Application of Spectral Decomposition Technique to Delineate the Evolution of Karst on Carbonate Platforms of Central Luconia, Offshore Sarawak, Malaysia

Karstification in carbonate platforms of the Miocene age in Central Luconia province, offshore Sarawak, Malaysia, has been discussed since the onset of exploration and initial discoveries in the region, with over 200 mapped platforms to date. An extensive drilling program over the last decade confirmed the existence of karst during the drilling process where issues such as total loss circulation and bit drops were common. Karst in Central Luconia has been proposed by several authors; however, detailed quantitative description of the observed features have not yet been conducted. This study involves systematic mapping of loss circulation depths, chalkified/rubble/vuggy zones described from cores, and vugs of >2 mm in size and moldic porosity observed on thin sections of the Jintan platform. These data supplement the interpretation of karst from multiple 3D seismic attributes. Seismic interpretation of the Jintan and M1 platforms revealed an extensive dendritic pattern which is on average 70–100 m deep and 3–5 km long, and circular geobodies of 1 km in width that exist on the upper part of the platform. Spectral decomposition, also known as time-frequency analysis, was used to enhance the interpretation of karst features on seismics within a specific wavelength. In this study, a comparison of three spectral decomposition methods applied on the 3D seismic cube of the Jintan and M1 platforms was undertaken to determine the method which allowed for better delineation of the karst features. The results show that the short-time Fourier transform (STFT) method using frequencies of 46, 54, and 60 Hz delineated most of the karst features compared to the continuous wavelet transform (CWT) Morlet and CWT Ricker wavelet methods. This paper aims to discuss the dimensions, evolution and geometry of the karst features quantitatively on three selected karst horizons named “K1”, “K2”, “K3”. Interpretation revealed that the dendritic karst features were found to be most prominent on the K2 horizon which lies below a conspicuous change of the external geomorphology of the platform. Backstepping of the platform margin by 12 km is observed in both platforms. Quantitative seismic interpretation shows that the karst observed in M1 platform is approximately 70–100 m deep, and the dendritic features are around 1–2 km in length and approximately 500 m wide; whereas, in the Jintan platform the dendritic features observed are up to 5 km in length with several 1 km wide circular/sinkhole features. More than 20 dendritic features orientated SE and NS were mapped mainly in the transitional area as well as the center of both platforms. The nature of the karst morphology in Central Luconia remains controversial; however, it is proposed to be of mixing zone karst origin.

Shallow vs. Deep Subsurface Structures of Central Luconia Province, Offshore Malaysia Reveal by Aeromagnetic, Airborne Gravity and Seismic Data

Across the Luconia continental shelf, the nature and structures of the crust are lacking geological understanding and precise characterization. Newly acquired, aeromagnetic, and airborne gravity data were used to assess deep and shallow sub-surface signals within the Central Luconia Province, off the coast of Sarawak, offshore Malaysia. Regional aeromagnetic anomalies appear to primarily reflect deep crustal features while depth (Z) tensors of airborne gravity anomalies evidence shallow subsurface structures. Strike directions of the interpreted structural trend on aeromagnetic and airborne gravity anomalies maps are measured and plotted into rose diagrams to distinguish the structural orientations for all datasets. Signature patterns extracted from the depth profiles were correlated with parallel seismic lines and nearest exploration wells and coincide well with the top of carbonate for Cycle IV/V and structures seen within the Cycle I and II sediments. The orientation of faults/lineaments at shallower depth is dominated by a NW-SE orientation, similar with the faults extracted from two recently published structural maps. Deeper subsurface sections yielded E-W to NWW-SEE dominant directions which were never presented in the published literature. The E-W oriented anomalies are postulated to represent the remnants of the accretion between the Luconia crustal block and southern boundary of the Palawan block. The NW-SE trend follows the same direction as prominent faults in the region. The insight into shallow and deep subsurface structures in Central Luconia Province imaged through airborne gravity and aeromagnetic data should provide guidelines and complementary information for regional structural studies for this area, particularly in combination with detailed seismic interpretation. Further evaluation on the response of Air-FTG® gravity and aeromagnetic could lead to the zonation of potential basement highs and hydrocarbon prospects in this area.

Seismic expression of miocene carbonate platform and reservoir characterization through geophysical approach: application in central Luconia, offshore Malaysia

A substantial proportion of proven oil and gas reserves of the world is contained in the carbonate reservoir. It is estimated that about 60% of the world’s oil and 40% of gas reserves are confined in carbonate reservoirs. Exploration and development of hydrocarbons in carbonate reservoirs are much more challenging due to poor seismic imaging and reservoir heterogeneity caused by diagenetic changes. Evaluation of carbonate reservoirs has been a high priority for researchers and geoscientists working in the petroleum industry mainly due to the challenges presented by these highly heterogeneous reservoir rocks. It is essential for geoscientists, petrophysicists, and engineers to work together from initial phases of exploration and delineation of the pool through mature stages of production, to extract as much information as possible to produce maximum hydrocarbons from the field for the commercial viability of the project. In the absence of the well-log data, the properties are inferred from the inversion of seismic data alone. In oil and gas exploration and production industries, seismic inversion is proven as a tool for tracing the subsurface reservoir facies and their fluid contents. In this paper, seismic inversion demonstrates the understanding of lithology and includes the full band of frequency in our initial model to incorporate the detailed study about the basin for prospect evaluation. 3D seismic data along with the geological & petrophysical information and electrologs acquired from drilled wells are used for interpretation and inversion of seismic data to understand the reservoir geometry and facies variation including the distribution of intervening tight layers within the Miocene carbonate reservoir in the study area of Central Luconia. The out-come of the seismic post-stack inversion technique shows a better subsurface lithofacies and fluid distribution for delineation and detailed study of the reservoir.

FEP Based Model Development for Assessing Well Integrity Risk Related to CO2 Storage in Central Luconia Gas Fields in Sarawak

Underground storage of CO2 in depleted gas reservoirs is a greenhouse gas reduction technique that significantly reduces CO2 released into the atmosphere. Three major depleted gas reservoirs in Central Luconia gas field, located offshore Sarawak, possess good geological characteristics needed to ensure long-term security for CO2 stored deep underground. Long-term integrity of all the wells drilled in these gas fields must be ensured in order to successfully keep the CO2 stored for decades/centuries into the future. Well integrity is often defined as the ability to contain fluids without significant leakage through the project lifecycle. In order to analyze the risk associated with all 38 drilled wells, that includes 11 plugged and abandoned (P&A) wells and 27 active wells, probabilistic risk assessment approach has been developed. This approach uses various leakage scenarios, that includes features, events, and processes (FEP). A P&A well in a depleted reservoir is a very complex system in order to assess the loss of containment as several scenarios and parameters associated to those scenarios are difficult to estimate. Based on the available data of P&A wells, a well has been selected for this study. All the barriers in the example well have been identified and properties associated with those barriers are defined in order to estimate the possible leakage pathways through the identified barriers within that well. Detailed mathematical models are provided for estimating CO2 leakage from reservoir to the surface through all possible leakage pathways. Sensitivity analysis has been carried out for critical parameters such as cement permeability, and length of cement plug, in order to assess the containment ability of that well and understand its impact on overall well integrity. Sensitivity analysis shows that permeability of the cement in the annulus, and length of cement plug in the wellbore along with pressure differential can be used as critical set of parameters to assess the risk associated with all wells in these three fields. Well integrity is defined as the ability of the composite system (cemented casings string) in the well to contain fluids without significant leakage from underground reservoir up to surface. It has been recognized as a key performance factor determining the viability of any CCS project. This is the first attempt in assessing Well Integrity risk related to CO2 storage in Central Luconia Gas Fields in Sarawak. The wells have been looked individually in order to make sure that integrity is maintained, and CO2 is contained underground for years to come.

Assessing Controls on Isolated Carbonate Platform Development in Central Luconia, NW Borneo, from a Regional 3D Seismic Facies and Geomorphology Investigation

The Central Luconia province in the South China Sea, offshore Sarawak, features extensive development of Middle to Late Miocene isolated carbonate platforms. This study presents a regional seismic architecture - seismic facies review of platform flank geometries and off-platform depositional styles, with the goals of understanding their patterns and exploring the controlling processes. The information on flank steepness and predominant shedding direction is contextualized with respect to extrinsic and intrinsic factors of carbonate platform growth, including tectonics, eustatic sea level fluctuations, hydrodynamics and regional paleogeography. Results reveal that flank geometries are consistent throughout the Middle to Late Miocene. In the north of the province eastern flanks are dominantly aggradational, steep, and sediment-starved escarpments, whereas western flanks show more progradation and are accretionary in nature. Discrepancies from this pattern are observed among closely spaced platforms. The eastern aggrading flanks are interpreted to have been highly influenced by monsoonal wind driven currents. Further south in the province tidal currents, antecedent topography and syn-depositional tectonics were more important controls on platform architectural development. This study of regional processes and geomorphic products provides a basis for enhanced conceptual facies models and reservoir quality predictions.

Unravelling an abandoned giant in Central Luconia Province, offshore Sarawak, Malaysia — Success story of Lang Lebah

In 1994, two exploration wells were drilled consecutively to explore for gas prospectivity in Lang Lebah, a Miocene carbonate buildup in the geologic province of Central Luconia located in the Sarawak Basin in Malaysia. High overpressure and operational problems prevented both wells from fully evaluating the target. Postdrill analysis concluded that Lang Lebah has limited potential due to poor reservoir quality, small gas column, and challenging drilling conditions. For these reasons, it was left dormant for 25 years. In 2016, new 3D broadband seismic acquisition and megamerge reprocessing of 3D seismic data sets followed by an integrated application of multidisciplinary workflows successfully derisked key petroleum system elements of the Lang Lebah structure, yielding a more optimistic view of its potential. A new well was justified at Lang Lebah and resulted in one of the major gas discoveries of 2019.

A new approach to petroelastic modeling of carbonate rocks using an extended pore-space stiffness method, with application to a carbonate reservoir in Central Luconia, Sarawak, Malaysia

Pore geometry plays an important role in the elastic response of carbonate rocks. Diagenetic processes in carbonate sediments generate a range of pore-type distributions. Hence, the petroelastic modeling (PEM) of carbonate rocks is more complex than for clastics. Petrophysical properties connect to elastic properties through PEM or, in general terms, rock-physics modeling. Pore types cause variation in P-wave velocity — up to 40% for a given porosity. A variety of pore types with different aspect ratios such as vuggy, moldic, interparticle, intraparticle, fracture, and crack makes the porosity-velocity relationship complex, and empirical models fail to handle it properly. We propose a new, easy-to-implement approach for PEM of carbonate rocks that leads to more accurate elastic properties estimation. It offers a novel PEM method that reduces the number of defined parameters and equations. In it, the Xu-Payne rock-physics modeling equations are replaced with an extended pore-space stiffness equation. Instead of including a pore's aspect ratio as is done when using the Xu-Payne inclusion model formulation, in our proposed technique only the appropriate value of pore-space stiffness for each pore type is considered, together with the corresponding volume fraction of pore types. However, parameters are optimized by calibrating the estimated elastic properties with corresponding information from well-log measurements. This inclusion model yields acceptable predictions of elastic properties at wells that do not have measured elastic logs. The method was tested using well data from a carbonate reservoir in Central Luconia, offshore Sarawak, Malaysia. Here, one well has a complete suite of log data needed to calibrate the model. The calibrated model was then used to predict the missing shear velocity log in the other well. Next, simultaneous elastic seismic inversion was performed on 3D seismic data covering the area of the carbonate reservoir, and elastic property volumes (acoustic impedance and VP/VS ratio) were estimated. From these results, a posterior probability distribution of stiff pore types was determined, which validated the outcome of this approach using a blind test.