Clay minerals consist of a fine-grained material which is classified according to their crystal structure into four major groups: kaolinite, chlorite, illite, and smectite. These minerals are important constituents in many types of rocks, like shales and sandstones. The last one is considered a very common type of reservoir rock for oil and gas.
The clay presence in reservoir rocks, such as sandstones, can significantly affect one’s capacity to extract oil from these reservoirs. This happens because many rock properties are commonly dependent on the clay content within the rock pore space. The occurrence of clays implies erroneous measurements of physical properties that are important for oil and gas exploration; therefore, without the knowledge of the clay types and its distribution within rock pores, one struggles to describe the quality of reservoirs. As an example, rocks with high porosity and high permeability are promising candidates for reservoir rocks.
Studies revealed a severe reduction in permeability and porosity readings due to the presence of smectites. The same phenomenon is significantly reduced when the clay mineral is kaolinite. So, if a rock sample is full of smectite, one should be aware of the mentioned problems and be really careful during the definition of the exploration region.
How to identify clay minerals in rocks
There are several methods to identify different types of clay minerals, mostly based on the analysis of different physical and chemical properties of each type of clay. A very important one is the Cation Exchange Capacity, referred to here as CEC, which is the ability of a clay mineral to adsorb cations in a  saline solution. Each group of clay mineral presents a different CEC response when placed in a saline solution, as seen in the table.
Unfortunately, CEC laboratory readings are financially demanding to accomplish and in most cases, the rock samples of a target are not available for such studies.
What is proposed in this study?
To work around the previously-mentioned drawbacks, a few researchers proposed alternative empirical methods to compute CEC values without the need for rock samples and laboratory measures. In this research, an empirical expression was obtained from old CEC measurements collected from rock samples in the Mexican Gulf. The idea consists of using a volume of shale data, which is easily acquired in a well log investigation, compared with laboratory data to fit an exponential curve of how CEC responds to the expected volume of shale. This methodology was then applied to Namorado Oilfield, one of the most important exploration fields of the Campos Basin, located in Southeast Brazil.
The studies made by our team revealed that there is a massive predominance of kaolinite in the selected depth interval, with thin layers of illite and chlorite at shallow zones. More importantly, there is no presence of smectite in the region studied, which is quite promising for exploration of oil and gas in the region. These findings show that the empirical method worked fine for the studied area and could be considered for other similar regions in the world.
These findings are described in the article entitled Estimation of clay minerals from an empirical model for Cation Exchange Capacity: An example in Namorado oilfield, Campos Basin, Brazil, recently published in the journal Applied Clay Science. This work was conducted by Monique Lopes da Silva, Jorge Martins, Mario Ramos, and Rodrigo Bijani from the ObservatĂłrio Nacional.