Deep-Water Pebbly Sandstones And Conglomerates - Facies And Reservoir Characteristics

ABSTRACT 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 cross stratification. 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 5m) shale blanket, deposited when one suprafan lobe is abandoned and receives only fine deposits (in a manner analogous to sub aerial 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. INTRODUCTION Although classical turbidite reservoirs are fairly well known in many areas, their coarse grained counterparts (massive and pebbly sandstones, and conglomerates) are not. However, these coarse rocks may make many potentially excellent reservoirs, for example in the offshore California area. The object of this paper is to describe the facies, and to fit the facies into a model that will help predict reservoir size, geometry and facies changes. The facies to be described below can all be associated with thick sequences of classical turbidites, implying deposition in water consistently much deeper than storm wave base, at the bottom of a major slope into the basin. Sediment gravity flows of sand, gravel and cobbles accelerate down this slope and deposit their load on submarine fans at the base of slope. Transport processes can involve turbidity currents and debris flows.1 MASSIVE SANDSTONES The massive sandstone facies [Fig. 1] is gradational with classical turbidites, but typically, massive sandstone beds are thicker (0.5 to 5 m) and composite. There is more channeling between beds, and normally no shaly partings are preserved. Graded bedding within beds is uncommon, and the only common sedimentary structures reflect fluid escape during deposition - vertical fluid escape tubes and "dish" structures.2 The fluid escape features suggest very rapid deposition of the sandstones from turbidity currents with high grain concentrations at their base, typically in an environment more proximal (closer to source) than classical turbidites. PEBBLY SANDSTONES Pebbly sandstone beds range in thickness from about 0.5 to over 5 m [Fig. 2]. They are characterized by sharp bases and absence of shaly interbeds.3 The Bouma model of classical turbidites cannot be applied to pebbly sandstones.