The nature and significance of regional unconformities in the hydrocarbon-bearing Neogene sequence offshore West Sabah

Abstract

The Teniary West Sabah basin is a trench-associated sedimentary basin containing up to 12 kIn of predominantly siliciclastic sediments. The basin history can be divided into two phases: I. Pre-Middle Miocene deposition of deep marine deposits with tectonic imbrication related to south southeastward convergence along the fore-runners of the Palawan TroughlNW Borneo Trench.2. Middle Miocene and later deposition, after the cessation of subduction, by a series of northwestwardprograding shelf/slope sequences associated with important wrench-faulting in the basement.Although a small amount ofoil and gas has been discovered in the Pre-Middle Miocene deep water deposits, all commercial accumulations discovered to date are in the Middle Miocene and younger deposits.

Seismic images of a collision zone offshore NW Sabah/Borneo

“…1) to late Middle Miocene off SW Palawan and unconformity B has been dated as 14-12 Ma old (Schlu¨ter et al, 1996). However, given the time transgressive nature of this horizon in the study area off NW Sabah, this unconformity might be as young as Late Miocene and thus roughly coincide with the 'Shallow Regional Unconformity' (SRU; Levell, 1987;Tan and Lamy, 1990;Rice-Oxely, 1991). In our interpretation, unconformity B (Late Miocene) was formed by marine erosion because it partly onlaps unconformity C (Fig.…”

“…For location see Fig. 2. break in the sedimentary history (Bol and van Hoorn, 1980;Levell, 1987). Unconformity B of Hinz et al (1989) and Schlu¨ter et al (1996) is interpreted as a time transgressive erosional horizon that forms the top of the accreted wedge, or what was the top of the individual thrust sheets at the time of deformation.…”

“…This seismic stratigraphy is based on the previously interpreted seismic successions developed for the shelf to NW Borneo Trough sediments (Bol and van Hoorn, 1980;Levell, 1987;Hinz et al, 1989;Schlu¨ter et al, 1996). The timing of each of these marker horizons and their relationship to other studies is listed in Table 1.…”

Tertiary stratigraphy and basin evolution, southern Sabah (Malaysian Borneo)

“…This is earlier than the Early to Middle Miocene boundary age (17 Ma) given by other authors for the Deep Regional Unconformity (DRU), a widespread surface recognisable on seismic profiles offshore NW Sabah (Bol and van Hoorn, 1980;Levell, 1987;Hinz et al, 1989Hinz et al, , 1991. However, as noted by Clennell (1996) the time period represented by the DRU is variable, and the early Miocene was a period of widespread melange development triggered by submarine slope failures (Clennell, 1991(Clennell, , 1992(Clennell, , 1996.…”

“…The unconformity between the Kapilit and Simengaris formations may be attributed to a widespread base level drop during end-Miocene which is known from offshore data as the Shallow Regional Unconformity (Bol and van Hoorn, 1980;Levell, 1987). The Simengaris Formation consists of shallow marine to fluvio -deltaic sediments deposited during a subsequent transgression.…”