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Source Rock Properties and Hydrocarbon Generating Kinetics of Lacustrine Shales in Ordos Basin, China


Upper Triassic Yanchang shale in Southeast Ordos Basin (SOB) is a main potential source rock for conventional petroleum fields and has been recently recognized as an important unconventional reservoir. The hydrocarbon potential of lacustrine Yanchang shales stemmed from SOB was investigated using bulk and quantitative pyrolysis techniques. The rock samples were taken from Chang7 and Chang9 intervals of upper Triassic aged cores. The analytical program included total organic carbon (TOC), Rock-Eval, pyrolysis gas chromatogram (Py-GC), source rock analyser (SRA) and micro-scale sealed vessel (MSSV) pyrolysis. The results were used to determine petroleum type, hydrocarbon composition, bulk kinetics and phase behavior of fluid generated from Yanchang shales. The investigated shale rocks proved to contain type II/III kerogen with quality organic abundance. The heterogeneity was identified and total sulphur content is lower than 1.5%. These black shales generate paraffinic-naphthenic-aromatic (P-N-A) low wax oils, whereas samples with increasing maturity show a potential for gas condensate generation. Bulk kinetic parameters reveal a relatively broad distribution of activation energies and indicate that Chang9 kerogen is more stable. Hydrocarbon generation could be characterized by a frequency factor A (1/S) =2.20E+12 and a main active energy at 50 kcal/mole. Computed onset (transformation ratio=10%) and peak generation temperatures were 121oC and 132oC respectively using a 3oC /million years constant heating rate. Compositional kinetic modelling predicts that 75% of the hydrocarbons were generated as oil and 25% as gas. The liquid is predominated by boiling ranges C7-15 and C16-25, while n-C1, n-C2 and n-C3 contribute to the gas composition. The saturation pressure (Psat) and formation volume factor (Bo) display a linear correlation as a function of transformation ratio. And the property of the generated hydrocarbons is in agreement with naturally occurring petroleum fluids. The pressure-temperature (P-T) envelope is defined for hydrocarbons generated from Yanchang shale. It is possible to determine if a single or two-phase fluid could occur at different TRs. This research provides the first compositional kinetic description of lacustrine Yanchang shale in the studied area and demonstrates excellent shale gas and oil potential.