Facies and Bedding Styles in Basin-Floor Fan Deposits of the West Crocker Formation, West Sabah: Implications for Deepwater Reservoir Facies Distribution

Abstract

The Oligo-Miocene West Crocker Formation in the Kota Kinabalu area, West Sabah, is often described as a sand-rich turbidite system. A field programme was undertaken to study the sedimentary facies of the West Crocker as a possible analogue for the deepwater reservoirs of the NW Sabah Basin and elsewhere. The formation consists predominantly of thick-bedded sandstone facies (beds are commonly >1 m thick) deposited by high-density turbidites. In places, there are the “classical” flysch-like, thin-bedded turbidite sequences deposited on the basin floor. In the thick-bedded successions, sandstone beds are commonly 1.5-3 m thick, while ‘megabeds’ may reach anomalous thicknesses of up to 35 m. The presence of amalgamation surfaces within some of the megabeds suggests that they were produced by multiple flow events. Despite the abundance of thick sandstone beds, there is a general lack of large-scale channelized scours, even at the bases of the megabeds. Almost all the sandstone beds have tabular or sheet geometries. Evidence of channel-fill deposits is found only in the innermost (eastern) part of the system. This suggests that the West Crocker Formation, at least in the vicinity of Kota Kinabalu, represents the non-channelized deposits of a basin-floor fan. The internal architecture of basin-floor fan succession are governed by the vertical (and possibly lateral) distribution of the three dominant facies types. Each facies type is the product of gravity flow events, identified in outcrop as turbidite, debrite and slump. Turbidite beds are dominated by massive to poorly laminated sandstone (Bouma Ta/Tb divisions), which are relatively mud-poor and normally graded with common dewatering features. The beds fine upward into parallel and ripple laminated heterolithics (Tc/Td), which are sometimes burrowed. More commonly, the massive beds have ‘floating’ shale clasts near or at the top, indicating deposition by high-density turbidity currents. Debrite beds generally consist of internally chaotic mud-rich units with scattered shale/mud clasts. They generally have sharp bases and directly overlie the massive sandstone beds, often filling hollows or subtle topographic lows at the top of the massive sands. Debrite beds, of varying thicknesses, tend to overlie turbidite beds with sharp, and often irregular, contacts. A third facies type is slump, which comprise generally muddy or shaly intervals displaying pervasive softsediment deformation (folding and faulting) and remobilization of pre-existing deposits. All three facies types are intercalated with thin ‘distal’ turbidite (Bouma Tc/Td) and hemipelagic shale intervals. Reservoir architecture and, consequently, heterogeneity of basin-floor fan succession, are ultimately governed by the distribution of these different facies types. Predictive depositional models for these facies types are important for effective reservoir characterization. We observe that the sand-rich West Crocker outcrops around Kota Kinabalu span a distance of 40 km along strike of the Kinabalu coastal plain, and we interpret them as representing the medial part of a basinfloor fan system, which is dominated by the thick-bedded sheet sandstones. Although there may be stratigraphic repetition in this steeply dipping succession, due to the thrust-related deformation, it is estimated that the outcrops represent a minimum total thickness of 5000 m of stacked mid-fan section. Since there is evidence for major channelization only in the most “updip” (eastern) outcrop, we speculate that the upper/proximal to base-of-slope part of the West Crocker system must occur east of our studied outcrops. By the same token, the lower/distal parts of the system must lie to the west beneath the coastal areas and beyond, and may possibly crop out on the islands off Kota Kinabalu and Klias peninsula. Hence, the West Crocker Formation is indeed sand-rich (estimated net-to-gross ratio > 70-80%) only insofar as the outcrops around Kota Kinabalu represent the exposed medial fan lobes, and are not necessarily representative of the entire West Crocker depositional system. Further characterization of both updip and downdip sections relative to the more exposed and accessible medial fan belt is required for a more complete understanding of the West Crocker depositional system.