Inferring Hydrocarbon Migration and Sealing Potentials of Some Shales of the Anambra Basin (South-Eastern Nigeria) from Their Paragenesis and Geotechnical Index Properties

Shale samples from designated units of the Imo, Nsukka, Mamu, Enugu and Nkporo Formations of the Anambra Basin were subjected to X-ray diffraction and geotechnical index tests to determine the potential of the Formations to either create hydrocarbon migration pathways or to form seals for hydrocarbon entrapment. The X-ray diffraction results indicate that the shales are made up of silicate minerals with carbonates and sulphates. Results of the geotechnical index tests showed that the plasticity index (PI) for the Enugu shale ranges from 16 to 24 (average of 20) while the toughness index (TI) ranges from 0.4 to 0.8 (average of 0.7). The PI for the shales of Imo and Nsukka Formations ranges from 62 to 72 and 34–59 with averages of 67 and 46 respectively while their TIs range from 0.8 to 1.7 and 1.5–2.2 with averages of 1.3 and 1.9, respectively. These properties dispose the Enugu shale to easy amenability to loss of cohesion, deforming in a brittle manner, when subjected to shearing stress with the ability to create joints and faults that are pathways of hydrocarbon migration. Conversely, the properties of the shales of the Imo and Nsukka Formations give them the capacity to absorb high shearing stress without loss of cohesion thereby deforming in a ductile manner, which prevents the leakage of fluids in fluid migration. Hence, Enugu shale has the highest potential of creating hydrocarbon pathways while the shales of the Imo and Nsukka Formations are better seal rocks than others.