Basic Reservoir Characteristics
The alluvial deposits of braided rivers occur as blankets of coarse, sandy sediment. Grain rounding is moderate at best, and sorting is often intermediate to poor; thus, despite high, gross porosity, these deposits typically show a good deal of permeability variation. At the same time, due to sediment coarseness, intercommunication between individual sandstone bodies (channels and bars) is usually excellent. Due to their continental setting, braided alluvial deposits are normally capped by an unconformity in the sedimentary record, and as a result, often occupy stratigraphic positions with good reservoir potential.
Postburial percolation of acid meteoric waters through this type of deposit has almost an equal chance of destroying primary pore space by mineral precipitation as enhancing it through dissolution. In cases where such fluids have been especially active, (usually just below an erosional surface), an upper “leached zone — within which cemented material has been partly removed — may overlie a tight interval where reprecipitation of this material has occurred. A high degree of “tightening” due to this type of early diagenetic process is shown in northern Libya’s Sirte basin, where Paleozoic and Cretaceous non-marine sandstones are much more silicified on structurally elevated basin flanks and highs (in areas closer to the major unconformity) than in the deeper, basinal region. We’ll look more closely at this particular setting below; it forms one of the several giant field areas that produce from braided alluvial sands.
As discussed, shale layers in this facies are usually inconsequential in terms of fluid migration blockage, and therefore, do not promote or aid stratigraphic trapping. Neither do they serve as good source rocks. Moreover, the development of braided stream systems relatively far inland, deep within continental shield and intracratonic basin regions, limits their eventual accessibility to petroleum generated from marine source rocks. The ideal reservoir setting for this facies is immediately beneath a major unconformity over which rapid marine transgression has occurred. If subsequent tectonism were to auspiciously juxtapose these potential host rocks against overlying marine shales in a regime of high heat flow, so much the better. (Figure 1, Idealized cross section to show how braided alluvial sand blankets may act as sites for large oil accumulations on truncated horsts, sourced and sealed by transgressive marine shales.
In such a setting, tectonism-possibly rift-related-would generate the high heat flow necessary to mature organic material into hydrocarbons.) In fact, in a very basic way, this has been the case at Sirte and at Prudhoe Bay, another giant field area.
