Quantifying the uncertainty of Gross Rock Volume: a decade of time-to-depth conversion in Sepat Field, Malay Basin

Gross Rock Volume (GRV) range and uncertainty estimation from depth maps is important for resource assessment, and represents a significant contribution from seismic interpretation into the reservoir model. Subsurface uncertainty comes from different factors such as errors in time and depth maps, number of wells drilled, property and facies modelling. This paper focuses on the sensitivity of GRV to various velocity models as the implementation of seismic technologies changed from 2002 to 2013 and as conventional time-to-depth migrated multi-component data was used for depth conversion. GRV estimates from different depth maps are captured and utilized for volumetric and structural uncertainty analysis. Sepat Field was discovered in 1983 and has been interpreted structurally as a 30 km long by 10 km wide, E-W trending, four-way dip closure dissected by normal faults. Like many of the oil and gas fields in the Malay Basin, it is significantly affected by shallow gas clouds especially in the crestal area of the structure (Figure 1). Shallow gas clouds cause amplitude and frequency attenuation and time structure sagging effects which lead to significant uncertainties in structural and stratigraphic mapping of reservoirs below the shallow gas area, and subsequently in the hydrocarbon resource assessment of these fields (Ghazali, 2013). A decade of various seismic vintages and studies are applied to the field in order to achieve better and more accurate depth predictions and quantify uncertainty. As a solution to the shallow gas cloud issue, a 4C seismic survey was acquired in 2012 with the objective of recording both compressional and mode converted shear wave (PS). Acquisition and Processing of PS mode conversions enables improvement in subsurface imaging and subsequent reservoir characterization (Barkved et al., 2004).