Resumen

CASTRO-CASTINEIRA, Olga; GARCIGA-ACOSTA, Anabel  y  HERNANDEZ-LEON, José. Electrotypes identification for sandy-clayey rocks in formations of the Cuban North Oil Belt. Bol. cienc. tierra [online]. 2022, n.52, pp.37-47.  Epub 13-Mar-2023. ISSN 0120-3630.  https://doi.org/10.15446/rbct.n52.102033.

The study of seals and reservoirs is the main objective of petrophysics, in which borehole geophysical logs play an important role. The best option to characterize formations in wells is the lithological description of cores and channel samples, which is complemented by the continuous information provided by the well logs. Among the sealing rocks, clays are the mostand those that are mostly clayey, and also are characterized by low resistivity values, very high porosity, high natural radioactivity values, and the presence of landslides in conventional logs, whereas, in borehole image logs, the darkest colors of the range are typical. In practice, these characteristics are not as evident in all rocks that are defined as clay seals because their composition and properties vary, and it is often a problem to identify from the log display the formation to which these rocks belong. The Placetas Tectonic Stratigraphic Unit within the North Cuban Oil Province is made up of a series of formations. The Constancia Formation is considered a transition between the synrift clastic rocks and the carbonate rocks of the formations that constitute reservoirs within the Unit, while Vega Alta is the synorogenic cover that preserves the hydrocarbons of the productive formations.

The Constancia and Vega Alta formations are usually confused in the wells, in the complex geology of mantles and scales in which they appear. In this work, the available geological-geophysical elements are integrated to study the differences and similarities of the clay rocks in different contexts, and the identification with the mentioned formations is analyzed. The cross-property plots of geophysical logs help in the lithological identification in conjunction with the representation of the composite logs. A prominent role in the characterization is played by the image log because it allows for defining the texture of the rocks. Finally, the variation of the different geophysical logs can be summarized in the star plots as a statistical study of each analyzed package. As a result, four lithological groups of clayey rocks were characterized and were called electrotypes because of their relationship with the well profiles, graphs, and images and their relationship with the Vega Alta, Constancia, or other clayey formations in other tectonostratigraphic units.

Palabras clave : lithology; well logs; formations Vega Alta and Constancia.

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