Static Modelling and Fault Seal Analysis of the Migrant Rollover Structure, Sable Subbasin, Offshore Nova Scotia, Canada

Abstract

Abstract Crestal faulting can lead to breach of trap integrity and leakage. The Migrant structure is an example of a potentially breached trap due to fault leakage and juxtaposition. In this paper we use 3D geocellular modeling, populated with new interpretation of input parameters, including shale volume, to examine the possible mechanism for leakage (crestal faulting). A fault plane profile (Allan diagram) was constructed, which can be taken a further step into dynamic modelling and simulation (not presented in this study). Located in the Sable Sub-basin, the Migrant structure is a fault controlled, four-way dip anticlinal closure, which formed as one of a series of related structures during rift basin extension, sediment loading and salt mobilization in the Cretaceous. Genetically related rollover structures (e.g., the Distal Thebaud Field) in a similar structural and stratigraphic setting have proved viable as a commercial trap. The Migrant N-20 well was drilled to test for hydrocarbons trapped in Late Jurassic to Early Cretaceous deltaic and fluvial-deltaic reservoirs in the structure. The well encountered gas from a deep sand reservoir during drill stem testing (DST 2) with a reported flow rate of 10 million standard cubic feet per day. However, over the duration of the test, an associated decline in flow rate and pressure depletion was observed, which led the operators to consider the target reservoir as non-commercial. In this paper we present a re-appraisal to assess why this trap failed by integrating well data (logs, checkshot and pressure) and 3D seismic to produce a static model demonstrating the trapping mechanism in the Migrant Structure. Initial observation of the 3D seismic shows shallow crestal faults while preliminary observation of well logs from the Migrant N-20 well suggests a diminishing sand/shale ratio from the shallow to deep sections of the trap. This study of the Migrant Structure contributes to the understanding of the relationship between reservoir and seal thicknesses relative to fault displacement and its role in subsurface fluid trapping or cross-fault leakage, through upward and outward displacement (stair-stepping) between reservoirs of different ages across a given fault. The paper shows how data integration and workflows have been combined effectively and is an important contribution for risk assessment in the Sable Subbasin. The proposed model can be applied in other basins including the similar salt cored basins like those offshore Brazil.