Abstract: Past and Present Oil Migration Pathways in the Yacoraite Formation, Argentina

Cesaretti, N. N. - UNS Argentina; Parnell, J. - QUB, UK; Dominguez, E. - UNS, Argentina

Pores, microfractures and fractures were all migration pathways for organic fluids in the Yacoraite Formation (Cretaceous-Paleocene) in the NW Cretaceous, Lomas de Olmedo Oil Basin, in Salta, Argentina. Four drill cores from Pozo Escondido (x-1, e-2, e-4) and Escondido Este (x-1) (64°, West Long., 24° South Lat.) were studied for the distribution of hydrocarbons. Different migration pathways functioned to varying degrees during the basin history.

Lomas de Olmedo is an Early Jurassic rift basin whose subsidence started during the Cretaceous. The end of the subsidence is recorded by the Yacoraite Formation, the most important oil bed. It is integrated by quartz sandstones, mudstones, boundstones and grainstones that act as a reservoir and source rocks.

In sandstones pore sizes were partially reduced first by quartz overgrowths, then by anhydrite cementation, and finally by bitumen as droplets and grain-coatings in the reservoir rocks. In the chalk level the pore sizes were increased by dolomitization and reduced by anhydrite cementation and bitumen precipitation.

Microfractures functioned only in the past. In the chalk level they crosscut some quartz grains in a very fine grain carbonate matrix. The microfractures were generated by high fluid pressure caused by organic matter maturation and were sealed by silica cementation. These microfractures are considered to have been primary migration pathways. In the sandstone level the microfractures crosscut detrital quartz grains and predated silica cementation. The microfractures were generated by high fluid pressure caused by migration of the oil present in the pores and were sealed by silica cementation in the presence of oil (Fig. 1). These microfractures are considered to be secondary migration pathways. In both cases the microfractures host asphaltic hydrocarbon fluid inclusions.

Fractures function only in the chalk level at the present time. They are open and clean and were generated by dilation. These fractures host the oil produced from the well.

The predominant mechanism of secondary migration in the sandstone level was as globules or bubbles of oil driven by water movement (Fig. 2). Both aqueous and hydrocarbon fluid inclusions are observed. The distorted oil globules (Fig. 2) could be attributed to a greater buoyant force than the pressure inside the globule, forcing the globules to move through pore throats. When this pressure exceeded the mechanical strength of the rocks, microfractures in quartz were developed and secondary oil migration was induced.

Present day migration is probably linked to open pores and fractures, mainly in the limestone level which hosts the present napthenic oil and has no evidence of bitumen cement.

In this basin there is a relationship between oil maturation and the migration pathways. Initially a heavier oil migrated through the pores and microfractures. Then the heavier fraction (asphaltic bitumen) remained in the pores and the lighter fraction moved through the microfractures. After cementation reduced the porosity, fractures became the pathways for the present napthenic oil migration.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil