Azimuthal Anisotropy Evaluation Based on Wide Azimuth 3D Data for Fracture Analysis in Chonnabot Carbonate Reservoir

Abstract

Abstract Chonnabot area is located in northeast Thailand. The main reservoir target in northeast of Thailand is Permian platform carbonate. The Chonnabot structure is a four-way dip closure, large elongate in shape with NNE - SSW trending anticline. The present-day stress orientation of the Khorat Plateau is measured and interpreted to have a maximum horizontal stress (Shmax) in N-S to NNW-SSE direction. The existing wells indicate that quality of carbonate reservoirs depend on fractures, which provide secondary porosity for hydrocarbon accumulation and consequently identifying fracture network within reservoir body will be very crucial for prospect evaluation. The existing 2D data is unsuitable to detect fracture in this area and unreliable for imaging of complex structures beneath the Top Carbonate layer. Thus, higher standard seismic data such as wide azimuth 3D volume is really needed to find high fracture density zones within the platform facies and identifying Permian platform carbonate at target level. In order to facilitate fracture analysis in Chonnabot structure PTTEP has acquired for an approximately 200 km2 of wide azimuth 3D dataset over Chonnabot area in year 2009 with aspect ratio 0.86, maximum offset 4,375m and mean fold coverage 187. Offset Vector Tile (OVT) based PSTM processing workflow is adopted to obtain reliable seismic quality data for azimuthal anisotropy analysis. Using stress orientation information from well data as reference the migrated OVT gathers from processing are azimuthally split into sectors. Sector dependent velocity analysis is performed to individual azimuth sector to generate sub-stacks, full azimuth stacks, angle stacks, OVT gathers and azimuth-dependent velocity cubes. Seismic anisotropy analysis techniques cover amplitude variation in incident angle and azimuth (AVOAz), travel time and interval velocity anisotropy. AVOAz is to observe amplitude variation at certain incident angle range (especially far angles) as a function of azimuth. To predict fracture direction using AVOAz technique, a synthetic AVOAz model is generated from well data as reference for calibrations. According to this model amplitude variation in near incident angles due to azimuth is considered insignificant but the variation occurs more obviously in far incident angles data range. The lowest magnitude of amplitude based on velocity anisotropy model represents the direction of the fracture. Interval velocity anisotropy is mainly focus on azimuthal velocity variation and this variation is obtained from NMO velocity data which are picked from various azimuth sectors. Amplitudes, travel time and interval velocity attributes extracted from each azimuth sector at target levels are combined together and used it as initial information to model azimuthal anisotropy effect from WAZ data by fitting them to ellipse model of anisotropy velocity (Greckha and Tsvankin, 1998). Ellipse fitting of interval velocity dataset is to estimate "fast and slow trends" of horizontal velocities. Ellipse fitting of amplitude dataset is used to estimate "maximum and minimum trends" of azimuthal amplitude variation, while ellipse fitting of travel time dataset is used to determine "maximum and minimum trends" of azimuthal travel time variation. The outcome of fitting process is combined together to map the behaviour of the seismic anisotropy from wide azimuth data to predict fracture's orientation and intensity. The maps are normally presented as magnitudes of max-min trend differences of the observed anisotropy attribute based on certain approach and overlaid with arrow icon to indicate fracture orientations. In relation to fracture orientation, for simple fracture system normally the azimuth direction of fast velocity trend represents the direction of the fracture. High magnitude of amplitude indicates high fracture density and short travel time difference indicates low fracture density. Combination of interval velocity, travel time difference and magnitude of amplitude attributes will provide better information to analyse seismic anisotropy effect and their possible relationship with fracture orientation and intensity.