--> ABSTRACT: Basin Prospecting Using Porosity and Permeability from Surface Outcrop Samples, by Jorge H. Poncetta, Adolfo Gonzales Torres, and John MacPherson; #91022 (1989)

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Basin Prospecting Using Porosity and Permeability from Surface Outcrop Samples

Jorge H. Poncetta, Adolfo Gonzales Torres, John MacPherson

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.

The relationship of porosity as a function of depth within the basin was reviewed in a comparison between data from wireline logs and surface outcrops. The compaction trend for claystones from a sonic log was developed. The porosity data from claystone outcrop samples was then projected onto the compaction curve at the appropriate stratigraphic levels within the Olmedo-Tunal and Yacoraite Formations. Results confirmed a similar compaction trend for the surface outcrops.

Surface outcrop samples for basin evaluation can now be analyzed for porosity and permeability index. These data, along with surface geochemical and surface radioactivity techniques, can assist in successful hydrocarbon exploration.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.