Abstract

The Pab turbidite system was deposited along the Indo-Pakistani passive margin during the late Maastrichtian. The turbidite system (Lower Pab Formation) is the lowtand system tract of a third-order sequence, deposited while an unconformity developed on the shelf. The basin-floor fan was composed of at least three stacked channel complexes separated by extensive hemipelagite units. The fan was fed by a canyon incised into slope deposits. The reservoir geometry of the uppermost of these channel complexes has been studied along exceptional exposures in a 6-km (3.6-mi)-long continuous cliff, in a midfan setting. The cliff is roughly oriented in the dip direction of turbidite transport, and several side gullies that locally cut the main cliff provide nearly strike-oriented sections of the channel complex. The reservoir architecture was reconstructed in three dimensions using detailed interpretations of the outcrop photopanels, logs of 11 sedimentological sections, and the physical correlation of the main surfaces in outcrops and line drawings on photomozaics.

The channel complex consisted of several amalgamated turbidite channels. Single channels were 800–1500 m (2624–4920 ft) wide and 20–70 m (65–230 ft) thick. Each base corresponded to a simple erosion surface, which was then draped by various bypass deposits. During the erosional stage of the single channel, levees and crevasse lobes were deposited laterally to the main channel axis. The single channel was then filled by massive to laminated sandstone beds during the fill stage, and spill-over lobes capped the channel during the spill stage. Finally, the channel was abandoned and draped with muddy turbidites interbedded with hemipelagites. This spatial organization suggests erosion and then backstepping dynamics for single channel growth.

Single channels were amalgamated into channel complexes, with each of the channels partially eroding the underlying ones. They also laterally migrated with a significant aggradation component. The 3-D reconstruction of the channel complex in the midfan setting shows that the total width of the channel complex probably exceeded 4 km (2.4 mi), with an axial sand-rich zone 3 km (1.8 mi) wide. Along its axis, the channel complex was approximately 150 m (490 ft) thick and consisted of massive amalgamated sandstones. Towards its margin, the channel complex thinned, and the proportion of overflow deposits increased.

The internal heterogeneity of the channel complex consists mainly of (1) mud-clast lags and thin-bedded bypass deposits draping erosional surfaces, (2) mud-clast breccias resulting from channel-margin collapses, (3) abandonment deposits, composed of thin-bedded and low-density turbidites preserved from erosion, and (4) internal levees and crevasse lobes developed laterally to the channel axis.

Most of these heterogeneities are below seismic resolution but may have a major impact on fluid behavior during the production of similar reservoirs in the subsurface.