An Integrated Approach to Calibrated Shale Volume Calculations, from Example from Niger Delta Oil Field, Nigeria

John Bosco Onyeji¹, Norbert E. Ajaegwu¹, Frederick Ogboi², Prince Abangwu², and  David Nwabor^2
1 Department of Geological Sciences, Nnamdi Azikiwe University, Awka, Nigeria.
² DSC, Schlumberger, Lagos, Nigeria.

Clastic hydrocarbon-bearing formations in the Niger Delta commonly contain shaly sands as well as sandstones, making the distinction between productive and nonproductive intervals difficult. The relative ease of identification and quantification of hydrocarbons in these shaley and sometimes low-porosity reservoirs has changed with the exciting development of algorithm used in computing composite shale volume. This represents a minimization of the uncertainties that associates with traditional shale indicators commonly used in the Niger Delta Basin.

The methodology of composite shale volume was developed after systematic evaluation of the shale indicators commonly used in the Niger Delta, considering uncertainties associated with each one. Putting the uncertainties into consideration, long interval was subdivided and assigns separate end point values. Neutron and density logs were used in combination to remove porosity effect and correct neutron-density separation values was assigned based on fluid content of the sand package. In areas affected by washouts, resistivity interpretation was employed since modern resistivity logs are rarely affected by washouts. The composite shale volume algorithms were developed using the best values of the results obtained from the selected shale indicators. The composite shale volume generated was calibrated using shale volume calculation standards- X-ray diffraction (XRD) and Nuclear Capture Spectroscopy (NCS) measurement which are direct and indirect mineralogical measurements respectively. When this information is combined with other measurements, definite improvements in the accuracy of effective porosity, water saturation and productivity predictions are observed and stratigraphic interpretation is significantly enhanced.

Consequently, the applications of this approach have successfully quantified shale volumes independent of washout and light hydrocarbon effects; determined permeability and bound fluid based on mineralogy; lead to the modification of completion techniques in clastics based on improved clay type and volume and provided stratigraphic information for enhanced geologic correlation and reservoir modeling.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009