Post Drill Contacts Analysis: A Detailed Analysis on the Degree of Uncertainty and Reliability of Amplitude-Based vs. Stochastic Trap Analysis and Risking (STAR) Based Hydrocarbon Fluid Contact Prediction

Abstract

Abstract In general, unconsolidated Tertiary deltaics in Niger Delta are characterized by a strong amplitude hydrocarbon response and are used successfully to predict hydrocarbon fluid contacts. Accurate definition of fluid contact is critical at every stage of prospect maturation. In areas, where amplitude support is inadequate, Stochastic Trap Analysis and Risking (STAR) tool has proven to be a reliable substitute in calculating the fluid contacts. STAR is based on Shell's global hydrocarbon column height database that shows a correlation between column height and fault shale gouge ratio (SGR). The programme calculates the SGR of trap defining faults in a Petrel 3D model and predicts a low-mid-high column height range for each reservoir. In this study, we compared the pre-drill column heights derived from seismic amplitudes and STAR with well results at XY-South Prospect for various reservoir sands. The study showed in general that the actual HC columns were consistently smaller than the prognosed results. A postdrill HC column height estimation study was carried out to address the under estimation of column heights. Findings indicate that the HC columns are strongly dependent on the fluid phase and gradient used. The predrill STAR estimate was for oil with a default gradient of 0.35psi/ft. However, the actual fluids found were more gas rich and the oil had lower gradients than the STAR default values, hence shorter predicted columns. Also a comparison of the amplitude-based versus STAR-based contact predictions was made on this XY-South prospect. Predrill STAR predicted contacts are very close to the predrill amplitude estimated contacts (differences are between 12 to 20ft). Thin oil rims cannot be resolved on the amplitude depth plots (GOC vs OWC).