Influence of igneous intrusions on thermal maturity of Late Cretaceous shales in the Tuma well, Chad Basin, NE Nigeria

This study evaluates the influence of igneous intrusions on the thermal maturity of Late Cretaceous (Turonian–Santonian) potential source rock shales within the Gongila and Fika formations penetrated by the Tuma well. Twenty representative shale samples of the two formations between 1000 m and 3195 m were analyzed by bulk organic geochemical and palynofacies methods.A positive excursion in Rock-Eval Tmax values and an increase in the percent of non-fluorescent amorphous organic matter (AOM) were observed between 1500 m and 2900 m. These anomalies are moderately correlated with the abundance of igneous intrusive fragments. At depths greater than 2900 m where igneous intrusive fragments are absent, Rock-Eval Tmax values return to the same trend as observed above 1500 m and the percentage of fluorescent AOM also increases. These observations suggest that the thermal maturity of the shales between 1500 m and 2900 m was elevated by the heating due to the emplacement of the intrusives.Further studies should be undertaken to differentiate reliably the effects of regional burial and those of more localized contact metamorphism, and to determine the effects of each on the maturation of the Late Cretaceous potential source rocks.

► Late Cretaceous shales of Chad Basin affected by igneous intrusions.
► Cuttings samples with igneous rock fragments exhibit higher Tmax values.
► Elevated ratios of non-fluorescent to fluorescent amorphous kerogen.
► Tmax results are compatible with published vitrinite reflectance data.