Characterization of dense zones within the Danian chalks of the Ekofisk Field, Norwegian North Sea

The Ekofisk Field is a giant field which has been producing at a high level for more than forty years and, since 1987, this production has taken place with the support of sea-water injection. The Danian-aged chalk deposits of the Ekofisk Formation and the Maastrichtian Tor Formation form the main reservoir units in the Ekofisk Field. The Ekofisk Formation principally consists of porous resedimented chalks intercalated with relatively thin and lower porosity beds, called dense zones. A multi-scale study of dense zones, from scanning electron microscopy to wells and seismic impedance data, has allowed the characterization and mapping of these deposits. Five main dense zone lithotypes have been identified: (1) argillaceous chalk; (2) chalk with abundant flint nodules; (3) chalk beds cemented with silica/nano-quartz; (4) calcite-cemented chalk; and (5) stylolitized chalk. The different types of dense zones tend to cluster in certain stratigraphic intervals, such as the EE and EM reservoir units at the base and in the middle part of the Ekofisk Formation. Dense zones have different mechanical properties compared to porous chalks and, depending on the connectivity of their fracture networks, they can act as preferential conduits or baffles for the reservoir fluids. An increased understanding of the distribution, characteristics and geological factors at the origin of the dense zones is fundamental to better define the reservoir architecture and ultimately identify unswept zones for future infill drilling targets.

埃科菲斯克油田 (Ekofisk Field) 是一座巨型油气田，已持续高产开采四十余年，且自1987年起，其开采作业均依托海水注入技术提供支撑。埃科菲斯克组 (Ekofisk Formation) 的达宁阶 (Danian) 白垩沉积与马斯特里赫特期托尔组 (Maastrichtian Tor Formation) 共同构成了该油田的核心储集层单元。埃科菲斯克组主体由多孔再沉积白垩组成，其间夹有厚度较薄、孔隙度更低的岩层，这类岩层被称为致密带 (dense zones)。研究人员通过覆盖扫描电子显微镜 (scanning electron microscopy)、测井数据与地震阻抗数据 (seismic impedance data) 的多尺度研究，实现了对这类沉积体的精细表征与空间编图。目前已识别出5类主要的致密带岩相 (lithotypes)：(1) 泥灰质白垩；(2) 富含燧石结核 (flint nodules) 的白垩；(3) 以二氧化硅/纳米石英 (silica/nano-quartz) 胶结的白垩；(4) 方解石胶结白垩 (calcite-cemented chalk)；(5) 压溶白垩 (stylolitized chalk)。不同类型的致密带往往集中赋存于特定地层段 (stratigraphic intervals) 中，例如埃科菲斯克组底部及中部的EE、EM储集层单元。与多孔白垩相比，致密带具备独特的力学特性；依据其裂缝网络的连通性差异，致密带可作为储层流体 (reservoir fluids) 的优先渗流通道或遮挡屏障。深化对致密带的分布特征、成因地质要素的认知，对于精准刻画储层构型、最终甄别未动用储层以作为未来加密钻井靶点 (infill drilling targets) 具有核心指导意义。