EXPLORATION • DRILLING • FIELD SERVICES
and/or faults interpretation. Proportional horizons were used for iso-stratigraphic property during automatic calculation for stacking interpreting 3D horizons.
GRID CONSTRUCTION, STRUCTURAL AND FACIES PALEO- TREND GRIDDING Initially the potential reservoir zones were identified using the integrated seismic analysis (ISA) within the horizon modelling phase. Tis incorporation ultimately placed the zone boundaries of all modelled layers of the study field.
Although the data provided for the study was limited only to a seismic cube, incorporation of present-day deposition pattern within the block and previously studied depositional settings were used as inputs for the FPTM trend to be developed. Tis study was completely based on geometrical inputs that control particular body shapes (thickness/width, etc.). Te facies modelling workflow for the studied reservoir followed three robust steps based on the abovementioned technical steps. During the initial stage, depositional-bodies were generated using FPTM algorithm; then the boundaries of the interpreted geobodies were defined; finally, the internal geometry and heterogeneity of the facies were developed to generate the final FPTM of the study reservoirs. Tree horizons were interpreted for the study using OrbStrata’s advanced geomodelling algorithm. An empirical geodatabase incorporating seismic amplitude anomaly along with amplitude normalised factor (ANF) was used to determine the target horizons for the study. As the study was based exclusively on seismic data (in depth), the determination of ANF helped delineating probable lithofacies classes within the studied field. For the study, amplitude class was developed based on the zone of interest and exclusively implicates the classification within the zone boundaries. Structurally the reservoir zone
interpreted in between the top and bottom reservoir surfaces is devoid of any faults. It is a classical four-way dip closure structure with an average orientation of the structure being N25E and S220W. Te structure is thick at the central part of the structural model and thin on the sides of the structure. Te average azimuth of the structure is N40°E.
PALEO-DEPOSITIONAL MODELLING Te subsurface structure, its horizons and the seismic facies interpreted all lead in the same direction in re-evaluating the paleo- depositional settings of the field. From the seismic facies, it is observed that the facies probability model (P50 to P90 sands), from bottom to the top horizon indicates two major sediment supply directions (one from the Northwest and the other from Northeast, as evident from the facies geomodel) in the modelled grid. Analysing the seismic facies carefully, it was concluded that the facies tend to get dirtier (mixed sand to shale in nature) from the bottom layers gradually towards the top layers of the model. Tis also signifies that the facies altogether might have retrograded (transgression) to the younger successions (within the studied interval). Te facies geomodel also depicts a probable paleoshoreline shift parallel to the initial NNE-SSW orientation down further South towards the present-day coastline of Brunei. Te algorithm used a combination of sequential indicator simulation (SIS) for flexibility in defining different variables within the model, truncated Gaussian simulation (TGS without any preferential trending) along with DSSB’s own facies paleo-trend modelling (FPTM) algorithm
to develop 3D seismic-based facies putting more weight of the seismic amplitude anomaly for determining logical facies distribution spatially. It is also to be noted that the study used all available modelling parameters only with the help of seismic data (in depth domain). Taking into consideration the current limitations set for the study, the output geomodel(s) that it has produced are of considerable accuracy, as are the subsurface settings within the target zone of the study field. Determining three-dimensional geometries of the interpreted facies was a key component of this study. As the facies and facies classes were directly associated with the quality of the prospective reservoir zone, facies geometries were carefully interpreted. According to the facies associations and literature review from previous studies performed on this area, the study concluded that the prevalent depositional setting of the area is offshore deep-sea fan sedimentation. It was also evident from the seismic facies modelled within the facies model. From the topmost surface of the target reservoir zone, it was evident (considering the amount of data provided for the study) that different sand facies distributed in a fan shape along the Southwest and Southeast portion of the model.
Structural orientation of the interpreted horizons
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