Deepwater Thin-Bed Depositional Settings : A Geological Framework from NW Sabah

Deepwater clastic depositional systems can have various thin-bedded depositional elements from a variety of settings. A systematic geological observation and classification using image-logs, cores and conventional/ sharpened logs, from the K Field, NW Sabah Province has been performed in this study. The thin-bedded deposits were looked at to assign depositional settings which could be one of several environments like proximal and distal levee, passive channel-fills and distal sheets. Available core demonstrates visible sedimentological characteristics like grain-size variations and trends, nature of bedding contacts, bedding-density and trends, sedimentary structures and post-depositional imprint. These observations when integrated with log and borehole-image response, at various scales, allow for a geological framework of thin-bed groups within the study area. The thin-bedded intervals typically lack the Bouma Ta division and commonly comprise of the parallely laminated Tb and rippled Tc divisions along with ome Td-e divisions, in combination or in isolation. The occurrence of these sedimentary beds in association with other features like mud-clasts, convolute-bedding, and climbing-ripples is indicative of the affinity of these beds to belong to one thin-bed depositional setting versus another. Core-based observations when calibrated against OBMI image-logs become a strong geological tool to help propagate validated observations into other wells or uncored intervals, where only image and log data is available. With these fine-scale observations it has been possible to package thin-bedded zones into genetic units at a coarser scale. These units are bound by surfaces that show distinctive breaks in well-log signatures and are potential strong reflectors in the seismic domain. It is possible to generate a model that predicts the stacking of these genetic units which are related to the cyclical depositional style in deepwater settings, primarily driven by relative sea-level fluctuations. A commonly observed cycle of deposition in deepwater during one lowstand cycle starts with mass transport deposits (MTD) at a sequence boundary, overlain successively by thickly-bedded good quality sheet sands followed upward by channel-levee-overbank complex capped by a condensed section (Fig. 1). In the K Field area this cycle is clearly seen. Mass transport complexes (MTDs - thickness range ~30-100m) are overlain by thick-bedded well developed massive sands (ThkSstn - thickness range 10-30m) which is followed upward by thinly-laminated sand-shale heterolithic package (thin bedded turbidites, TBT - thickness range 30- 100m) with variable net:gross (Fig. 2). H43 H86 Accommodation space in the toe-of-slope and proximal basin-plain controls facies distribution and architecture. Sea-floor radient, sea-floor mobility and rugosity, sedimentary budget and volume, net:gross in the system controls the type and distribution of accommodation space (Prather, 2001). In the K Field area local sea-floor rugosity is created by deposition of MTDs during early lowstand, which leaves an uneven depositional plain on the sea-floor. Following this event, typically a sheet-like well-developed sand unit is laid down which partially ponds the local sea-floor rugosity in the form of thickly-bedded sand packages, i.e. ThkSstn. Thereafter a graded to nearly graded system takes over, during which a channel-levee overbank complex forms which deposits thin-bedded turbidites, the TBTs (Fig. 2b and Fig. 3). This system is somewhere between the two extremes of a graded basin-plain setting and a successive fill-and-spill slope minibasin. Another category of sea-floor topography other than rugosity created by MTDs, albeit at a much slower but bigger scale, is attributed to active tectonic compression of the basin and the creation of toe-thrusts and the associated relief. The effect of this tectonic imprint on the sea-floor and hence on the depositional architecture is not very clear on the reservoirs of interest in the K Field area. The thinly-bedded (TBT) packages have been observed to represent two classes, one is high net:gross sand-shale package and the other is a low net:gross sand-shale package. Both classes are inferred to represent levee-overbank complexes, with the former being proximal levees and latter distal levees. The sedimentary characteristics that support the thinly-bedded deposits to be levee-overbank deposits are as follows: • interbedded sand and shale package, occurring as heteroliths (in analogy with modern observations) • consistent, parallel stratigraphic dips without much variance (Fig. 2a), unlike the other units especially MTDs • parallel-laminated to ripple cross-laminated sands and climbing ripple cross-laminations having mostly Bouma Tb-Tc-Td units (Fig. 3) • rip-up mud-clasts • convolute lamination. The sedimentary attributes mentioned above are not all common in all the thin-bedded intervals but they are typically observed and are presented here in order of importance. The occurrence of thin-bedded turbidite sections within a lowstand cycle of deepwater deposits hold a key for facies modeling, determination of depositional architecture leading to reservoir simulation and producibility estimates. Due to lack of good quality seismic, affected by shallow gas cloud effects, geobody xpressions are not always clear and high-resolution observations like image and core should be employed for reducing uncertainty in facies analysis.