Formation evaluation in thinly-bedded sands has always been a challenge especially as individual sand beds thin below the resolution of logging tool measurements. Evaluation of these reservoirs is often further complicated since the various log measurements have differing vertical resolutions and depths of investigations and the wells normally have somewhat disparate log datasets.
In this study of a field located in the Gulf of Mexico (GoM) Deepwater the authors will demonstrate how a consistent and cross-verified approach was taken to evaluate thinly-bedded sands which resulted in increasing interpretation confidence and a more realistic net pay when compared with prior techniques that utilized conventional log interpretations.
The field for this study has a total of five wells including one well with whole core, three wells with nuclear magnetic resonance (NMR) logs, including the cored well, and two wells with standard log suites. The methodology applied begins with creating a continuous sand count curve from the visual evaluation of whole core photographs. This sand count curve is then compared with the net-to-gross (NTG) curve obtained from a Thomas-Stieber analysis and then cross-verified via NTG obtained from a NMR log. The calibration from the three wells with whole core or NMR was applied to the Thomas-Stieber analysis performed on the two wells with standard suites of logs. Whole core porosities and water saturations are then used to constrain the log-derived porosity and water saturation values. Finally saturation height modelling has been performed using porous plate capillary pressure data and compared to log-derived water saturations.