Deep-water pebbly sandstones and conglomerates - Fagies and reservoir characteristics

The deep-water massive sandstone facies includes thick sandstones with individual beds normally thicker than 50 cm. Shaly partings between beds are normally absent, in contrast to the classical turbidite facies where shaly partings are ubiquitous. Beds commonly lack primary sedimentary structures. Pebbly sandstones also lack shaly partings between beds, but are coarser, better graded, and commonly contain parallel lamination or crossstratification. The pebbles commonly show a preferred fabric (imbrication) indicative of flow direction. Conglomerates can show grading, inverse grading, and stratification, the stratified types normally occurring farther basinward than unstratified or inversely graded types. Facies relationships suggest that massive and pebbly sandstones occur on the braided portion of suprafan lobes, in the mid-fan area of large submarine fans. Coarser facies occur within the braided channels, but all facies tend to coalesce laterally into a continuous sand/gravel sheet. This potential stratigraphic trap can be sealed by a thick (greater than 5 m) shale blanket, deposited when one suprafan lobe is abandoned and receives only fine deposits (in a manner analogous to subaerial delta lobe switching and abandonment). The coarse deposits are potentially ideal reservoirs - good source rocks may occur immediately basinward, and the reservoir can be sealed on both sides by lateral facies passage into fines, and sealed above by the shale blanket.