Salt structure characteristics of Block G in the western Central Graben, the North Sea Basin and their impact on oil-gas accumulation

A set of Permian salt rock layers is widely distributed in the western central graben of the North Sea Basin. Previous studies on salt movement in this area tended to focus on regional and mechanistic geology without in-depth study on the stages of salt tectonic activity in this area and its impact on oil-gas accumulation. Therefore, 3D seismic data, slice data, and balanced profile recovery techniques were used to study deeply the tectonic evolution, salt layer movement and its impact on sedimentary sand bodies and oil-gas migration in Block G of the western Central Graben in the North Sea Basin. Results show that: ① the Block G underwent two episodes of fault depression and three phases of structural inversion during the Triassic–Eocene. ② The salt movement lasted from the Triassic-Eocene, running through the fault depression and inversion periods, during which salt floor splitting, transition type salt rolling, and piercing type salt plants, as well as different types of traps of anticlinal traps, stratigraphic pinch-out traps, and fault block traps were formed. ③ In addition, the salt movement affected the formation of oil-gas accumulation by controlling the paleogeomorphology and oil-gas migration.The slope formed by salt movement during fault depression provided topographical conditions for the deposition of turbidite sand in the Late Jurassic, while the salt movement during structural inversion period provided structural dynamics for the recessive activities of faults and the formation of stratigraphic micro-fractures, and a geological foundation for the migration and accumulation of oil and gas.

北海盆地西部中央地堑内广泛分布一套二叠系盐岩层。以往针对该区域盐岩运动的研究多聚焦于区域地质与地质机理层面，未对该区域盐构造活动期次及其对油气成藏（oil-gas accumulation）的影响开展深入探讨。为此，本次研究以北海盆地西部中央地堑G区块为研究对象，采用三维地震数据（3D seismic data）、切片数据（slice data）及平衡剖面恢复（balanced profile recovery）技术，深入剖析了该区域的构造演化、盐岩运动过程，以及其对沉积砂体展布与油气运移（oil-gas migration）的影响。研究结果表明：① G区块在三叠纪至始新世期间经历了两期断陷作用与三期构造反转事件。② 盐岩运动持续于三叠纪至始新世，贯穿断陷与反转演化阶段，期间形成了盐层底板拆离、过渡型盐滚构造、刺穿型盐株等多种盐构造样式，以及背斜圈闭、地层尖灭圈闭、断块圈闭等不同类型的油气圈闭。③ 此外，盐岩运动通过调控古地貌与油气运移过程，对油气成藏过程产生了显著影响。断陷期盐岩运动形成的斜坡带为晚侏罗世浊积砂体（turbidite sand）的沉积提供了有利古地形条件；而构造反转期的盐岩运动则为断层隐性活动（recessive activities of faults）与地层微裂缝（stratigraphic micro-fractures）的形成提供了构造动力，同时为油气运聚成藏奠定了地质基础。