Cause and timing of the thermal over-maturation of hydrocarbon source rocks of the Ecca Group (Main Karoo Basin, South Africa)

Abstract

The source rocks of the Main Karoo Basin, South Africa are believed to have undergone thermal over-maturation close to the Jurassic-aged Karoo Large Igneous Province (KLIP) dolerite intrusions. Yet, the cause and timing of this source rock exhaustion event likely to have extensively destroyed their shale gas potential remain controversial. Based on drill core samples from the central Main Karoo Basin, we investigated the clay mineralogy of the 2:1:1 and 2:1 phyllosilicate groups, and have carried out 40Ar/39Ar radiometric dating of illite. The X-ray diffractograms (XRD) and mineral chemistry reveal that chlorite and illite phases of samples close to the dolerite intrusions are characterized by higher crystallinity with a chemical composition indicative of formation in a metamorphic environment. In contrast, chlorite and illite distal to any dolerite intrusion show lower crystal growth rates with chemical compositions suggesting formation in a diagenetic environment. 40Ar/39Ar radiometric dating indicates that illite in samples distal to the dolerite intrusions formed between 292 and 245 Ma with some ages clustering around 281 Ma. The oldest ages are consistent with a detrital fraction probably recycled from the erosion of the ash bed at the top of the underlying Dwyka Group. However, ages around 281 Ma can be linked with estimated depositional age of the Ecca Group at ∼280 Ma, while the youngest ages may reflect late diagenesis to regional metamorphism. Illite fractions from samples close to dolerite shows formation age between 204 and 170 Ma, consistent with the time of the KLIP dolerite sills and dykes intrusions at ∼180 Ma. This illustrates that the ages of these illites were partially or totally re-set due to recrystallization in a late thermal anomaly induced by the intrusion of dolerites. The correlation between the timing of the youngest mineral transformation processes and the KLIP represents strong evidence suggesting that source rocks of the Ecca Group experienced a thermal overprinting event ∼180 Ma ago. This late thermal anomaly is consistent with previous data illustrating thermal over-maturation and metamorphic devolatilization of Ecca Group shales situated close to the dolerite intrusions.