The undersigned declare that they have read, and recommend to The Postgraduate Studies Program for acceptance entitled "3D Seismic Interpretation and Development of Sequence Stratigraphic Model of a Carbonate Buildup in Central Luconia, Offshore Sarawak" submitted by Mohamad Faizal Bin Idris for the meet the requirements for the MSc Petroleum Geoscience degree. The stacking pattern of the system tracts and the stratigraphic unit reveal the growth architecture and evolution of the superstructure. Three main factors have been identified in controlling the growth, evolution and architecture of the build; which are tectonics (faulting and subsidence), relative sea level and also paleowind direction in addition to the rate of carbonate production.
Using the seismic facies approach, the hilly reef and progradation facies may be associated with good reservoir properties. They are extensive in the peripheral region of the superstructure, while the tight lagoon facies are usually found in the center of the superstructure. The enhancement of porosity and permeability through secondary processes such as karstification and late leaching has been identified as one of the major contributors to reservoir formation.
These facies are distributed relative to the formation of sequence boundaries during low sea levels. This attribute illustrates a back-rock (lagoon) facies displayed by the continuous parallel reflector which is fertile in the central area of the assemblage.
Overview
Therefore, their potential contribution to the understanding of the carbonate reservoirs in the area has not been used. Through the understanding of the stratigraphy and the development of structures, prediction of reservoir distribution can be made through understanding the systems and their related facies. The main areas of work for this project are 30 seismic interpretation, seismic attribute analysis and seismic sequence stratigraphy.
CHAPTER TWO: REGIONAL GEOLOGY
Geological Setting
Stratigraphy and Sedimentation History
This data set completely covered the 'W' structure (central area) and partially covered another 'X' structure in the south. The data quality varies from fair to good, with the central part of the dataset remaining poorly visible due to a wipe-out zone with poor seismic quality - (Figure 2.4. This central wipe-out zone is caused by strong signal absorption by a very shallow event, likely a recent reef and also shallow gas clastic reservoir effects above the main carbonate section.
The study area was penetrated by seven wells; six wells on structure 'W' and one well in the vicinity of 'X' structure to the south. Note: The central part of dataset is characterized by poor data quality - delete zone;. Analysis of wells W-1, W-2, W-3, W-4 and W-5 indicates that the carbonate sections in this study area consist of cycle III, IV and V carbonates.
This well was selected as a key well for this study, other wells penetrate only the Cycle IV and V Carbonates. The thick Cycle IV carbonates could be associated with the period of prolific carbonate growth during the Middle Miocene as a result of the rising relative sea level.
W-2 Well
CHAPTER THREE: 3D SEISMIC INTERPRETATION
Seismic to Well Tie
- Top Cycle V
- Intra Cycle V
- Base Carbonate
High Carbonate was selected at the red border, High Cycle III at the zero crossing, and Base Carbonate at the black peak. Dashed lines indicated selected markers; High Cycle V, Upper Carbonate, High Cycle III and Base Carbonate respectively from top to bottom. Although the focus is on the carbonate section, additional horizons within the clastic sequence were interpreted to better understand the overlying sequence in the study area.
The results from the horizon interpretations were used for attribute analysis in the next chapter. The presence of gas in this reservoir indicates the existence of an active petroleum system in the studied interval. The seismic sections on Figures 3.3 and 3.6 illustrate that the faults were formed during early late Miocene time.
A flattened seismic section at the horizon within cycle V (Figure 3.6) shows the original state of the underlying sequences. Information from wells confirmed reservoir connectivity between wells W-2, W-5 and X-I (western section) and in the eastern section between wells W-1, W-3 and W-6. Horst and graben topography controlled lateral facies variation, while high-elevation horst blocks provide a suitable site for shallow-water carbonate and deeper-water carbonate facies to accumulate at a much lower position in grabens.
Carbonates continue to grow until mid-cycle V time, but only to a limited extent in the western area; with the geometry of an isolated peak (Figure 3.8). The Upper Cycle III horizon is an intermediate horizon between the Upper and Lower Carbonate. In well W-2, cycle III sequence is characterized by shallow water carbonate, while in well W-4 an interbuildup (deeper water) facies is typical, shown by siliciclastic intercalation between tight argillaceous and muddy limestone.
As the position of this horizon is only 100 msec below the overlying carbonate, there is a similarity in structural style to the overlying carbonate map, which is clearly shown on the horizon map. Visibility slices below this interval illustrated the complexity of the fault pattern of the deeper section (Figure 3.11. Visibility slice at 2100 msec (below basal carbonate) shows a strong fault zone in the deeper zone.
CHAPTER FOUR: ATTRIBUTES ANALYSIS
The boundaries displayed from RMS attributes usually refer to the transition from reef to fore reef zone near the ramp position. However, there is also an indication of the strong amplitude anomaly (black) generated by the overlapping of transgressive sediments on the older layers. The build-up area is defined by a series of alternating reef, fore and aft reefacies.
This shallow accumulation of water is separated by a relatively deep edifice zone, from which it was filled with carbonate mud facies characterized by fine-grained carbonate materials and sometimes interbedded with clastic sediments. The RMS attribute extracted from the high carbonate horizon shows a typical elongated geometry of the assemblages with size ranges from I km2 to more than 21 km2 (Figure 4.3). These fault blocks provide a template and allowed carbonate to grow into the uplifted horst blocks.
The interpretation of the geomorphology from the RMS attribute appears to match the image from the seismic sections very well. Three seismic sections were chosen to illustrate the geometry of the buildings (Figure 4.4). The area between the outcrops from these sections shows a typical sediment-filled zone showing continuation and parallel reflectors with an overlap pattern.
The construction geometry is wider in the central area and narrower in the peripheral area. The results from the W-4 well (Figure 4.5), which was drilled in the saddle area (within the intermediate structure), confirmed the presence of deeper marine facies, with mainly chalk and fine-grained limestone.
TOP CARBONATE RMS AMPLITUDE
W-4 well was drilled in the interbuildup zone; in the depositional slave (basin area), characterized by deeper marine carbonate facies. This deeper carbonate facies in the interbuildup area has the potential to act as a permeability barrier, which could provide a stratigraphic trap for hydrocarbon accumulation. The position of the top carbonate marker (W-2: 6060 tvdss, W-4: 6350 tvdss) in both wells recorded at differential depth with a difference of 290 ft (88m); these variations in depth absolutely allow deposition of deeper marine facies in the interbuildup area (W-4).
The RMS attribute depicted from Top Cycle III illustrates the initial phase of build-up extension and coalescence. The arrangement of the structures seems to be controlled by the topography of the fault blocks and oriented in a N-S (slightly easterly) direction. The sizes of the structures range from 0.25 km2 to more than 12 km2• The small and rounded carbonates located close to the elongated structures.
There is also evidence of step-back growth pattern (dotted arrow - Figure 4.6) which is indicative of the transgressive nature of the accumulation. The dummy feature and time slice at 1710msec were used to illustrate the fault pattern and build-up characterization (Figure 4.8). The time slice image shows the possibility of karstified surface near the Top Cycle III horizon, which is related to a sequence boundary.
This is confirmed by sequence stratigraphic analysis, which confirmed the presence of a main sequence boundary in this interval.
TOP CYCLE III RMS AMPLITUDE
Two attribute extractions were made from this horizon (0 to +lOmsec and +10 to +50msec; Figure 4.9 & Figure 4.10) to capture the image of the structures at two different stratigraphic intervals.
BASE CARBONATE RMS AMPLITUDE (II)
APPENDIX 1
APPENDIX2
APPENDIX3
APPENDIX4
APPENDIX5
