity of the reservoir which resulted that not only forecast of sandbody distribution is very difficult, but also the development result is greatly restricted. Sedimentary facies of the Member 1 of Liushagang Formation of WZ Oilfield is finely studied based on cores observation and analysis, in combination with the data of well logging and masterlog, the results show that it is dominated by steep slope fan delta of fault-depressional lacustrine basin, which is mainly the fan delta front subfacies containing abundant gravity flow sediments. The fan delta front subfa- ties can be subdivided into subaqueous distributary channel, mouth bar, distant bar and interchannel mi- crofacies. The gravity flow sediments include gravity slump, debris flow, turbidity flow and fluidized flow, which are all generally small scale. The sedimentary microfacies maps of the key oil units of IV in- terwll and II interval of the Member I of Liushagang Formation are made by ingoing interpretation of the sedimentary microfacies on a single-well section and fine multi-well correlation. According to the maps the whole sedimentary framework of the area is a stable fan dela front and the provenances are from north and northwest. The gravity flow deposits are common from fan delta front to prodelta areas because of steep slope. Moreover, the depositional model of deeper water of steep slope fan delta is established based on characteristics and distribution of microfacies. Analysis of physical property and oil potential of the differ- ent microfacies sandbody shows that the subaqueous distributary channel is the best, mouth bar is better, distant bar and distant turbidite are bad. A favorable development area and an adjustment well are pro- posed which achieves several dozens of meter oil beds and play an important role in production and increas- ing reserves.