Mass transport deposits in deep-water minibasins: Outcropping examples from the minibasins adjacent to the Bakio salt wall (Basque Country, Northern Spain)

Highlights

• MTDs in the Bakio area are assumed to represent locally-derived MTDs sourced from adjacent salt structures.

• Seal potential for locally-derived MTDs depends on facies found on diapir roofs and at the seafloor along MTDs trajectories.

• Muddy siliciclastic-dominated MTDs tend to have relatively good seal potential.

• In sandstone clast-rich siliciclastic-dominated MTDs, the seal potential decrease away from diapirs, where they initiated.

• In carbonate-dominated MTDs, the seal potential increase away from diapirs, where they initiated.

Abstract

Recent subsurface studies show that mass-transport deposits (MTDs) in salt-controlled basins may correspond to local or regional bodies induced by either regional tectonics, or diapir growth. These MTDs are commonly considered as muddy bodies but they may alternatively incorporate a high amount of clasts and reworked beds with good reservoir properties and thus they are often challenging deposits in oil and gas exploration. The minibasins adjacent to the Bakio salt diapir, in northern Spain, provide a unique opportunity to study up to seven outcropping MTDs comparable in size to subsurface examples. Detailed structural analysis was used to reconstruct the transport direction for each MTD and to infer their source locations. In addition, facies analyses enabled the estimation of their percent of mud or matrix, allowing for a discussion on their potential reservoir and seal properties. At least six of the studied MTDs correspond to locally-derived MTDs sourced from the Bakio diapir or from the footwall of the adjacent sub-salt extensional faults. The primary trigger for these MTDs may be halokinesis, probably with contributions from other secondary processes, such as carbonate platform aggradation, high sedimentation rates and regional extension. Transport directions together with palaeoflow analysis suggests that regionally-derived turbidites flowed along the minibasin axis, while MTDs were transported laterally from the minibasin margins at high angle with the turbidity flows. We identified three types of MTDs: muddy siliciclastic-dominated MTDs, sandstone clast-rich siliciclastic-dominated MTDs and carbonate-dominated MTDs. Using this classification and subsurface analogs we propose a model of locally-derived MTDs according to the nature of the source area and the sedimentary facies reworked along the MTD downslope trajectories. This model suggests that reservoir and seal properties could be suggested for MTDs in subsurface studies by characterizing the nature of the diapir roof and the facies at the seafloor found along the MTDs trajectories.

Introduction

Mass-transport deposits (MTDs) are common gravity-induced units in deep-water environments triggered by slope failure, that include a range of facies from slides, slumps and debris flows (Ogata et al., 2014; Festa et al., 2016). MTDs linked with growing salt structures were previously documented by seismic surveys in: the Central Graben of the UK North Sea (Davison et al., 2000; Back et al., 2011; Arfai et al., 2016), the Gulf of Mexico (Tripsanas et al., 2004; Madof et al., 2009; Prather et al., 2012; Wu et al., 2020), Offshore Brazil (Gamboa et al., 2011; Jackson, 2012; Omosanya and Alves, 2013; Gamboa and Alves, 2016; Ward et al., 2018) and Offshore Angola (Gee and Gawthorpe, 2006; Doughty-Jones et al., 2019). Two type of MTDs are usually described in salt-controlled basins: regionally-derived or extra-basinal MTDs and locally-derived or intrabasinal MTDs (e.g. Madof et al., 2009; Doughty-Jones et al., 2019; Wu et al., 2020). Following the classification of Moscardelli and Wood (2008, 2016), regionally-derived MTDs, usually reach thousands of square kilometers in area and hundreds of meters in thickness, and are sourced from up-dip basin shelves or deltas. Failures of these up-dip locations, may be triggered by sea level variations, regional tectonics, or high sedimentation rates, causing the mass failure which results in the MTD (Moscardelli and Wood, 2016). Locally-derived MTDs correspond to more limited units, only reaching tens of square kilometers in area and are often sourced from margins of minibasins or localized bathymetric highs (Moscardelli and Wood, 2008, 2016). In salt controlled basins, these MTDs are assumed to be triggered by flank steepening during diapir growth (e.g. Madof et al., 2009).

Facies variability in MTDs is a key uncertainty in subsurface studies for oil and gas exploration in deep-water settings, in part because MTDs can be good seals or good reservoirs dependent on primary lithology. For example, MTDs made of reworked chalk derived from the roof of diapirs in the Central Graben, UK North Sea, can form producing reservoirs (Davison et al., 2000; Arfai et al., 2016), while MTDs in a minibasin in the Mexico Gulf are assumed to have a high content of mud and thus to have preferential sealing properties (Madof et al., 2009). Subsurface data are useful for the characterization of the length and width of MTDs and of their lateral and vertical distribution inside minibasins. However, seismic datasets fails to assess the sub-seismic scale facies heterogeneity within the remobilized deposits. These elements may affect the sealing potential of MTDs. Therefore, integrating field-analogs and subsurface data is vital for de-risking seal potential.

This study presents field-based structural and sedimentological analysis of seven MTDs exposed around the Bakio salt wall, which was growing during the Albian (Rowan et al., 2012; Ferrer et al., 2014; Poprawski et al., 2014, 2016, 2016; Roca et al., 2020), in the Basque-Cantabrian basin, northern Spain. The aims of this study are to assess the seal potential of MTDs in salt-controlled deep-water basins and to identify the genetic relationship between MTDs and coeval growing salt structures. These aims will be achieved by addressing the following objectives: (i) documenting the transport direction for each MTD exposed around Bakio, (ii) describing the nature of the clasts and folded beds, (iii) identifying their source areas and discussing their triggering mechanisms. The main findings of this study are then compared with other MTDs from salt-controlled basins worldwide.