Seismic net-to-gross estimation for a geologic model update: A case study from a turbidite lobe reservoir in the deepwater of the Niger Delta

Abstract

Geological model updates are routinely performed in mature fields to obtain improved descriptions of facies distributions and reservoir properties such as volume of shale, net-to-gross (NTG), porosity, etc., for better understanding of the static and dynamic behaviors of reservoirs for effective well placement, improved production, and monitoring. A simple but integrated seismic NTG estimation approach, using detuned seismic amplitudes was employed in guiding NTG modeling during the geological model update of a thin turbidite lobe reservoir in a mature oil field in the deep offshore Niger Delta. The objective was to address NTG overestimation and gross rock volume (GRV) uncertainty in a previous model arising from seismic amplitude tuning effects. A seismic NTG approach was chosen relative to sophisticated deterministic or stochastic inversion techniques to avoid tuning effects which usually bias NTG estimates in thin turbidite reservoirs. The primary dataset was a 2019 reprocessed Prestack Depth Migrated (PSDM) seismic data vintage which had better resolution, higher signal-to-noise ratio (SNR), and more appropriate angle-stack apertures for amplitude versus angle (AVA) fidelity, than the older 2011 PSDM seismic data that was used in the previous geological model. The methodology involved rock physics analysis, seismic data quality checks, tuned area determination, detuning of composite seismic amplitudes of the top and base reservoir, and their direct calibration to NTG at wells. Good correlations were obtained between the detuned composite seismic amplitudes and NTG at wells. The seismic NTG map showed good calibrations at wells and provided a robust trend for net sand modelling in the oil pool and aquifer. Static model QCs and dynamic simulations proved that the seismic NTG attribute addressed the GRV uncertainty in the earlier model, thus giving confidence for use of the updated model for the planning and geosteering of infill wells, sand completions, reservoir monitoring, and production.