5.1. Division of thermal evolution stages based on the pyrolysis products
The pyrolysis process can be divided into four stages based on the variation characteristics of the yield of expelled oil, residual bitumen, and gaseous hydrocarbons with increasing maturity (Fig. 6): 1)Ro = 0.82–1.09% (Ts = 335–360 °C), which was the lower maturity stage dominated by the generation of liquid hydrocarbons. This stage is characterized by rapid oil and lower gas generation (Tissot et al., 1974; Lewan, 1997; Hill et al., 2007; Sun et al., 2019b). The slope was used to measure the inclination of the steep curve. In mathematics, the slope of a line is the same for any point, and it is a measure of the direction and steepness of the line. The slope of a line can be calculated using principles of algebra and geometry. The slope of the curve at a point reflects how quickly the variables of the curve change at that point. When the slope (k ) is positive, a greater value indicates a more rapid increase. When k is negative, a smaller value indicates a more rapid decrease (Matuszewski, 2006; Taylor et al., 2019). The slope of the curve for increased hydrocarbon yield (kn ) was equal to (Yieldn+1 - Yieldn ) / (Ro n+1 - Ro n), as shown in Table 2. At this stage, the curve of the slope for increased expelled oil (ke1 = (Yield2Yield1 ) / (Ro 2Ro 1)) reached 101.48, indicating the characteristics of rapid expelled oil generation. 2) Ro = 1.09–1.65% (Ts = 360–400 ℃), which was the maturity stage dominated by the generation of lighter liquid hydrocarbons and wet gaseous hydrocarbons. At this stage, the yield of residual bitumen oil decreased sharply (kb = -20.20), and the yield of expelled oil did not change significantly (ke = 6.07, which was significantly lower than that in the first stage). The yield of hydrocarbon gases began to show an increasing trend, and the corresponding value of k also increased rapidly from 14.52 to 39.39. 3) Ro = 1.65–2.3% (Ts = 400–480 °C), the yields of gaseous hydrocarbons increased continuously, expelled oil almost did not change, and the residual oil decreased to a lower value, which could be seen as the stage of thermal cracking to wet gases. 4) Ro = 2.3–3.24% (Ts = 480–575 °C), which was the post-maturity stage. A large amount of methane was produced, and the oil and gas further cracked into methane. The increase in the amplitude of gaseous hydrocarbon was larger, and the corresponding kgreached 172.89, providing proof of higher methane production. However, the yield of expelled oil was further reduced when the negativeke and residual bitumen were in equilibrium withkb near zero.