Oil-generation characteristics of Mesozoic syn-rift Madbi source rock in the Masila Basin, Eastern Yemen: New insights from kerogen pyrolysis and bulk kinetic modelling

Abstract

Organic-rich shale layer is present within the syn-rift Madbi Formation of the Masila Basin, representing excellent oil source rock. Twenty-five representative immature to very early-mature source rock samples from the Upper Jurassic Madbi shale sequences were investigated using bulk and quantitative pyrolysis techniques. The analytical program included TOC, pyrolysis (SRA), pyrolysis GC, bulk kinetics, and kerogen microscopy. The results were used to characterize the kerogen composition and type of petroleum generated during thermal maturation from Madbi source rock and to clarify the variability of the organic matter and their effect on the timing of petroleum generation.Pyrolysis data identified two major organic facies with different petroleum generation characteristics; Type II kerogen with HI values of >400 mg HC/g TOC; Type II/III facies with HI values <400 mg HC/g TOC. This is supported by the abundant liptinitic materials (i.e., alginite, structureless (amorphous), and spores). Pyrolysis GC analysis on extracted rock samples show a dominance of a marine aliphatic-rich organic matter with variable contributions from transported terrestrial organic matter and proved to produce mainly paraffinic–naphthenic–aromatic oils with low wax contents.Bulk kinetic analysis on Madbi shale samples indicate that the Type II facies mostly derived from a homogenous organic matter, spans relatively narrow distribution of activation energy (50–54 kcal/mol) and predicted petroleum formation temperature is very different, with onset (TR 10%) temperatures of ∼125 °C (geological Tmax ∼138 °C). In contrast, the Type II/III facies reveals a relatively broad distribution of activation energies (40 kcal/mol to 62 kcal/mol) that are directly related to the heterogeneity in the organic matter, onset (TR 10%) temperatures of ∼106 °C (geological Tmax ∼139 °C). The bulk kinetic analysis also suggested that the peak generation temperatures (geological Tmax) are variable and ranging from 135 °C to 142 °C, reaching a significant transformation ratio (∼50% TR). Therefore, the Madbi source rock potential could have be generated and expelled mature oils in the Masila Basin.