Deep-Water sediments are those sediments deposited below the wave base, on and at the base of the continental slope and transported on to the abyssal plane. The sediments are transported through a number of mechanisms ranging from sediment creep, hyper pycnol flows, turbulence to mass transport deposits. Large quantities of sandstone and shale are remobilized from the shelf in to the deep ocean basins. Sediments can either pool behind topographic barriers forming thick, structureless sandstone, or can travel great distances to form clean, laterally extensive thin beds.
Thin Beds
The Challenge: Thin bedded turbidites are a very productive play, due to the highly porous, clean laterally extensive beds. However, these beds can be centimetres thick and below the resolution of traditional open-hole logs and therefore only the high resolution borehole images can identify the thin beds. Although, calculating porosity and permeability requires additional processing to sharpen low resolution logs.
Solution: Textures and resistivity contrasts from borehole images, show clear difference between resistive sandstones and conductive shales, critical to sandstone volume calculations. Sharpening neutron and density logs can enhance the resolution to calculate porosity and permeability.
Structureless Sands
The Challenge: Thick accumulations of sands in deep-water settings, require either a large sandy source – such as clean winnowed shelf sands – and sporadic slope failure to mobilise the sand or a continuous supply of sand to the coast over a period of time, either from a fan delta or a major river. Sandstones and other course-grained facies mostly occur in clearly erosive channels and channel complexes that cannibalise previously deposited sands to form large amalgamated bodies. Evidence of the sinuous nature of these channels in planform comes from high quality seismic time slices and varied paleaocurrent directions from dip meter tools.
Solutions: Three-dimensional seismic provides spectacular resolution of turbidite channels, although it is not always clear on seismic whether these channels are mud or sandstone filled? In addition thick mass transport deposits characterized by low acoustic impedance can mask the channel form. Thick bodies of amalgamated, structureless sands are very difficult to correlate laterally on log data, and the lack of flow indicators on borehole image date unhelpful. However, this type of play can contain large reserves if exploited for reserves.
Mass Transport Deposits
Challenge: Mass Transport Deposits (MTD’s) are chaotic often muddy sediments, that form through a variety of processes ranging from slumping, slope creep, to slurry and chaotic, debris flows. Characteristic of these thick beds are shear plane faults, forming multiple ridges. The MTD can form a good reservoir seal to the sandy reservoirs, but can be a drilling hazard due to fault shear reactivation during drilling.
Solution: Based on observations from recent wells drilled offshore Brunei and Sabah, it appears that quiescent sedimentation with thick bodies of conformable bedding planes, with consistent structural dip interspersed with sheet sands and sporadic debris flows form the majority of the deposits. However, enveloping this uniform sedimentary environment, is massive structural failure, involving slides, slumps, and debris flows, followed by post depositional creep. Overlaying these chaotic beds are often thick, massive structural sandstones, possibly suggesting the slope failure is onset by sea level lowering and followed by low stand sandstone deposits.
