Circular Depressions Trails in Late Neogene/Early Quarternary Deepwater Slope Deposits Offshore Angola: Implication for the Interpretation of Turbidity-Current vs. Fluid Escape Dynamics

Abstract

High-resolution seismic data have enabled geoscientists to understand geological events recorded in subsurface strata including subsurface fluid flow (SFF) and sedimentary dynamics. Both have significant implications for petroleum exploration. Recognition of SFF may give an indication of hydrocarbon prospectivity and shallow geohazards, whilst sedimentary processes largely may control the distribution and quality of reservoir and seal. As reported in previous studies, both SFF and sedimentary processes can lead to the presence of circular depressions, whose characteristics and distribution can be analyzed using seismic geomorphology. Circular depression may represent fluid expulsions from an SFF conduit onto the seabed or may be erosional features created by turbidity currents or the flows. In this study, paleochannels, trails of circular depressions, and giant crescentic bedforms (~1.5 km in diameter) along the slope were seismically interpreted on different surfaces within Neogene and Quarternary strata Offshore Angola. Neogene paleochannels extend for ~50 km in E-S direction following the paleo-topography; they are vertically stacked with a width range of 0.4 to 2 km. The trails of circular depressions are up to 12 km long, comprising up to 15 depressions, and overlie the Neogene paleochannels. The depressions range 0.2-1 km in diameter and 10 to 100 ms TWT in depth. Above the trail of circular depressions (0.8-1 km in diameter), smaller scale depressions (0.2 km in diameter) and high amplitude ‘soft’ anomalies inside them were interpreted as gas pockets. The trails of circular depressions are interpreted to form through a sequence of amplification and depositional processes of early depressions by turbiditic current dynamics. The early depressions were thought as a result of either (1) SFF and expulsions or (2) sedimentary dynamics such as cyclic step at the seafloor. The seafloor ‘defect’ was found promotes the development of the giant crescentic bedform. Understanding these circular depressions and their interaction with buried stratigraphy and sedimentary slope processes is critical to an evaluation of seals in many subsurface plays.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018