Shales: A review of their classifications, properties and importance to the petroleum industry

Shales are fine-grained, laminated or fissile clastic sedimentary rocks with predominance of clay and silt as the detrital components. They may be classified as clayey, silty or sandy shales on the basis of texture. Other criteria used in the classification of shales include mineralogical composition, cementing materials, organic matter content, depositionalenvironment and strength. Generally, shales have moderate to high clay content (average, 57%), low strength (range, 5-30MPa), low permeability(range, 1 x 10-6 - 10-12 m/s) and are water  sensitive(susceptible to hydration and swelling when in contact with water).Shales are important to the petroleum industry because of their usefulness as source rocks in petroleum generation, seals in petroleum traps and reservoirs. Problems associated with drilling oil/gas wells in shale formations include slow rate of penetration and wellbore instability. These problems are generally caused by pressure gradients between the oil/gas wells and shale formations, and shale hydration due to drilling fluid/shale interactions. The problems can be prevented or controlled by adequate monitoring of the drilling fluid density and use of potassium-base drilling fluid that is insensitive to shale hydration. In the Niger Delta petroleum province, the source rocks and seal rocks are the marine/deltaic, plastic and over-pressured shales of Akata and Agbada Formations