Hydrocarbon Generation Kinetics of Lacustrine Yanchang Shale in Southeast Ordos Basin, North China

The 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. Here, we report on the hydrocarbon potential of this lacustrine shale using bulk and quantitative pyrolysis techniques. The rock samples were taken from the Chang7 and Chang9 intervals of upper Triassic-aged cores. The analytical program included total organic carbon (TOC), Rock-Eval, pyrolysis gas chromatography (Py-GC), source rock analyzer (SRA), and microscale sealed vessel (MSSV) pyrolysis. Phase kinetic modeling was also employed on the basis of these data sets. The results were used to determine the petroleum-type organofacies, bulk hydrocarbon composition during maturation, bulk and compositional kinetics, and phase behavior of fluids generated in the Yanchang shales. The shales proved to contain type II2 kerogen with organic matter in high abundance and generate paraffinic-naphthenic-aromatic (PNA) low wax oils when mature, whereas samples with increasing maturity show a potential for gas condensate generation. Bulk kinetic parameters of the immature Yanchang shale reveal a relatively broad distribution of activation energies and indicate lower stabilities than marine Cambrian type II shale in south China. Hydrocarbon generation could be characterized by a frequency factor A = 2.20 x 10(12) S-1 and a main activation energy at 50 kcal/mol. Extrapolation to the geological heating rate of 1.0 degrees C/Ma in SOB, the onset (transformation ratio = 10%) and peak generation temperatures were 115 and 124 C, respectively. Compositional kinetic modeling predicts that the generated gas fraction mainly consists of C-1, C-2, and C-2, while the liquid phase is predominated by compound groups of C7-15 and C16-25. Furthermore, the gas/oil ratio (GOR) varies between 83.6 Sm-3/Sm-3 (97.1 m(3)/t) and 168.2 Sm-3/Sm3 (195.3 m(3)/t). The saturation pressure (P-sat) and formation volume factor (B-o) display a linear correlation as a function of the transformation ratio (TR). The property of the generated hydrocarbons is in agreement with naturally occurring petroleum fluids. Using the pressure temperature (P-T) envelope defined from these experiments, only a single liquid phase (black oil) is predicted at different TRs (10-70%). This research provides the first case study with respect to phase kinetics description of Yanchang shale oil and shale gas in the study area.