uIntroduction

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uPlate tectonics

uHalf-grabens

uFault movement

uDepositional models

uRift stratigraphy

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uSummary

uReferences

uIntroduction

uFigure captions

uPlate tectonics

uHalf-grabens

uFault movement

uDepositional models

uRift stratigraphy

  uModel

uSummary

uReferences

uIntroduction

uFigure captions

uPlate tectonics

uHalf-grabens

uFault movement

uDepositional models

uRift stratigraphy

  uModel

uSummary

uReferences

uIntroduction

uFigure captions

uPlate tectonics

uHalf-grabens

uFault movement

uDepositional models

uRift stratigraphy

  uModel

uSummary

uReferences

uIntroduction

uFigure captions

uPlate tectonics

uHalf-grabens

uFault movement

uDepositional models

uRift stratigraphy

  uModel

uSummary

uReferences

uIntroduction

uFigure captions

uPlate tectonics

uHalf-grabens

uFault movement

uDepositional models

uRift stratigraphy

  uModel

uSummary

uReferences

Figure Captions

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Plate Tectonic History of the Region 

As a result of the hydrocarbon potential of this region, much research has been done on the plate tectonic history of the overall South China Sea area and the interaction of the several micro-plates that comprise this complex area. The deepwater offshore Sarawak and northwest Sabah areas have developed in response to the complex tectonics related to the opening of the South China Sea which began about 45 Ma with an initial rifting phase. This rifting which appears to have occurred along a northeast to southwest trend resulted in the separation of the Luconian and Dangerous Grounds micro-plates from the major Eurasian Plate (Maddon et al, 1999). These micro-plates moved in a relatively south-southeast direction away from the Eurasian plate across the proto-South China or Rajang Sea. This movement initiated a series of syn-rift, half-graben, subbasins that are generally developed downthrown to moderate to large, normal and sometimes listric type growth faults that trend northeast to southwest or subperpendicular to the direction of rifting motion.  

With continued extension, through Oligocene time these micro-plates approached the continental block of Kalimantan as the lead edge of the micro-plates was subducted below the Kalimantan continental block. This active seafloor spreading promoted extensional movement along these half-graben-bounding faults.  

The continued southeastward drift of the Luconian and Dangerous Grounds Blocks (Figure 1) eventually led to a collision of these blocks with the Kalimantan Block, first by the Luconian Block about Middle Oligocene time and subsequently by the Dangerous Grounds block by Middle Miocene time. With the collision of these plates the extensional or growth phase of these half-grabens came to a close.  

Intense rifting during the rift-drift transition has created a series of characteristic asymmetric wedge-shaped half-grabens. These half-grabens are bounded by both normal faults and large-scale listric faults, which expand the stratigraphic section and often appear to sole-out at depth. The sedimentary section contained within these half-grabens is the primary focus of the petroleum potential in this deepwater area.

Half-Graben Structures 

Intense rifting during the rift-drift transition has created a series of characteristic asymmetric wedge-shaped half-grabens. These half-grabens are bounded by both normal faults and large-scale listric faults, which expand the stratigraphic section and often appear to sole-out at depth. The sedimentary section contained within these half-grabens is the primary focus of the petroleum potential in this deepwater area. Figure 2 is a NW-SE oriented seismic profile 95 kilometers long, which illustrates the common structural form of the North Luconia Province. Here you can see the edge of the Middle to Upper Miocene Luconia Platform and the wedge of Upper Miocene to Plio-Pleistocene bathyal sediments above the regional Mid Miocene Unconformity (MMU) dated at approximately 16 Ma. Below the MMU lies a series of tilted half-grabens which appear best developed close to the Luconia Platform margin and less well developed or less well preserved, farther from the margin.  

Figure 3 shows the typical form of a half-graben with a large controlling listric fault on the SE margin. This listric fault flattens out with depth and in this case continues upwards to the sea floor. Additional extensional synthetic and antithetic faulting occurred at the time of the Middle Miocene Unconformity and has subdivided the half-grabens into multiple block-faulted compartments. In some cases the fault compartmentalization has been intense. Another characteristic of many of the half-grabens is the tilting and erosion at the toe of the half-graben. This is combined with an overall regional tilt and erosion at the Middle Miocene Unconformity that preserves much of the sedimentary section close to the Luconia Platform Margin, but has eroded much of the 25 Ma to 16 Ma section further from the margin.

Deposition and Rates of Fault Movement 

Slip rates on the bounding listric fault surfaces are highly variable but slip rates of up to 43 cm/1000 years have been calculated from depth corrected and structurally balanced models of the half-grabens. During this same time interval sedimentation rates observed in wells were averaging 10 to 20 cm/1000 years. The low sedimentation rates of about 10 cm/1000 years indicate two things:  

• In an actively extending rift system such as this one, the tectonic subsidence is likely to be of greater significance on internal seismic facies characteristics than eustatic sea level changes.

• The half–grabens were primarily in a starved condition with new accommodation space outstripping sediment supply.

There is clearly a strong tectonic overprint on the stratigraphy of the half-grabens.

Depositional Models 

Several authors have since looked at the specific question of sequence stratigraphic concepts as they relate to tectonically active fault-bounded asymmetric grabens where the tectonic effects may outweigh the eustatic effects. In these settings it may not be possible to identify the eustatically controlled systems tracts. In such extensional basins the problem is complicated by complex basin margin topography and significant, along strike variations in subsidence and accommodation creation (Howell and Flint, 1996). Gawthorpe et al. (1994) argue that high rates of hanging wall subsidence close to the center of normal fault segments may cancel out the effects of sea-level fall, so that accommodation development is normally characterized by the continual addition of new space. The resulting sequences would lack type 1 sequence boundaries and lack the typical characteristics of lowstand systems tracts but would tend to stack into aggradational sequence sets which would be dominated by the characteristics of highstand systems tracts. In the instance where sea level would drop, exposing the footwall block, lowstand fan deposits could be deposited in the deeper water hanging wall trough.

Rift Basin Infill Stratigraphy 

Prosser (1993) examined how extensional rift basins are infilled and proposed a four-fold division of rift evolution. The four stages are rift initiation, rift climax, immediate post-rift, and late post-rift.

Rift Basin Infill Model  

• During rift initiation, deposits are often continental in nature with sedimentation rate greater or equal to subsidence.

• The rift climax phase is the time of maximum rate of fault displacement and sedimentation is outpaced by subsidence. As a result, most of the basin has been drowned either in a lacustrine or a marine setting. Sediments deposited are primarily fine-grained with the exception of coarser grained talus and fan deposits derived from the upthrown footwall.

• The immediate post-rift stage marks the end of active tectonism and fault displacement. Sedimentation in this stage is associated with basin infill in a generally coarsening-upward succession.

• Late post-rift is related to the final infilling of the basin with the erosion of the footwall high and general peneplanation of the surrounding area. In our study this peneplanation occurred at the Middle Miocene Unconformity at 16 Ma. In the study area there has been several periods of fault re-activation, and as a result the rift stages described above can be repeated, creating two or three or more repetitive tectonostratigraphic sequences of rift infill. Figure 4 shows this repeated pattern of infill in a large half-graben. The section between the Magenta and Orange horizons marks the first sequence while the interval from Orange to Dark Green is a second tectonostratigraphic sequence of rift climax to post rift infill.

At the time of the Orange horizon (Figure 4), fault displacement increased resulting in a new rift climax phase with associated talus and fan development adjacent to the footwall. This was followed by a long period of apparent deep-water sedimentation indicated by the low amplitude, low frequency seismic facies. The Blue horizon estimated to be at 25 Ma, marks an unconformable surface where another pulse of fault slippage caused the tilting of the hanging wall and then onlap of the Blue surface by the high amplitude post-rift section. The onlap of the Blue horizon on the Orange horizon is direct evidence for a starved basin model for this half-graben. At some time between 16 Ma and 5.5 Ma the fault was rejuvenated once again to create a third pulse of rift infill. This Upper Miocene section shows a new rift climax phase with associated talus and fan sediments followed by immediate and late post-rift intervals. This pulse occurred in deep water and remains in bathyal depths and as such does not display the usual coarsening upward appearance of the pre-unconformity cycles. Figure 5 shows two oldest cycles well developed and preserved on a horst on which a well was drilled.

Summary 

The extensional half-grabens of the deepwater Sarawak and Sabah area are primarily composed of two or more regressive cycles of rift infill strongly influenced by regional tectonics. The first is related to early extension and rift development from approximately 43 Ma until 30 Ma and the second is related to the period between the initiation of sea floor spreading in the South China Sea at 30 Ma and the end of sea floor spreading at 16 Ma. The lower portion of each cycle is predominantly composed of fine-grained bathyal sediments due to the rapid subsidence of the hanging wall and resulting sediment starvation of the basin center. This interval could theoretically contain lacustrine or marine source rocks. The upper portion of each cycle is characteristically more sand prone inner neritic to fluviomarine sediments related to the post-rift infill. This section is seen to contain reservoir quality sands in well penetrations. It is difficult to identify classic depositional systems tracts within these rift infill stages but it is inferred from the model and our observations to consist primarily of aggradational highstand systems tracts.

References 

Gawthorpe, R.L., Fraser, A.J., and Collier, R.E., 1994. Sequence stratigraphy in active extensional basins: Implications for the interpretation of ancient basinfills. Marine and Petroleum Geology, 11, p. 642-658.

Holloway, N.H., 1982, North Palawan block, Philippines—Its relation to

Asian mainland and role in evolution of South China Sea: AAPG Bulletin, v. 66, p. 1355-1383.

Howell, J.A., and Flint, S.S., 1996. A model for high resolution sequence stratigraphy within extensional basins. High Resolution Sequence Stratigraphy – Innovations and Applications, Geological Society Spec. Publ. 104, p. 129-137.

Madon, M.B.Hj., 1999a, Plate tectonic elements and evolution of Southeast Asia, The Petroleum Geology and Resources of Malaysia: Chapter 4, Petronas, p. 59-76.

Prosser, S., 1993. Rift-related linked depositional systems and their seismic expression. Tectonics and Seismic Sequence Stratigraphy. Geological Society Spec. Publ. 71, p. 35-66.

Taylor, B., and Hayes, D.E., 1980, The tectonic evolution of southeast Asian seas and islands: AGU Geophysical Monograph, p. 89-104.

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