Evolution of Fault-Related Folds in the Contractional Toe of the Deepwater Niger Delta

Scot W. Krueger¹ and Neil T. Grant^2
1 ConocoPhillips, Houston, TX
² ConocoPhillips (UK) Ltd, Aberdeen, United Kingdom

The toe thrust belt of the deepwater Niger Delta is an ideal natural laboratory for studying the evolution of contractional folds. The belt typically consists of an in-sequence set of 10 to 20 fault-fold structures. While the majority of the structures are forward verging, there are local domains dominated by backthrusting and a frontal wedge. The structures are predominantly fault-propagation folds, with hints of initial break thrusting out of early low-relief buckles. Detailed reconstructions of the western belt suggest that the timing of progressive initiation of motion on the basal detachment is strongly linked to the timing of onset of highly elevated fluid pressures near the fracture gradient. These elevated pressures are driven by disequilibrium undercompaction in response to rapid burial by the advancing delta.

Detailed analysis of the growth history of several fault-propagation folds suggests that once initiated they propagate rapidly to near their ultimate length. Subsequent deformation increases the fault slip and structural relief without significant increase in length. In the waning stages of growth, deformation frequently retreats to localized deformation near the structural culminations, while the extremities are successively abandoned. It is proposed that this pattern of growth is being driven by lateral pressure transfer (centroid effects) within the interbedded sands. Pressures are elevated above the background shale pressure near the structural crest and suppressed at depth. This has the mechanical effect of weakening the shales near the top of the fold and strengthening the shales near the tip line of the fault. Such a driving mechanism could explain both the increasing resistance to lateral tip propagation after initial rapid growth and the late retreat of deformation to the crest of the structures.