Advanced Spectrometry and NMR Based Porosity Partitioning Integrated Technique in Evaluating Cretaceous Pinda Formation, Offshore Angola
Authors: Jacob Claire , Dutta Dhruba J. , Grau Jim , Abangwu Prince , Baig Mirza Hassan , Saikia Prodip , Santos Celia C. 
Published: January 2012

Characterization of complex Cretaceous reservoirs of offshore Angola, Lower Congo basin, poses similar uncertainties of evaluating carbonates in other parts of the world. The formation is a mixture of clastics, with carbonates at varying fraction that changes with depositional environment. With horizons of complete carbonates and fraction of dolomite and anhydrite, it poses a real challenge to any Petrophysicist engaged in evaluation of these oil bearing formations. The primary evaluation problem becomes even more complicated with porosity related issues since the primary oil reservoirs in general fall in the carbonate sections with moderate porosity and with varying secondary porosity like vugs and occurrences of fractures at different intervals. There is often a significant difference in permeability between testing results and those estimated from formation evaluation results or computed from formation tester data. These observations have also been verified with core results. A need for integrated evaluation could therefore be easily reckoned, with proper matrix evaluation to generate a better estimation of intergranular porosity and a complementary method for accurate estimation of secondary porosity from vugs, and then incorporating the results to generate a more realistic permeability.

Apart from evaluation challenges with conventional logs, the drilling mud system and logging dynamics generates some environment effect on the acquired data which impedes the efficient use of the data. Photoelectric factor is an important measurement in complex lithology, but its accuracy is very sensitive to mud systems and to borehole rugosity. The method of estimation of carbonates based on elemental yields from the new generation elemental logging devices has evolved significantly and in recent years magnesium can be separated out of the concentrations to provide a major input for estimating dolomitic fraction out of total carbonate in the formation. The remaining issue of porosity partitioning has been developed since 1995 with the incorporation of nuclear magnetic resonance (NMR) data. For carbonates, the porosity systems were assumed in a completely different way to accommodate the larger vugular fraction in the evaluation schemes. This new approach has been generated based on the knowledge and experience acquired in evaluating carbonates throughout the world. The approach in estimating permeability therefore takes advantage of the computation of vugular pore fraction from NMR. The recent technique provides significant accuracy while correlating with other answers from testing and core results.

Improvement of the results depends on the data acquisition and uncertainty can be reduced by proper planning. The acquisition for the current case was forced to a limit since the drilling was carried out with very saline mud. Although, higher salinity for mud and formation water is always environmentally challenging to logging, a proper approach in cancelling the effect of borehole chlorine can generate very reasonable log quality. In the current analysis for Pinda carbonates, the authors utilize the carbonate-dedicated workflow comprising of elemental spectroscopy and nuclear magnetic resonance data acquired in an offshore well. Integrating all data and information with a proper workflow, realistic estimation of permeability was possible with a rock typing framework based on calibrated porosity partitioning.