三角洲河口区沉积微相演化特征及成因分析——以鄂尔多斯盆地樊学地区长4+5油层组为例

Analysis on evolution and genesis of sedimentary microfacies in delta estuary:an example from Chang 4+5 pay zone in Fanxue area,Ordos Basin

前人对鄂尔多斯盆地延长组三角洲河口区沉积微相的类型、演化规律以及成因机理存在不同认识。依据岩心、测井等资料,对鄂尔多斯盆地樊学地区长4+5油层组沉积微相特征进行系统研究,并分析其成因机制,取得如下认识:(1)研究区沉积微相的主要类型为分流河道、河口坝和分流间湾;(2)在重点小层沉积微相展布特征研究的基础上,建立了樊学地区河坝共存的沉积模式,长4+5油层组沉积早期发育薄层毯状河口坝+窄带状分流河道,长4+5油层组沉积晚期发育厚层土豆状河口坝+宽带状分流河道;(3)基于密集井网资料,对河口坝砂体的空间形态进行了半定量统计;(4)分析了控制河口区沉积微相演化的主要因素,认为受控于沉积基准面的不断下降,分流河道的分叉、聚合、侧向迁移等自旋回作用更为强烈,限制了河口坝的侧向生长,河口坝与分流河道具有此消彼长的关系;(5)河口坝砂体是三角洲前缘的优势储层,应该引起更多的重视。

Previous understandings on types, evolution rules and formation mechanisms of the Yanchang Formation microfacies in delta estuary of the Ordos Basin are different. In this paper, a systematic study was conducted on microfacies of Chang 4＋5 pay zone in Fanxue area of the Ordos Basin and its formation mechanisms based on core and well logging data. The study provides the following conclusions. First, the study area is dominated by distributary channel, mouth bar and interdistributary bay in terms of microfacies. Second, the sedimentary model for the Fuxue area with co-existence of rivers and bars is established according to the microfacies distribution in key layers, and thin blanket mouth bars and narrow zonal distributary channels were mainly developed during the early sedimentation stage of Chang 4＋5 pay zone, while thick bulk mouth bars and wide zonal distributary channels during the late sedimentation stage. Third, a semi-quantitatively statistical analysis was carried out on the spatial forms of mouth bars using the data of dense well pattern. Fourth, it is concluded after analysis on main controlling factors for evolution of sedimentary microfacies that, due to the sustained failing of the sedimentary base level, the autogenetic cycle processes of distributary channels, such as bifurcation, convergence and lateral migration, were intensive to constrict the lateral growth of mouth bars, thereby resulting in a reciprocal relationship between mouth bars and distributary channels. Filth, more attentions should be made to the mouth bar sand bodes which are predominant reservoir beds in the delta front.