Deep-Water Sandstone Facies and Ancient Submarine Fans: Models for Exploration for Stratigraphic Traps: DISCUSSION

Abstract

Five main facies of deep-water clastic rocks can be defined: classic turbidites, massive sandstones, pebbly sandstones, conglomerates, and debris flows (with slumps and slides). The classic turbidites consist of monotonously parallel-interbedded sandstones and shales without channeling; internal sedimentary structures include grading, parallel lamination, and cross-lamination. Massive sandstones are thicker, coarser, and commonly channelized. They lack the sedimentary structures of classic turbidites, but do contain evidence of dewatering during deposition. Pebbly sandstones tend to be well graded, and can contain parallel stratification and large-scale cross-stratification. Conglomerates are characterized by inverse and normal grading, parallel and cross-stratification, nd commonly have a preferred clast fabric (imbrication). Both the pebbly sandstones and conglomerates commonly are channelized. The facies can be fitted into a model of submarine-fan deposition. Modern fans are subdivided into an upper fan (suprafan), characterized by (1) a single deep channel with levees, (2) a middle fan, built up from suprafan lobes that periodically switch in position, and (3) a topographically smooth lower fan. The suprafan lobes have shallow, braided channels on their inner parts, but the outer suprafan lobes are smooth, and grade basinward into the smooth lower fan and basin plain. The smooth suprafan lobes and lower fan are characterized by deposition of the classic turbidite facies, and the braided part of the suprafan lobes by massive and pebbly sandstones. When one lobe is abandoned and another starts to prograde elsewhere, the first lobe is blanketed by mud, forming a potential stratigraphic trap. The upper-fan channel is an area of coarse sediment deposition, or conglomerates where gravel and boulders are supplied to the basin. During fan progradation, thickening- and coarsening-upward facies sequences can be formed in a manner analogous to those of deltas. Fan channels also can be abandoned progressively, forming thinning- and fining-upward sequences similar to those of fluvial or distributary channels. These sequences can be identified on electric logs. Where basin shales act as hydrocarbon-source areas, the classic turbidites can act as conduits, leading the hydrocarbons to the thicker, laterally coalesced massive and pebbly sandstones of the braided suprafan lobes. These bodies can be of the order of 25 km in diameter, and up to 100 m thick. The coarse deposits of the upper-fan channel also might form good reservoirs, being bounded by shales (levee deposits) on either side, and possibly by shales above if the fan-channel system is abandoned. Such channels can be tens of kilometers long, several kilometers wide, and a few hundred meters deep. Reservoirs may be present in all of these environments.