Quantitative analysis of the structural evolution of salt diapirs and their impact on sediment routing and the architecture of deep-water channel reservoirs, Venus field offshore Angola

Abstract

The interaction between deep-water channels and topography on the slope due to faulting, folding or diapirism is well known. However, most studies show only qualitative observations on the development of the structures. In the Block 31 area, offshore Angola, previous studies have established a chronostratigraphic framework where ten regional horizons can be assigned absolute ages from 89 Ma to 9.7 Ma utilizing biostratigraphic markers correlated to a global chronostratigraphic framework. This study focusses on the area around the Venus structure in Block 31 and over the 51-13.3 Ma time interval. The Venus structure is a salt-cored anticline with a syncline and a further salt-cored anticline to the east.Shortening analysis has been performed on 15 dip lines across the Venus structure for each of the ten horizons. During the focus interval, the Venus structure is divided into three areas; the crest of the structure which has a cumulative shortening of 680 m and interval shortening rates between 10 and 25 m/Myr, the northern low which has cumulative shortening of 180 m and interval shortening rates of 2–8 m/Myr and the southern low which has cumulative shortening of 450 m and interval shortening of 5–18 m/Myr. Over the same intervals eight main channel systems are identified (CS10-80). Each of the channels has been imaged on amplitude maps from spectral decomposition processed seismic data. The channel systems enter the study area from the east and take three routes down-dip; passing to the SW along the syncline between the two salt-cored anticlines or crossing the Venus structure through the northern of southern lows.Integrating the shortening analysis and the channel system routes shows that the channel systems are only able to cross the Venus structure through the southern and northern lows when the interval shortening rates in those areas are less than 10 m/Myr. At other times the channels are directed to the southwest along the synclinal axis. Off-set stacking of subsequent channel systems can be observed both at the entry point of the systems and through the northern and southern lows. This may be due to more subtle shifts in structurally controlled sea-floor topography or could also be due to stratigraphic compensational stacking. Within the CS20 system the sinuosity of higher order channel complexes and elements appear to be controlled by crestal faults extending into the southern low.