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Semi-Quantitative SEM Analysis of Vaca Muerta Formation, Neuquén Basin, Argentina


More than 100 samples were studied in thin section and with a scanning electron microscope (SEM) from samples taken from two long cores (>200m) from the organic-rich Vaca Muerta Formation in the central Neuquén Basin, Argentina. One core is in the light oil window (Ro ~1.0) and the other core is in the dry gas window (Ro ~1.6). All features described were estimated semi-quantitatively and captured in Excel so they can be plotted with logs and correlated within a high-resolution sequence stratigraphic framework. There are at least ten different rock types found in the Vaca Muerta with different reservoir characteristics which are vertically partitioned within the high-frequency sequences. The most favorable facies are silty organic-rich mudstone, radiolarian and coccolith pellet wackestone and the least favorable facies are dolomite, laminated lime mudstone and volcanic ash beds.

Pore types found include: organic, leached feldspar, interparticle, moldic, intraparticle and intra-clay. The most abundant pore type is organic porosity, but about half of the total porosity found was matrix porosity. Organic pores are present in both cores but are larger and better developed in samples from the same facies in the core from the dry gas window suggesting that further maturation has led to development of more pores and enhanced existing porosity. Organic pores developed within organic matter and those lined with bitumen are considered to be part of the reservoir because the presence of bitumen indicates that hydrocarbons have migrated into those pores. Pores with no OM lining them may or may not be part of the reservoir and may be water saturated.

Kerogen can be discriminated from migrated bitumen by its appearance. Kerogen has associated clay and is typically found in subparallel laminations. Bitumen migrated out of the kerogen in earliest stages of maturation and it plugs pores and decreases permeability in the heavy oil window but with increasing thermal maturity organic pores grow larger and more abundant and eventually make up the main porosity system in the lighter oil and gas windows.

There are multiple types of clay found in the reservoir. Most of the clay is in the volcanic ash beds, but there are some beds where terrigenous clay is common. In the lower, more organic-rich parts of the formation, most clay in the samples is found in intraclasts that were probably ripped up from the ash beds that have less of an impact on reservoir quality than clay disseminated in the matrix.

Diagenetic quartz is very common in both cores and appears to be more common in intervals with higher organic content. Diagenetic quartz crystals are significantly larger (up to 10μ) in the core from the dry gas area and these larger crystals occur in seams of bitumen with large organic pores. The ideal landing zones for horizontal wells may be where the seams of coarser diagenetic quartz crystals and bitumen with large pores are most abundant.