3D Seismic and AUV Data Integration for Deepwater Geohazard Assessment: Application to Offshore Northwest Borneo, Brunei

Abstract

A 3D seismic and AUV data integration has been performed for geohazard assessment at an early stage of a field development. The study area is located on the South China Sea, approximately 100 km northwest of the coast of Brunei Darussalam margin characterized by a delta toe fold-thrust belt. Elongated mini-basins are separated by shale-cored ridges created by basin-directed thrusting. The continental slope is characterized steep slopes and affected by large channels, and mass transport deposits (MTD). A regional geohazard assessment was performed on the 3D exploration seismic data highlighting the presence of numerous seabed an sub-seabed features potentially affecting E&P activity. In order to better assess the possible impact of these geohazards on the field development, a detailed AUV survey has been performed on target areas. The study area ranges from 830 m to 2050 m of water depth and is characterized by seabed gradient up to 45°. Several events of buried MTD including more cohesive blocks which result in the hummocky signature on the present-day seabed are observed. The presence of such MTDs may induce vertical and lateral soil variability that may impact both drilling operations or pile/anchor installation and flowlines installation passing through the hummocky seabed. In addition, the slopes are affected by recent instability processes covering 2 km2 to 10 km2. Larger slide scarps with an amphitheatre-like source area are observed and appear to be created by various retrogressive events. The slides initiate on various slope angles, from 2° to 16°, and measured run-out distances show a large range of values, from hundreds to thousands of metres. Evidences of seafloor expulsion features (craters, pockmarks), shallow gas, gas hydrates are also observed within the study area. The 3D seismic dataset has been used to identify at a regional scale particular geohazards areas requiring a more detailed investigation via AUV at an engineering scale. Such engineering scale allows producing favorability maps created from selection criteria and allowing mitigation strategies for safe E&P activities.

A GIS-based decision-support approach to deepwater drilling-hazard maps

“…Similar but complementary approaches have been taken to identify and avoid geohazards or other conditions that might have an adverse effect on deepwater oil and gas well drilling, subsea development, and pipeline route determination (e.g., Campbell, 1999;Haneberg et al, 2013;Dan et al, 2014). The methods used to create the maps range from simple summation of the number of attributes mapped or inferred at different locations to more sophisticated techniques such as neural networks and logistic regression (Lu and An, 1999;Harris et al, 2003).…”

“…Although rule-based approaches such as the one described by Dan et al (2014) consider some combinations of hazards, the number of possible combinations of hazards and their implications can quickly become unmanageably large. In such cases, formal procedures such as multiple-attribute utility theory (MAUT) adapted from multicriteria decision-support literature offer a promising solution (e.g., Walls, 1995;Suslick et al, 2009;Munier, 2011).…”

“…Areas with gentle slopes (< 5° and beyond the steep slope buffers) but underlain by mass-transport deposits (MTD) and small faults were classified as yellow. Figure 1 shows the resulting maps from Dan et al (2014) for seafloor geohazards, shallow-subsurface geohazards, and favorability.…”

“…Campbell (1999) did not, however, propose a specific workflow for weighting and combining different kinds of geohazard information to create the favorability map. Dan et al (2014) illustrate in greater detail the preparation of contemporary early-stage favorability maps based on seafloor and shallow-subsurface geohazard maps for a deepwater development off the coast of Brunei, in this case based on 3D exploration seismic data supplemented by higher-resolution geophysical surveys by autonomous underwater vehicles (AUV). The authors used a rule-based system to classify areas as yellow (most favorable), orange, and red (least favorable).…”

“…We define slope angles less than 6° to present no hazard and thus assign them a score of zero. Following Dan et al (2014), we define slope angles greater than 12° to be completely unacceptable for exploration and production activities. As such, favorability of the unacceptably steep slopes is undefined because they are off limits regardless of how much steeper than 12° they are or whether the impact of their steepness is compounded by the presence of other hazards.…”