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Abstract

The application of sequence stratigraphy to the clastic oil and gas reservoirs in the Sarawak offshore has not lived up to the initial high expectations. To find a cause and remedy for the often-disappointing results, outcrops of the Nyalau, Lambir, Miri and Liang Formations were studied in the exposed Neogene of the Tatau, Bintulu and Miri regions of Sarawak. The outcrops appear to be providing good analogues for the shallow marine and coastal reservoirs in the offshore subsurface, as far as they were deposited within the coastal reach (between high- and low-stand shorelines). All formations consist of a series of regressive-transgressive tongues of coastal and coastal-plain sediments in marine shales. Five key facies-associations can be distinguished with similar characteristics in all four formations and in cores and logs from offshore wells: (1) shallowing shelf; (2) regressive shoreline; (3) coastal plain; (4) incised valley; (5) transgressive shoreline, and (6) deepening shelf. These associations are separated by (a) the<br>shoreface transition zone; (b) the emerging shoreface surface; (c) the sequence boundary (d) the marine flooding surface; (d) the mud line and (e) the maximum flooding surface. The transgressive shoreline association can usually be subdivided into an inshore tidal (backbarrier/ lagoon) and an offshore tidal unit, separated by an erosive marine-flooding (ravinement) surface left by the landward migration of the shoreface. Even in outcrops, with a wealth of geological information at hand, two practical problems become<br>immediately clear: – It is very difficult to correlate the major (4th to 5th order) regressive-transgressive units between outcrops. The resolution of the currently available bio-stratigraphy (based either on planktonic foraminifera or palynomorphs) is insufficient to tell individual tongues apart, let alone to relate them to global sea-level fluctuations. Even differentiating between the Lambir and Miri formations on bio-stratigraphic grounds is difficult. – Only four of the six bounding surfaces can usually be identified in outcrop. This does unfortunately not include the two key bounding surfaces of classical sequence stratigraphy: the sequence boundary and the maximum flooding surface. In most outcrops the sequence boundary is not present as an erosive surface, but as a more subtle “surface of maximum regression”. The position can often only be inferred where an a-sequential pattern in the succession of lithofacies indicates a discontinuous seaward shift of the depositional system. In many cases, especially in the deposits of low-stand coastlines, this surface may be represented by a sand-on-sand contact, precluding recognition from logs. The problem is compounded by the presence of several –more frequent- alternative candidates for the sequence boundary: (1) the sharp erosive base of the shoreface, as found in several forced regressions in the Lambir Formation; (2) the ravinement surface of the transgressive shoreline found in many new outcrops of the Nyalau and Liang Formations; (3) the erosive base of distributary channels, common in many coastal plain associations (4) low-angle, sub-horizontal thrust faults, commonly observed in the Lambir Formation, and presumable also present in compressive structures in the subsurface. Possible remedies for these problems include: A more rigid lithofacies analysis to preclude erroneous picks of the sequence boundary. For wells without core data, this implies the design of a more refined logfacies scheme using overlays of wireline logs rather than Gammy Ray logs only (see Budding, 2006,<br>elsewhere in this volume). In addition, improving the resolution of bio-stratigraphy might enable the distinction of individual 4th order sequences and possibly lead to a calibrated sea-level curve for the region. Nanoplakton may provide a viable solution.

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/content/papers/10.3997/2214-4609-pdb.256.R16
2006-11-27
2024-03-28
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