Uncertainty analysis of hydrocarbon in place calculation using 3D seismic and well data during appraisal stage – Case study of Goldie Field, offshore Sarawak

Abstract

Goldie Field located offshore Sarawak was previously estimated to be less than 200 Bscf of gas initially in place (GIIP) and well-4 of a major reservoir in the field was classified as a dry hole. In this study, the uncertainty of hydrocarbon in place of a major reservoir in Goldie Field is estimated with limited seismic and well data to determine if an oil column is present in the reservoir. 3D Stacking velocity incorporated into well velocity from check shots were analysed and the best method or equation for time-to-depth conversion was determined. Gross bulk volume (GBV) was developed using area depth method. Both deterministic and probabilistic calculations were conducted and analysed with the uncertainties. The plot of true vertical depth (TVD) versus two-way time (TWT) velocity function derived from seismic interpretation time at well location and velocity function derived from pre-stack time migration (PSTM) were used to determine the accuracy of the time-to-depth conversion. Pickett and Hingle graphical solution equation was used to derive the water resistivity (Rw). Log base saturation height function was calculated using Cuddy's method and Harrison-Skelt's equation. The study has produced new depth structure maps that shows a highly correlated values up to a regression coefficient of 0.99 and seismic attributes that assisted in modelling of the reservoir in Goldie Field. Check shot analysis shows that a linear function Vavg versus TWT and a second order polynomial function for TVD vs TWT are accurate for seismic time conversion. Revaluation of well logs and other well data has produced new porosity, water saturation, net to gross ratio and formation volume factor. Well-4 was classified as a dry hole in PETRONAS in-house report, but the cuddy's plot for R3 sand in well-4 indicates existence of low saturation gas. This study has produced an estimated GIIP of 360 Bscf for R3 sand and well-4 has penetrated deeper sands previously interpreted to be wet. The seismic interpretation and petrophysical evaluation showed that Excel graphics and mathematical analysis improved the quality of the analysis, and the results show good comparison to other specialized software provided by vendors.