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ABSTRACT
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, depositional environment and strength. Generally, shales have moderate to high clay content (average, 57%), low strength (range, 5-30MPa), low permeability(range, 1 × 10^sup -6^- 10^sup -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.
KEY WORDS: Shales, Classification, Strength, Composition, Petroleum Industry, Niger Delta.
INTRODUCTION
Shales are fine-grained laminated or fissile clastic sedimentary rocks with predominance of silt and clay as the detrital components (Krumbein and Sloss, 1963). They are formed from silts and clays that have been deposited and compacted or hardened into rocks. On the basis of texture therefore, the most common types of shales are silty shale (silt dominant) and clay shale (clay dominant). These two types of shales are also called argillaceous shales. Occasionally, shales may also contain appreciable amounts of sands in which case they may be called sandy shale (arenaceous shale). Black shales with high proportion of organic matter content are called carbonaceous or bituminous shale. Shales that contain high amount of lime are known as calcareous shale.
Shales are the most abundant of all sedimentary rocks (constituting about 60%), and are distributed in wide range of geologic ages from Paleozoic to Cenozoic (Boggs, 1995; Greensmith, 1975). Their colours may range from white through green to...