Assessment of fault activity at offshore carbon storage prospects off northwestern Taiwan

The offshore area of northwestern Taiwan is considered a potential site for carbon capture and storage (CCS). However, the containment integrity—particularly the risk of faults breaching the reservoir–seal system—remains insufficiently constrained. Based on stratigraphic and sedimentological investigations, the Late Miocene Nanchuang and Pliocene Kueichulin formations are identified as the primary reservoir units, effectively sealed by the overlying thick Late Pliocene Chingshui Shale. This study aims to analyze the variation trends of fault displacement and reconstruct the temporal evolution of regional faulting, thereby assessing the resultant seal integrity. We integrated 48 multichannel seismic profiles with data from one exploration well. Fault geometries were delineated by identifying high-density contour zones on isopach maps and verifying them against seismic reflection characteristics. A total of 16 normal faults were identified. Quantitative analysis reveals that the maximum vertical displacements observed within the Chingshui Shale, Kueichulin Formation, and Nanchuang Formation are approximately 0.025 s (~40 m), 0.022 s (~35 m), and 0.020 s (~30 m), respectively. Crucially, the maximum displacement within the regional seal (~40 m) is consistently smaller than the shale’s thickness defined at the well location (~81 m). The results indicate that fault activity was episodic and closely linked to the regional tectonic framework. The initial phase of elevated activity occurred during the Late Oligocene to Early Miocene (Mushan, Daliao, and Shihdi stages), associated with rapid subsidence during the post-rift phase. A subsequent phase of reactivation was observed during the Pliocene to Late Pliocene (Kueichulin and Chingshui Shale stages), corresponding to the foreland basin flexure. Consequently, the insufficient fault throws and the long-term decreasing activity trend imply that seal integrity is preserved, supporting the feasibility of the study area for CCS.

Keywords: offshore northwestern Taiwan, fault activity, multichannel seismic (MCS), carbon capture and storage (CCS), seal integrity.