Application of Seismic Attributes in Estimating Gas Resources in Complex Fluvial Channel Reservoirs

Abstract

Abstract The purpose of this paper is to show the reserve estimation procedures used by KUFPEC on the NW Shelf of Australia production license WA-49-L, which is currently under development as part of the Chevron's Wheatstone LNG project. The gas in the WA-49-L permit was discovered in a combination of structural and stratigraphic traps of a regional fluvial channel sand complex within the Triassic Mungaroo Formation. These channel sands contain gas trapped on a large structural ridge with up-dip terminations below a regional, angular unconformity and overlain by the Cretaceous Muderong Shale. Some of the deeper channel sands are trapped against the bounding fault of the ridge. These channel sands are stacked as a result of amalgamation of numerous channels, with the individual sand thickness of up to 60 meters but typically in the 15 to 30 meters range. These channel sands are discrete with individual gas-water-contacts, and clearly visible on seismic and inversion data as low impedance, Class 3 AVO (amplitude versus offset) anomalies. The estimation of gas resources is delineated by 20 wells drilled through April 2011 on WA-49-L. Limitations of the seismic frequency content result in channel thicknesses of less than 5 meters not being visible. Seismic inversion attributes are used to estimate gas sand probability, sand thickness and porosity. Rather than using large scale net-to-gross assumptions commonly used in the industry for reserves calculations, KUFPEC used maps of individual sand bodies based on the inversion impedance amplitudes and gas sand probability (GSP) attributes to compute each sands' likely range of resources values.