Geohazard detection using 3D seismic data to enhance offshore scientific drilling site selection

Abstract

Abstract. A geohazard assessment workflow is presented that maximizes the use of 3D seismic reflection data to improve the safety and success of offshore scientific drilling. This workflow has been implemented for International Ocean Discovery Program (IODP) Proposal 909 that aims to core seven sites with targets between 300 and 1000 m below seabed across the north-western Greenland continental shelf. This glaciated margin is a frontier petroleum province containing potential drilling hazards that must be avoided during drilling. Modern seismic interpretation techniques are used to identify, map and spatially analyse seismic features that may represent subsurface drilling hazards, such as seabed structures, faults, fluids and challenging lithologies. These hazards are compared against the spatial distribution of stratigraphic targets to guide site selection and minimize risk. The 3D seismic geohazard assessment specifically advanced the proposal by providing a more detailed and spatially extensive understanding of hazard distribution that was used to confidently select eight new site locations, abandon four others and fine-tune sites originally selected using 2D seismic data. Had several of the more challenging areas targeted by this proposal only been covered by 2D seismic data, it is likely that they would have been abandoned, restricting access to stratigraphic targets. The results informed the targeted location of an ultra-high-resolution 2D seismic survey by minimizing acquisition in unnecessary areas, saving valuable resources. With future IODP missions targeting similarly challenging frontier environments where 3D seismic data are available, this workflow provides a template for geohazard assessments that will enhance the success of future scientific drilling.

Expedition 400 Preliminary Report: NW Greenland Glaciated Margin

“…The core sites initially submitted for IODP drilling Proposal 909-Full1 were located using 2-D seismic data, including four regional seismic surveys collected by high-resolution seismic survey completed in 2019 as a collaboration between Geological Survey of Denmark and Greenland (GEUS) and Geoscience-Aarhus University using the Danish R/V Lauge Koch (LAKO) (Pearce and Knutz, 2019). Both 3-D surveys were utilized in a compilation of seabed geomorphology (Newton et al, 2017(Newton et al, , 2021 and geohazard assessments for further refining drill site locations (Cox et al, 2020(Cox et al, , 2021.…”

Progress and Prospect of Seafloor Instability Research

“…响 [41] 。地层内局部异常流体超压是海底麻坑发育的 关键，而快速沉积的欠压实沉积物、良好的沉积盖 层和流体运移通道(断层、沉积边界、不整合面和 地层薄弱面)也是不可或缺的重要条件。地层内流 体的热增压作用、活动断层作用以及频繁的地震、 潮汐和海平面变化引发的静水压力改变是海底麻坑 发育的主要触发因素 [42] 。 了解海底麻坑的起源、发育和分布是至关重要 的。海底麻坑在油气田上方大量存在，可以被用作 油气聚集的指示物。另外，通过麻坑逸散的气体可 能导致海底局部岩化并形成甲烷衍生的碳酸盐建 造，碳酸盐建造和凹陷地形可能会阻碍输油管道和 海底电缆的铺设。麻坑的存在表明该区域内海床不 稳定，因为流体渗漏会导致诸多海底失稳现象。此 外，麻坑还可以充当地下水活动、陆坡失稳和海底 不稳定性的潜在指示物，为海底滑坡稳定性监测提 供重要信息 [43] 。墨西哥湾的自升式钻井平台倾倒后 形成了一个宽 500 m、深 12 m 的凹坑，事件期间观 察到大量气体泄露 [14] ，这也表明，麻坑通常预示着 浅层气的存在，有助于浅层气井喷风险评估和海上 工程设施选址。 (五)海底蠕变 海底蠕变是在重力荷载驱动下发生的缓慢的、 持续的、长期不可逆转的下坡沉积物运动与变形， 其相间排布的地形隆起、沟槽与海底不稳定性有 关，是海底沉积物受到挤压而蠕变形成的 [44,45] 。海 底蠕变在全球诸多海域大陆坡均有记录，广泛发育 在主 / 被动大陆边缘的富黏性土海底，是陆架 -陆 坡系统的重要组成部分 [46] 。目前主要的研究区大多 集中在北半球，如以色列近海、加拿大波弗特海、 巴芬岛廷金峡湾、韩国东海、地中海、马尔马拉 海、里海、南海北部及东南部等 [45,47,48] 。海底蠕变的 形成演化与构造作用、重力作用、风暴作用以及区 域沉积作用等关系密切，记录了丰富的海洋地质 学、海洋沉积学和海盆构造演化方面的信息，对海 底地形地貌塑造、沉积物搬运和再沉积研究具有重 要意义 [45,48] 。 海底蠕变的识别特征不同于沉积物波等海底起 伏地貌(见表 2) 。在多波束测深数据中，海底蠕变 一般以隆起和沟槽交替出现的海底起伏为特征，隆 起的脊线大多与等深线近平行，连续性较好、延伸 较长且少有交叉，形态变化无明显规律 [49] 。在地震 剖面中显示隆起的两翼沉积物厚度基本相同，无上 坡迁移特征、地层内部反射同相轴被断层切断而不 连续，随地层深度增加海底起伏尺度和规模无明显 减小趋势 [45,49] 。海底蠕变多见于峡谷群和陆坡区 ， 发育地层坡度通常位于 1~7°，其形成演化受地层岩 性与坡度、高沉积速率、深部流体运移、天然气水 合物分解、断层活动、相对温和且频繁的地震以及 自身重力荷载等诸多因素影响 [45,48] 。 海底蠕变产生的海底起伏是浅层沉积物变形与 重力荷载差异分布的表征，也是海底陆坡倾于发生 破坏失稳的前奏和指示。尽管海底蠕变滑移速率很 慢(约 10 cm/a) ，但受地震、构造抬升和火山活动 等因素的影响将持续发育，最终可能演变为大规模 的海底滑坡 [53] 。南海北部东沙群岛西南陆坡海底蠕 变区紧邻天然气水合物试采区、荔湾 3-1 深水气田 等重大海洋工程活动区，海底蠕变导致的海底不稳 定性(如海底滑坡)给附近海域的海洋石油钻井平 台带来了潜在的安全风险，如利用地球物理数据和 数值模拟方法，对南海北部蠕变区海底陆坡失稳破 坏后具备诱发大规模海啸的潜力进行了分析 [57] 。因…”

Expedition 400 Scientific Prospectus: NW Greenland Glaciated Margin

“…(2) a dedicated high-resolution seismic survey completed in 2019 as a collaboration between GEUS and Geoscience-Aarhus University using the Danish R/V Lauge Koch (LAKO) (Pearce and Knutz, 2019). Both 3D surveys were utilized in a compilation of seabed geomorphology (Newton et al, 2017(Newton et al, , 2021 and geohazard assessments for further refining drill site locations (Cox et al, 2020(Cox et al, , 2021.…”

“…None of the sites involve drilling into older strata that may have petroleum potential or structural elements (closures or faults), and the density of data means that high-amplitude anomalies and chimney fea- tures can be avoided. As further mitigation of drilling hazards, a gas anomaly study was carried out based on the 3D seismic volumes (Cox et al, 2020), and the information was used in the final round of site selection (IODP drilling Proposals 909-Full2 and 909-Add).…”