Abstract:

Subaqueous distributary channel systems in fan-delta front settings are characterized by non-stationary evolutionary dynamics. Their multi-stage migration and stacking significantly enhance reservoir heterogeneity, thereby increasing the uncertainty of reservoir prediction. To address this issue, this study utilizes a dense well pattern dataset from the upper member of the Jiufotang Formation in the Bao 1 block. Integrated core, logging, and seismic data are used to conduct a detailed single-sand-body architectural analysis, aiming to quantitatively characterize channel evolution and identify its controlling factors. The results indicate that: The study area develops four sedimentary microfacies, including subaqueous distributary channels, interdistributary bays, mouth bars, and distal sheet sands. Channel systems can be classified into three architectural styles: camalgamated, juxtaposed, and isolated patterns. Subaqueous distributary channels evolve from a concentrated stage to a bifurcation-dominated stage and further to a dispersed wandering stage. During this process, channel count increases from 23 to 36, the proportion of trunk channels decreases from 39.1% to 13.9%, while that of secondary channels increases from 60.9% to 86.1%. The bifurcation stage exhibits the highest width-to-thickness ratio of trunk channels, indicating enhanced lateral migration capacity. Channel evolution is jointly controlled by sustained transgression, reduced substrate gradient, and variable hydrodynamic energy; secondary channel width decreases from 64.6 m to 40.7 m, while the coefficient of variation of width-to-thickness ratio increases from 0.58 to 0.86 for the main channel and decreases from 0.49 to 0.30 for the secondary channel, reflecting differential hydrodynamic responses among channel hierarchies. Sand-body architecture evolves from stacked to laterally connected and isolated patterns, with a progressive reduction in connectivity, forming a retrogradational fan-delta architectural framework. Compared with conventional well-pattern datasets, the dense well pattern shows a significantly improved match between well spacing and architectural element scale, with S/W ratios of 0.75 for trunk channels and 2.31 for secondary channels, providing stronger constraints on channel boundaries and spatial distribution.

Key words: fan-delta front, subaqueous distributary channels, evolutionary processes, controlling factors, single sand-body architecture, dense well pattern

摘要：

扇三角洲前缘水下分流河道具有非稳态演化特征，其多期迁移与叠置增强了储层非均质性，进而加大了储层预测的难度。针对这一问题，本文依托包1块九佛堂组上段密井网资料，基于岩心、测井及地震资料，开展单砂体构型研究，旨在定量分析水下分流河道演化过程并揭示其控制因素。研究表明，该区块发育水下分流河道、分流间湾、河口坝及前缘席状砂4种沉积微相，水下分流河道分为近连片型、接触型及单一型3种组合形式。水下分流河道由集中发育阶段向分叉增生、弥散游荡阶段演化，该过程中河道数量由23条增至36条，主干河道占比由39.1%降至13.9%，次级河道占比由60.9%升至86.1%。分叉增生阶段主干河道宽厚比最高，河道侧向迁移能力较强。河道演化受持续水进、下伏地形坡度趋缓及水流能量变化共同控制，次级河道宽度由64.6 m降至40.7 m；主干河道宽厚比变异系数由0.58升至0.86、次级河道由0.49降至0.30，反映不同级别河道对水动力扰动的差异化响应。砂体空间分布由叠置型向搭接型、孤立型转变，连通性逐步降低，构成退积型扇三角洲构型模式。相较常规井网而言，密井网井距与构型单元尺度的匹配程度提高，主干河道S/W为0.75，次级河道S/W为2.31，对单一河道边界和空间展布具有更强的约束能力。

关键词: 扇三角洲前缘, 水下分流河道, 演化过程, 控制因素, 单砂体构型, 密井网