基于过程的分流平原高弯河道砂体储层内部建筑结构分析——以大庆油田萨北地区为例

Reservoir Architectural Analysis of Meandering Channel Sandstone in the Delta Plain Based on the Depositional Process

河道砂体内部结构已成为影响剩余油分布的决定因素。本研究旨在弄清三角洲平原高弯曲河道砂体内部结构,指导开发调整。解剖分为复合河道、单一河道、基本结构要素及要素内部结构等四个层次。基于三角洲平原上高弯河流的形成过程,依据河泛沉积分布及河道沉积体内部沉积特征识别单一河道。依据砂体厚度分布及砂体测井响应所体现的砂体所处的河道位置识别最终废弃河道,依据测井响应特征识别中间废弃河道,通过中间废弃河道及最终废弃河道位置关系,分析河道摆动特征,追踪河道迁移演化。按照河道演化过程识别点坝及其平面展布。在点坝内部通过岩芯、对子井分析确定侧积体的垂向厚度、倾向、倾角,进而获取侧积体平面发育频率,建立点坝内部的结构模型。研究基于河流演化过程,将河流过程与沉积结果统一起来,从过程角度解释了砂体形成,使解剖结果更合理更准确,可很好地指导剩余油预测分析。

Reservoir in Daqing Oilfield,the largest oil field in china,is mainly formed in the huge fluvial-delta system,with hign heterogeneities.Distributary channel and channel sandstone are key sandbodies for hydrocarbon accumulation.After more than 40 years of high speed development,it is now in matural stage with a very high water cut.A good understanding of remaining oil and bypassed oil,which are governed by the baffles and barriers,will be a key work for improving the recovery,while reservoir architectures will give a clue for the barriers and baffles prediction and the oil compartment identification. Architecture analysis method,widely used for the outcrop analysis,is a good method to understanding the architecture of fluvial deposit.Although some attempts have been tried for the subsurface reservoir analysis and resulting some progress of this method,there are still many difficulties in the subsurface architecture description.Especially it is hard to give a good explanation for the deposition process of the reservoir for this method focusing on the reorganization and distribution of the elements and not enough attention were paid for the process,which made it a little difficult for geologist to understand the architecture result. Here a new method for meandering channel sandstone architecture analysis was provided in the density well spacing area.It is hard to reconstruct the process of the highly cut super-complex sandstone,but it is possible to reconstruct the depositing process of moderately cut sandstone and a good explanation can be made for the relationship of the sandbodies in the channel framework.If the process is correct then the architecture will be more reliable. There is four hierarchies of architecture were reconstructed in this method,including composite channel belts,single channel belt,point bar and lateral accretion.The composite channel belt was reconstructed based on the widely spread flooding plain deposit between channel belts in the vertical profile.The flooding deposit maybe eroded by the later channel.The main work is to separate the deposit in vertical dimension and to reconstruct a detailed framefork.Single channel belt will bounded based on the flooding plain deposit between channel belts on the areal dimension.Sometimes the distributary channel had to be separated by the sand distribution. Point bar can be recognized on well logging,but it is difficult to make a correlation to get the 3 dimension of it.So it will be reconstructed based on the swing process of the channel.First the final abandoned position for the channel was inferred and abandoned cannels during the swing process were recognized based on the sand thickness and the well logging shape of each well and their areal combination.Then the channel swing direction and part of the channel shape can be guessed.Finally the original place for the channel would be induced based on the swing direction and the sand thickness before the first abandoned channel.After all of these works,the channel belt could be separated to several stage and in each stage there were several point bars. Lateral accretion architecture is reconstructed based on distribution of the areal density of accretion shale which can be quantified through its relation to its density and the dip in vertical profile.It is easy to get vertical accretion shale in core or well logging.The dip can be calculated based on the core or very densely spaced well which were drilled on the same well site. Two typical layer formed in the delta plain in North S3 block were select to reconstruct the process of the deposit and the detailed reservoir architecture.There are 38 and 23 point bars were recognized in the two layers each.And the dip of the accretion shale is between 4~10 degree and the areal space among shales is 16~20 meters.Combining these data and the shale distribution in the well,the architecture of the lateral accretion were constructed which gives a good understanding of the detailed architecture of the reservoir channel sandstone in the delta plain.