Basin prospecting using porosity and permeability from surface outcrop samples

Porosity and permeability values have been traditionally determined by post-drilling methods, namely wireline logging, conventional core analysis, and drill-stem tests. Determination of porosity and permeability index using a pulsed nuclear magnetic resonance (pNMR) device on drilled cuttings has been routinely carried out, both as a well-site and laboratory service. However, in a predrilling evaluation stage, it was felt that porosity and permeability analysis on surface outcrop samples could contribute to an integrated geological prospect evaluation. Surface outcrops of claystone, sandstones, and oolitic limestones and dolomite from the Olleros area, Salta Province, Argentina, were sampled and analyzed. The pNMR sample analyzer measured the absorption of energy by hydrogen nuclei contained within the pores of the rock sample. The amount of absorption is proportional to porosity. Permeability and free fluid indices are determined by measuring and comparing the rates at which the hydrogen nuclei return the absorbed energy. Using results from pNMR analysis, four sample populations were identified using pore space, pore size, and pore abundance criteria, which suggested potential seal rock, microfractured claystones, and potential reservoir rocks.