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PGCE 2010
- Conference date: 29 Mar 2010 - 30 Mar 2010
- Location: Kuala Lumpur, Malaysia
- Published: 29 March 2010
81 - 100 of 100 results
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Compensating for Gas Wipeout Effect Through Velocity Tomographic Inversion: A Case History from Malay Basin
More LessTomographic inversion has been widely employed to solve for the occurrences of gas accumulation problem in seismic imaging. In Malay Basin and offshore Malaysia, there are many imaging problem related to the so-called ‘gas wipeout” effects. The occurrence of the gas wipeout zone has creates difficulties in structural tracking and estimation of the hydrocarbon resource potential for reservoirs. We present here a case history of addresses the issue of gas wipeout effect through velocity tomographic inversion and its application prior to Pre-Stack Depth Imaging processes in the South of Malay Basin. Full azimuth hybrid tomography was used to derive the sediment velocity depth model and identify velocity changes across fault, structure and gas wipeout zone. Multiple passes of gas-flood modelling and imaging allowed accurate delineation of the anticlinal structure filled with gas. The key element of this method is the use of hybrid velocity model with control from the geological structures and well velocities.
The workflow of the hybrid 3D tomography involves automatically performs the focusing analysis on the depth-imaged gathers and updates velocity models through a global inversion process. Focusing parameters were measured and projected along the computed ray paths from the imaged subsurface reflectors back to the surface. The new velocity updates are derived from this inversion process will be used to improve the focusing of the seismic gather and generating new traveltimes for each of the reflectors in the next iteration of Pre-Stack Depth Imaging with the updated model. After 3 or 4 iterations of the tomographic velocity updates, the velocity depth model was ready and sufficient to image the structures and gas effect in the area. The final velocity model derived from the PSDM process is a geological product that can be used to identify lithology changes, overpressure zones and importantly, changes due to the fluid content of the reservoirs. The result of the study shows that the final velocity model has given significant improvement in the imaging and interpretation of the structures within the gas wipeout area. The issue has been compensated in producing a structural image, producing image-gathers, propagating the wavefield throughout the domain and determining the velocity via migration algorithms and tomographic inversion.
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Facies and Bedding Styles in Basin-Floor Fan Deposits of the West Crocker Formation, West Sabah: Implications for Deepwater Reservoir Facies Distribution
The Oligo-Miocene West Crocker Formation in the Kota Kinabalu area, West Sabah, is often described as a sand-rich turbidite system. A field programme was undertaken to study the sedimentary facies of the West Crocker as a possible analogue for the deepwater reservoirs of the NW Sabah Basin and elsewhere. The formation consists predominantly of thick-bedded sandstone facies (beds are commonly >1 m thick) deposited by high-density turbidites. In places, there are the “classical” flysch-like, thin-bedded turbidite sequences deposited on the basin floor. In the thick-bedded successions, sandstone beds are commonly 1.5-3 m thick, while ‘megabeds’ may reach anomalous thicknesses of up to 35 m. The presence of amalgamation surfaces within some of the megabeds suggests that they were produced by multiple flow events. Despite the abundance of thick sandstone beds, there is a general lack of large-scale channelized scours, even at the bases of the megabeds. Almost all the sandstone beds have tabular or sheet geometries. Evidence of channel-fill deposits is found only in the innermost (eastern) part of the system. This suggests that the West Crocker Formation, at least in the vicinity of Kota Kinabalu, represents the non-channelized deposits of a basin-floor fan. The internal architecture of basin-floor fan succession are governed by the vertical (and possibly lateral) distribution of the three dominant facies types. Each facies type is the product of gravity flow events, identified in outcrop as turbidite, debrite and slump. Turbidite beds are dominated by massive to poorly laminated sandstone (Bouma Ta/Tb divisions), which are relatively mud-poor and normally graded with common dewatering features. The beds fine upward into parallel and ripple laminated heterolithics (Tc/Td), which are sometimes burrowed. More commonly, the massive beds have ‘floating’ shale clasts near or at the top, indicating deposition by high-density turbidity currents. Debrite beds generally consist of internally chaotic mud-rich units with scattered shale/mud clasts. They generally have sharp bases and directly overlie the massive sandstone beds, often filling hollows or subtle topographic lows at the top of the massive sands. Debrite beds, of varying thicknesses, tend to overlie turbidite beds with sharp, and often irregular, contacts. A third facies type is slump, which comprise generally muddy or shaly intervals displaying pervasive softsediment deformation (folding and faulting) and remobilization of pre-existing deposits. All three facies types
are intercalated with thin ‘distal’ turbidite (Bouma Tc/Td) and hemipelagic shale intervals. Reservoir architecture and, consequently, heterogeneity of basin-floor fan succession, are ultimately governed by the distribution of these different facies types. Predictive depositional models for these facies types are important for effective reservoir characterization. We observe that the sand-rich West Crocker outcrops around Kota Kinabalu span a distance of 40 km along strike of the Kinabalu coastal plain, and we interpret them as representing the medial part of a basinfloor fan system, which is dominated by the thick-bedded sheet sandstones. Although there may be stratigraphic repetition in this steeply dipping succession, due to the thrust-related deformation, it is estimated
that the outcrops represent a minimum total thickness of 5000 m of stacked mid-fan section. Since there is evidence for major channelization only in the most “updip” (eastern) outcrop, we speculate that the upper/proximal to base-of-slope part of the West Crocker system must occur east of our studied outcrops. By the same token, the lower/distal parts of the system must lie to the west beneath the coastal areas and beyond, and may possibly crop out on the islands off Kota Kinabalu and Klias peninsula.
Hence, the West Crocker Formation is indeed sand-rich (estimated net-to-gross ratio > 70-80%) only insofar as the outcrops around Kota Kinabalu represent the exposed medial fan lobes, and are not necessarily representative of the entire West Crocker depositional system. Further characterization of both updip and downdip sections relative to the more exposed and accessible medial fan belt is required for a more complete understanding of the West Crocker depositional system.
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Sequence Boundary Recognition Criteria in Central Luconia Carbonate Cores
More LessNumerous third to sixth order sequence boundaries were recognized in selected cores from two Central Luconia carbonate buildups. Criteria to recognize a sequence boundary in these cores can be broadly grouped into three categories: (1) the nature of the surface itself, (2) features below the surface, and (3) features above or across the surface. The nature of the surface itself refers to whether the surface is subtle or sharp, apparently planar or irregular/erosive (Fig. 1), and with or without burrows, borings or encrustation. Features below the surface can be subdivided into macroscopic and microscopic features indicative of subaerial exposure and associated meteoric diagenesis and/or karst processes. The macroscopic features include (i) secondary pores such as
moulds, vugs, fractures and fissures, (ii) relict root systems e.g., carbonized rootlets and open or cemented root tubules, (iii) alteration features e.g., breccias with weathered, crumbly and/or chalky appearance or tight texture with brown mottling and/or pinkish colouration, and (iv) karst cavities or cave openings either cemented by speleothems (occasionally multi-phased) and/or filled with mudstone, dolomudstone, sediments or paleosols (Figs. 2 and 3). The microscopic features include (i) moulds and vugs which either still remain open or have been cemented, (ii) pendants or meniscus cements, (iii) bladed and/or equant calcite spar cements, and (iv) vadose silt geopetally filling secondary pores. Features across or above the surface include (i) presence of clasts of underlying rocks or rocks altered by meteoric diagenesis, and (ii) major change in depositional facies, parasequence set stacking pattern and/or microfossil assemblage. In general, the presence of a combination of the above-mentioned features characterizes individual sequence boundaries, and more severe diagenetic alterations or better developed karst features are observed below lower order sequence boundaries.
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Biofacies Assemblages of The Klang-Langat Delta, Selangor, Malaysia
More LessThe Klang-Langat delta is located on the west coast of Peninsula Malaysia, about 80 km from the capital city, Kuala Lumpur (Fig. 1). It is classified as a tide-dominated delta, having a tidal range of up to 4m (Coleman, 1970). This area is selected as one of the analogues for modern delta study because of its distinctive geological setting, accessibility, and the demarcation of the mangrove as a forest reserve. Being highly influenced by tide, the area provides a good analogue for assessing modern biofacies assemblages in the tidal dominated delta system. This paper present the results of foraminifera and palynological analyses on 181 bottom surface sediment and shallow core samples taken from selected transects in the shallow offshore and the lower reach
of Sungai Langat. The samples were collected during four field work programs conducted from April to December 2007. In the field, observations on the composition of vegetation along the rivers were noted (Fig. 2). Other field data measured include water depth, pH, salinity, and turbidity. The data of foraminiferal and palynological analyses were interpreted based on their distribution pattern, abundance and species diversities (Figs. 3 -5). The results were also evaluated by incorporating sediment grain size and other ecological parameters (Table 1). Six biofacies assemblages have been recognized. The biofacies assemblages are freshwater, upper brackish intertidal, lower brackish intertidal, channel banks, tidal channel and tidal flat/delta front.
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Application of High-Resolution Biofacies for Better Depositional Environment Interpretation in the Malay Basin Tertiary Sedimentary Successions
Interpreting detailed environment of deposition (EOD) as accurate as possible for the delineation of reservoir geometry and continuity is crucial in any field development plan, especially in reservoir geological modeling. Conventionally, a conceptual depositional model is developed from the sedimentological study of cored sections and constrained by wireline logs and seismic signatures. If available, any results from routine biostratigraphic analysis will be incorporated to provide additional constraints for the interpretation of EOD. In the Malay Basin, although biostratigraphic analysis is routinely undertaken on most wells, biofacies and sedimentological interpretations are often in conflict, and thus give contradictory results regarding to environment of deposition and salinities. For example, a peculiar anomaly observed in several cored sections shows that in numerous instances, upsection change to shallower water as suggested from sedimentological criteria coincides with increased in salinities based on microfauna/flora. In another observation, coal layers that were interpreted as brackish water coals from sedimentological study can be shown to be autochthonous freshwater coals when analysed using high-resolution biofacies. Our approach to high-resolution reservoir-scale biofacies analysis will be discussed in detail. For reservoir-scale interpretation using bed-to-bed biofacies analysis, biofacies signals are obtained through a carefully designed high-resolution systematic sampling program. Samples are selected from each lithofacies encountered in cored sections and analysed in a manner sufficient to display the temporal succession of biosignals. For each lithofacies, a minimum of three samples were taken to represent lower, middle and upper units, while lithofacies showing a continuous succession were sampled at regular interval. With respect to coals, samples were taken at a much closer interval as to ensure sufficient representative of biosignals are captured. As a result, the overall sample density may become greatly increased, sometimes with samples analysed at 1cm intervals, but more typically a sample every 50 – 100 cm. Since the results answer fundamental sedimentological questions, the work is easily justified. Using this approach, temporal biofacies successions are better understood, and interpretation of EOD is greatly improved to a higher degree of confidence than is possible using sedimentology alone. Also, shales associated with allocyclic and autocyclic sedimentary controls can be readily differentiated, allowing the sedimentary succession to be placed in a realistic sequence stratigraphic perspective. Examples of selected cored intervals from different stratigraphic units in the Malay Basin are discussed.
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Hydrocarbon Generating Potential of Terrigenous Shales and Coals of the West Balingian Province, Offshore Sarawak
Authors Awang Sapawi Awang Jamil and Abdul Jalil MuhamadThe Balingian Province is a proven hydrocarbon province in Sarawak and has been actively explored since the early 1920s. The Balingian Province is subdivided into West and East Balingian sub-provinces which are separated by N-S alignment of sub-basins or depocentres, namely the Acis, South Acis and Balingian sub-basins (Mazlan and Abolins, 1999). The sediments had been subdivided into 8 sedimentary units, called cycles, I - VIII, with Cycle I as the oldest (Ho, 1978). Major oil and gas accumulations are found in the western part of the province, whereas only minor accumulations occur in the eastern part. Geochemical characteristics of most oils indicate generation from mature source rocks containing high proportion of terrigenous organic materials (e.g. Awang-Jamil et al., 1991). Despite active exploration, uncertainty still surrounds the identity of the source rocks responsible for generating the oil and gas. Therefore, in this study, detailed geochemical investigation of the source-rocks of the West Balingian
Province was carried out. The objectives of this study were to determine their hydrocarbon source potential and to delineate the oil generation threshold through assessment of maturity. For this purpose, 90 rock samples (ditch cuttings and cores) from three wells (DA-2, DB-2 and DE-1) drilled in the northwest area of the Balingian Province was subjected to a series of organic geochemical analyses. The two main lithologies analysed were shale and coal, selected from thermally immature to mature sections of Cycles I, II and III of the Tertiary sediments. Rock samples from Cycles IV - VIII were not included because they are generally immature (below 0.4% Ro) and therefore are not thought to have contributed to hydrocarbons accumulation in this area.
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Regularised 1D Inversion of Marine Controlled-Source Electromagnetic Data
More LessWe present an algorithm developed for layered-earth inversion of data acquired using an inline horizontal electric dipole (HED) marine CSEM survey method. The inverse problem is solved using the Tikhonov derivative-regularization and the singular value decomposition method. The iterative scheme starts with a half-space resistivity model and finds the smoothest model that fits the data to 1 rms error. The inversion takes into account data errors, any available a priori information about subsurface structure, nonlinearity of the forward problem, and solution non-uniqueness thus providing a reliable constrained inversion model . Application to synthetic data is presented and a field example is discussed.
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Comparative Evaluation of Frequency and Time Domain Csem Methods for Malaysian Offshore Environments
We have undertaken an evaluation study of the capability of both frequency-domain and step-on time-domain CSEM methods for reservoir detection in both shallow-water and deepwater environments of Sarawak and Sabah in Malaysia. We selected some well sites for this evaluation study. The shallowwater well site (79 m water depth) consists of a 200 m thick reservoir of resistivity 100-200 ohm-m (Ωm) and the background resistivity is 2-5 Ωm. For the deepwater case (1137 m water depth), the well site consists of a 70 m thick reservoir of resistivity 100-200 ohm-m (Ωm) and the background resistivity is 2 Ωm. For modelling purposes, we constructed 4-layer models for each site. We used three frequencies (0.25, 0.5, 0.75Hz) for the frequencydomain method and a transient time window of 250s for the stepon time-domain modelling. The forward modelling studies show that both methods can potentially discriminate between the reservoir and non-reservoir anomalies in 79 m water depth when the resistivity contrast is greater than 20:1. In the deepwater case, the fCSEM and tCSEM methods show similar response patterns.
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Imaging Stratigraphic Channeling with Seismic Attributes in the Malay Basin
In Seismic interpretation a reflection is generally characterized by its arrival time (T) or (F), its reflection strength or amplitude (A) and by its phase. All other attributes are simply linear combination of these three. Each of these attributes represents different aspect of a seismic reflection where in turn brings out different aspects of the geologic features. Amplitude characterizes the reflection strength and can be used in finding sweet spot. A strong amplitude bright spot may be a Direct Hydrocarbon Indicator (DHI) if it is structurally confirmable. An amplitude shutoff may be an indicator of a hydrocarbon water contact. Frequency on the other hand signifies resolution to detect thin beds or an attenuation effect indicative of gas. Phase is the most sensitive of all the attributes and is primarily an indicator of structural discontinuities like faults, unconformity or pinch outs and other geomorphological features.
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3-D Tomographic Q Inversion for Compensating Attenuation Anomalies
Authors Kefeng Xin and Barry HungFollowing our previous work on Amplitude Tomography that deals with amplitudes alone, we extend our effort to include the compensation of bandwidth and phase of seismic signals that are distorted by seismic attenuation. Our new approach involves utilizing tomographic inversion for estimating the quality factor (Q) from prestack depth migrated common image gathers. By filtering the seismic data into different frequency bands and measuring the effect of attenuation on amplitudes in each band, the frequency dependent effect, which was ignored in our previous work, of attenuation is fully taken into account, allowing Q to be estimated from our tomographic method. By using the estimated Q volume in one of the migration methods that incorporates Q in the traveltime computation, we demonstrate, through examples, that our workflow provides an optimal compensation solution that resolves amplitude and bandwidth distortions due to seismic attenuation.
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Real Time Depth Determination for Drilling Using Seismic While Drilling Checkshots
By Aqil AhmedReal-Time Checkshots on a 3 Well / 3 Block Exploration Campaign help to accurately determine depth of key Formation marker offshore East Asia
CHALLENGE
An offshore 3 block / 3 well exploration campaign needed to determine the depth of a key formation marker as early as possible to reduce risk and save time. This meant acquiring good checkshots even inside casing on a vertical well. The target also had a possible pressure ramp associated with it.
SOLUTION
By acquiring checkshots while drilling using the seismicVISION* tool ,a real time update of the timedepth relationship was acquired thus the depth of the target was known accurately ahead of time and key drilling decisions made accordingly.
REDUCING UNCERTAINTY
An Operator in East Asia ran the seismicVISION* service to determine the depth of a key formation marker on 3 separate Exploration Blocks that were being evaluated. With an initial depth uncertainty of +/- 100m from the surface seismic, the client wanted to reduce this to less than 20m as early as possible so that casing could be set safely and accurately in a formation that had possible pressure ramp and well stability issues with possible pack-offs if the hole remained open for too long. Additionally as 2 of the 3 wells were in a Deep Water environment the need to make accurate real time drilling decisions was even more imperative. seismicVISION* - SVWD The seismicVISION* service acquires checkshots while drilling whilst taking up minimal rig time. Windowed Real Time Waveforms are transmitted uphole from the LWD seismic tool using mud pulse telemetry after acquiring the seismic station at connection, an acoustically quiet period. The source is a standard 3 air gun cluster at surface deployed from the rig. The checkshots are then processed from the waveform at the wellsite and real time depth updates calculated whilst drilling.
PRE-JOB PLANNING & OPERATIONS
Detailed pre-job modeling between Operator and SLB, and checkshot acquisition close to the target, enabled highly accurate updated depth prediction using Real Time answers products at the wellsite after checkshot processing.
RESULTS
For all 3 vertical Exploration wells in 3 different blocks, the target depth was predicted to within 10m each time allowing safe and successful drilling with casing being set at the correct point. These depths were confirmed by subsequent Wireline logging runs.
Well TD’s were set during drilling on the basis of the hydrocarbon sand depth prediction.
CONCLUSION
By employing seismic while drilling technology, the Operator was able to establish accurately, the depth of a key formation marker during drilling, hence reducing risk and uncertainty on exploration wells leading to a successful exploration campaign.
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Addressing Petrophysical Issues in a Low Resistivity Pay Environment with Modern Spectroscopy Logs and Nmr
Low Resistivity and Low Contrast (LRLC) Pay environments present significant problems for accurate petrophysical evaluation. The subject of this paper is a typical SE Asia LRLC reservoir with variable resistivity values in the pay zone which can sometimes be lower than those in the water zones. Low resistivity values are generally due to variable clay content. Variable clay content in the reservoir affects both the neutron and density logs to such an extent that using a basic logging suite porosity evaluation, fluid
identification and reservoir quality assessment are adversely affected, particularly for operational decision making. Clay conductivity effects in conjunction with low salinity formation water mean that resistivity-based saturations are frequently pessimistic and are heavily dependent on estimates of clay content. All of the basic petrophysical outputs are therefore compromised in some way in these environments. The addition of modern Spectroscopy logs on two recent wells has shown that some of these evaluation problems can be overcome by using elemental abundances derived from spectroscopy to define the response of the neutron and density logs to the rock matrix. Current generation Spectroscopy logs measure several major rock forming elements which impact the bulk properties of the rock such as grain density. After matrix correction on the basic logs, porosity values can be better computed and hydrocarbon identification is improved. Clay content predicted from the elemental Spectroscopy logs appears to show more consistency than predictions from other Vclay estimators- all of which have specific weaknesses. The presence of NMR in the logging suite addresses deficiencies in the assessment of reservoir quality and saturation through direct measurement of irreducible water and indirect correlations to permeability-shown to be reliable in the subject reservoir. The combination of NMR and spectroscopy corrected density porosity gives potentially the best estimate of porosity. The combination of Spectroscopy and NMR logs with the basic logging suite significantly enhances petrophysical evaluation capabilities in any environment where clay content and distribution is variable and difficult to estimate.
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Velocity Modeling in Gas Sagging Area
Authors Sia Chee Chuan and Munji Syarif and Achmad NurhonoVelocity model provides direct linkage between time domain and depth domain. Time to depth conversion of interpreted surfaces, faults and seismic data is an important part in the 3D geological modeling for volumetric assessment. The data used for velocity modeling may include seismic velocity data, well velocities (from checkshot, VSP, and/or synthetic seismograms), well markers, and surfaces. Conventionally geophysicist uses checkshot data, stacking velocity, or checkshot integrated with stacking velocity directly. These methods are widely used for depth conversion in a normal area (i.e. without gas sag or pull-up effect in surface seismic interpretation). In this project, the existence of shallow gas cloud in the area of study is causing sag effect instead of
anticlinal structure on the seismic data. The common solution for the gas sag is manually interpreting the anticline at the affected area. However, manual seismic horizons interpretation to correct the sag to anticline is not applicable for this project as seismic inversion work is involved. The manual correction in seismic horizons interpretation will introduce an artifact in the seismic inversion process. Thus the seismic horizons interpretation has to honor the sag effect. The depth conversion from the conventional
velocity modeling method above would not able to solve the gas sag effect, where the sag exists instead of anticline in the depth domain. A simple but effective workflow is introduced to overcome the problem where the seismic velocity was corrected first, and then calibrated with the well velocity. Considering a point - ‘X’, which is unaffected by shallow gas beside the gas sag is chosen as reference for time and velocity (TWT difference factor derivation and seismic velocity correction). A TWT difference factor is derived between the point ‘X’ and gas sag. Then the seismic velocity is corrected with the TWT difference factor, thus remove unreliable velocity within the gas sag area. This corrected seismic velocity is then calibrated with well velocity and the latter result is used as an input for velocity modeling. The structural dip of depth converted structure surfaces are QCed with average dip from the OBMI to ensure no anomaly on dip changes. The residual depth of the velocity model is within 10m.
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The Occurrence and Structural Characteristics of “Brick-Structure” Coal Fractures (Cleats) in Mukah-Balingian, Sarawak, Malaysia: a Probable Fault Reactivation Feature
Authors Wan Hasiah Abdullah and Sia Say Gee and Wong Yien LimThe Tertiary sequence of Northwest Borneo was gently folded with NNW- to SSE-trending axes during the Middle Miocene and the subsequent folding and uplift during the Pliocone-Pleistocene affected all the older stratigraphic successions (Haile, 1969). In this current study, a NNW- trending “coal-brick structure” (Figure 1) has been identified and is postulated to be related to the Pliocene-Pleistocene reactivation of preexisting structural fabrics. This interpretation is based on a number of supporting structural features which include the similar NW-trending fault bounded graben and horsts in the onshore Mukah area and the adjacent offshore area (Mazlan Madon, 1999). The “coal-brick structure” occurs at two locations; in the northern part it occurs in the Upper Miocene
Balingian Formation close to the contact with the Lower Pliocene Bergih Formation, while in the south it occurs in the Upper Pliocene Liang Formation close to the contact with the Eocene Belaga Formation (Figure 2). A few faults have been mapped close to the brick structure. Based on the similar orientation of the NWtrending of the long fractures (face cleats) of the “coal-brick structure” to the fault at the northern area, as well as being similar to the NW-trending adjacent offshore regional structural trend, it is postulated that the occurrence of the “coal-brick structure” is an indication of a fault-bounded zone associated with folding, uplift or unloading of the overburden, and could indicate proximity to an unconformity. It is observed that there is a variable cleat strike postulated to be within the main faulting region close to a hinge of the fold, while further away from the hinge the “brick structures” are larger in size and display a prominent long fracture pattern parallel to the NW-trending regional structural trend including the offshore West Balingian bounding fault line. It is postulated that the folding and faulting and the associated “coal-brick structure” in the Balingian and Liang formations point to possible structural reactivation of the Early-Middle Miocene regional faults and structural fabric within the Mukah- Balingan and adjacent offshore areas. Similar fracture patterns that are parallel to pre-existing regional faults has also been observed in basalt of Quaternary age at Pantai Batu Hitam, Pahang which has been postulated to point to the possibility of reactivation of regional Pre-Tertiary faults in the Malay Basin region (Tjia, 2008). The Pliocene formations (Begrih and Liang) rest directly on the Eocene Belaga Formation. Such major unconformities are common in onshore and offshore Sarawak. In the offshore area adjacent to the Mukah-Balingian region, an angular unconformity that rests directly on the Oligocene-Early Miocene (Cycle I-Cycle II) occurs at the base of Cycle VI/VII in Balingian Province (Mazlan Madon and Abolins, 1999). In
the adjacent Tatau Province, Cycle III/IV of Early–Middle Miocene strata were reported by Mazlan Madon and Redzuan Abu Hassan (1999) to rest directly on the Paleocene-Eocene basement (equivalent to onshore Belaga Formation). The shallow depth or immediate overlying proximity of the Pliocene Liang to the Eocene Belaga Formation may facilitate the Liang inheriting the Belaga fault directions and associated structural fabric upon reactivation. An important implication of the development of these cleats is their potential to act as conduits for hydrocarbons such as for coalbed methane gas.
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A Comparative Source Rock Study of Two Proven Petroleum Systems: The Marine Madbi Formation of Yemen and the Terrestrial Nyalau Formation of Sarawak, Malaysia
More LessTwo contrasting petroleum systems have been evaluated and compared. Marine shales of the Jurassic Madbi Formation in the Masila basin, Yemen (Fig.1a) and Tertiary coals and organic-rich sediments of the Nyalau Formation (offshore stratigraphic equivalent to the Cycle I & II of Balingian Province,) in Sarawak, Malaysia (Fig.1b) have been subjected to detailed organic petrological and organic geochemical studies. An assessment, based on organic facies characteristics, has been carried out on these sediments, in order to distinguish, characterise and evaluate source rocks deposited in marine versus paralic depositional sitting. The methods employed include evaluation of organic carbon content (TOC), biomarker distributions, pyrolysis-gas chromatography analysis and petrographic data. The maturity assessment of the samples analysed is mainly based on vitrinite reflectance (%Ro). The organic geochemical and organic petrological approach here has been able to clearly differentiate between marine and terrestrial depositional setting. Organic facies parameters such as Tm/Ts, Pr/Ph, Pristane/n-C17, Phytane/n- C18 and oleanane/C30 hopane ratios appear to reflect variation in depositional conditions and/or source input. Although there is a mixture of land-derived and
marine-derived organic matter in both sediments, the depositional conditions of these formations can be distinguished based on these organic facies parameters, whereby the Madbi shale samples were deposited in a reducing suboxic marine condition while the terrestrial Nyalau sediments in suboxic to oxic paralic condition of deposition (Fig.2) The Madbi shale samples possess vitrinite reflectance (% Ro) values ranging from 0.74-0.88% thus indicating an early mature to peak mature range, while the Nyalau sediments possess vitrinite reflectance values of 0.50-0.66% which suggest the samples are early mature for oil generation. The level of thermal maturity attained by these samples is also reflected in their biomarker distributions as indicated by the approximate 60/40 ratio of the S to R isomerisation of the C31 and C32 hopanes (Fig.3). With regard to oil generation potential, good source rock potential is suggested by the high TOC values for Madbi shales and the organicrich sediments of the Nyalau Formation, as well as owing to their liptinite-rich nature (based on petrographic data) (Fig.4) and Py-GC dominated by n-alkane/alkene doublets (Fig.5). Within early to peak oil window maturity, the Madbi shale would be expected to be a better source rock for oil as indicated from its higher abundance of Type I and II kerogen compared to the Nyalau Formation which are dominated by Type II and III kerogen. Based on this study, good oil/gas generating potential is anticipated from the coals and carbargillite/coaly shales of the Nyalau Formation, owing to predominant n-alkane/alkene doublets and aromatic compounds (Fig.5).
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Coastal Facies Sand Composition and Beach Dynamics on Pulau Pangkor to Assess the Impact of Potential Oil Spills
More LessThis research is conducted in Pulau Pangkor to assess the impact of potential oil spills by studying the coastal facies sand composition and beach dynamics of the island. This study includes mapping coastal facies with remote sensing, field sampling and analyzing the sand composition.
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3D General Surface Multiple Prediction on Full Azimuth Coil Shooting Survey
Authors Andreas T. Waluyo and Steven Wiseman and Dmitry NikolenkoOffshore Kalimantan, Indonesia in the Tarakan Basin, seismic illumination has been a problem for interpreters for many years due to its complex geology. During the summer months of 2008 a full azimuth Coil Shooting survey of approximately 536 sqkm was conducted over the Tulip field. Unlike conventional race-track acquisition, in which the vessel traverses a straight line, with Coil Shooting the vessel traverses a circular pre-plot. This geometry fully sampled the full azimuth range to overcome a number of geological issues which resulted in very poor target imaging. With acquiring a true full azimuth survey benefits signal sampling it also acquires a multiple dataset fully sampled in azimuth. Due to the water-bottom shape and reflectivity complexity of the field, the resulting seismic data contained strong and complex surface-related multiples that conceal the underlying primary seismic energy. Due to the crossline rugosity of the water-bottom the downward Reflection Points (DRP) of the multiple travel paths are not in line with the source and receiver points creating 3D multiple effects. It is well known that the effect of azimuth errors on multiples is greater then on primary events. In theory, conventional 3D Surface Related Multiple Elimination (3D SRME) can provide an accurate solution to these types of multiples. However for Coil processing, conventional 3D SRME as typically implemented will not produce an accurate multiple prediction model. Without an effective de-multiple process, the resulting image from data processing can provide limited information. Recently WesternGeco introduced 3D General Surface Multiple Prediction (3D GSMP*) as a solution to attenuation of multiples in this type of environment. 3D GSMP is a practical approach to 3D SRME that can handle the irregularities in real life survey geometries and provide a superior multiple model prediction. It takes care of 3D effects and works in true azimuth. Without 3D GSMP, strong residual multiples obscure the seismic image of Tulip Coil data, making any kind of seismic interpretation extremely difficult. 3D GSMP produced a final image with minimal or no residual multiples. This paper will illustrate the successful use of this approach.
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Preliminary Assessment of the Coalbed Methane Potential of the Mukahbalingian Coal Field, Sarawak
Authors Sia Say Gee and Wan Hasiah AbdullahWorldwide increase in coalbed methane development began in approximately 1988, prior to this coalbed methane was considered a safety hazard and was intentionally vented to the atmosphere to prevent mine gas explosions. In 2007, the United States has produced more than 50 billion cubic meter of pipeline quality coalbed methane (EIA, 2007). To date no effort has been made to explore the coalbed methane potential in the Malaysia’s coal basins. Early coalbed methane exploration targeted thermally mature high rank coals, but the study carried out by Bustin and Clarkson (1998) on a series of Australian, Canadian and United States coals have indicated that there is no or little correlation between coal rank and methane adsorption capacity as commonly assumed.
With the successful development of coalbed methane in the Powder River Basin, San Juan Basin and the Greater Green River Basin of the United State of America (SanFilipo, 2000, Nuccio, 2001, Breland, 2004), low rank coals have also become exploration target. Basically, at the current state of knowledge on coalbed methane, the study of coalbed methane is an empirical process (SanFilipo, 2000). Coalbed methane is believed to be generated during three distinct stages of the parent coal's maturation history: Stage 1, early biogenic gas due to bacterial activity during the conversion of peat to coal, Stage 2, thermogenic gas due to volatilization of coals constituents as rank increases, and Stage 3, late biogenic gas due to bacterial activity after the coal has been
uplifted to near surface (Rice, 1993). The Mukah-Balingian Coal Field consists of Mukah Coal Basin hosting the Miocene Balingian Formation and Balingian Coal Basin hosting the Pliocene Liang Formation (figure 1). These coals are of low rank whereby the vitrinite reflectance of the exposed Mukah- Balingian coal ranges from 0.34%-0.54% (Chai and Wan Hasiah, 2004) indicating the Mukah-Balingian coals are at the early thermogenic methane generation stage (figure 2), which usually unable to generate enough
methane for development. However, the possibility of the coals also accumulating migrated thermogenic and late biogenic methane, which will enhance the development potential of the methane, especially for the Mukah Coal Basin, cannot be discarded. The deeper coals at Mukah Coal Basin may have already reached the peak of wet gas generation (Mazlan and Abolins, 1999), where the thermogenic methane generated has migrated up along the coal seams or faults, together with the late biogenic methane produced at near surface by the bacterial activity after the coal has been uplifted could have enhanced the coalbed methane development potential of the coal basin. The distribution and orientation of cleats in coal will serve as permeability pathways for migration and accumulation of thermogenic methane generated during coalification. Coal seams at Mukah-Balingian Coal Field have a well-developed cleats network (figure 3); with the face cleats trending NNW to NNE and the butt cleats approximately perpendicular to it. The coal resources of the Mukah and Balingian Coal Basins have been estimated to be of 550 million tonnes (Sia and Dorani, 2000) and 200 million tonnes (Hussein and Dorani, 2000) respectively. At a pessimistic gas storage capacity of 2 m3/tonne and an optimistic gas storage capacity of 10 m3/tonne, as of the biogenic gas in Soma lignites, Turkey (Inan, 2008), have translated the coalbed methane in place from 1,100 million m3 to 5,500 million m3 for the Mukah Coal Basin and from 400 million m3 to 2,000 million m3 for the Balingian Coal Basin, respectively.
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Variation in Biomarker Distributions for a Lower Coastal Plain Source Rock Sequence in the Malay Basin
In this study, an attempt is made to integrate biomarker parameters with biofacies interpretations determined from palynomorph and foraminifera assemblages, sedimentological features and bulk geochemical properties. This is to investigate the biomarker signatures for various depositional environments. For this purpose, a well preserved 100-meter cored section from the Tangga Barat-3 well drilled in the Malay Basin was selected. This section transects lower coastal plain depositional environments within the Late Middle Miocene. Vitrinite reflectance measurements ranging from 0.4 – 0.5% indicate that the source rock intervals are immature for hydrocarbon generation. The immaturity of these sequences makes identification of biomarkers more challenging.
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Integrated Approach to Maximizing Asset Value in the Mature Palas Field, Malaysia
The Palas field, located offshore Peninsula Malaysia, was discovered in 1977 and has been producing for over 20 years. The field was developed in two stages with an initial drilling program in 1985 focused on Lower Miocene Major I reservoirs with Minor I reservoirs as secondary targets, and a subsequent drilling program in 2000 focused on Group J reservoirs. With maturing production from the Major I and Group J reservoirs there has been a shift in focus towards the less depleted Minor I reservoirs. This paper describes the evolution of Minor I reservoir drilling targets, from secondary objectives with opportunistic completions, to key targets in a recent infill drilling campaign. The Lower Miocene Minor I reservoirs are comprised of tidally influenced, lower delta plain
sandstones and occur as discrete channels to amalgamated channel complexes. Reservoir compartments are separated by intervening shales resulting in multiple fluid contacts and oil columns ranging from 15 to 70 meters. Seismic imaging is challenged by the relatively thin nature (5 – 10m gross thickness) of the sandstones and numerous interbedded coals. Reservoir-scale mapping is primarily based on well data. Opportunity generation was guided by mixed but encouraging production performance from the
sparse oil completions taken in the Minor I reservoirs and included collaborative Geoscience and Reservoir Engineering construction of 3D geologic and reservoir simulation models to high-grade infill drilling opportunities. This effort resulted in several proposed development wells in the Minor I reservoirs, two of which were drilled as part of a recent infill program. Encouraging results from these two wells are being used to further the understanding of the Minor I reservoirs and mature additional infill opportunities.
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