THE NATURE OF MASS FAILURE ELEMENTS IN ASSOCIATION WITH SALT DIAPIRS: A CASE STUDY IN THE THUNDERHORSE AREA, GULF OF MEXICO SALT PROVINCE

Abstract

This study investigates the geometrical distribution, petrophysical characteristics, and block geometry of submarine mass failures in the salt minibasin of the Thunderhorse field, located in the northern Gulf of Mexico. By using 3D seismic data that covers an area of approximately 1,224 km² and well logs at shallow depths of approximately 1.2 km, ten mass failures are identified to assess sealing quality and reduce drilling risks within these failures. Our interpretations reveal strong evidence of three distinct types of failures. Type 1 failures originate from the diapir roof, they are characterized by muddy, low-amplitude chaotic facies and are classified as basin-sourced failures (MFs 3, 4, 7, 8, 9, and 10). Type 2 failures are larger than type 1 and consists of two distinct zones, a basal shear zone and an upper zone characterized by high reflection and consists of a series of pop-up thrust failure structures. The lower zone has low amplitude chaotic facies and is sandy in nature. Type 2 failures are driven from the slope to the basin. Type 3 failures are derived from the shelf or from the delta and are not affected by the mini-basin topography. This type is larger failures and covers all the study area. Lithology in type 1 failures are predominantly muddy, Type 2 are sandy at the base with muddy upper layers, and Type 3 are primarily sandy with some muddy intervals. Petrophysical logs show that density and velocity generally increase with depth. Mud-rich mass failures exhibit slightly higher velocity increases compared to background sediment and sand-mud failures, whereas sand-rich mass failures display lower densities than the background, except in the basal shear zone of Type 2 failures, which show a sandy composition with higher observed velocities and densities. The blocks within these failures are closely related to flow size; the smallest are derived from within the mini-basin (Type 1), and larger blocks are sourced from the slope to the basin (Type 2). These blocks pose drilling challenges due to varying pressures and lithologies compared to the surrounding matrix of mass failures. Furthermore, these blocks can create pathways for fluid migration, posing a significant risk of sealing failure.