3.2. Experiments
The simulation experiment was conducted on the DK-Ⅱ type (third-generation) formation porosity thermocompression simulation experiment instrument from the Wuxi Research Institute of Petroleum Geology, Sinopec. The instrument had the advantages of the simulated experimental conditions approaching the actual geological conditions, retaining the original columnar and pore space, and obtaining accurate experimental data (Zheng et al., 2009; Zheng et al., 2011; Qin et al., 2014; Ma et al., 2017).
Therefore, to investigate the hydrocarbon generation potential of coal source rocks in the Pinghu Formation, we designed an experimental process based on the parameters of sedimentary and burial history, thermal evolution history, depth, and lithostatic pressure. In addition, combined with the simulated temperature, time, and pressure of the instrument, the experimental process has been detailed subsequently. 1) Experimental parameter setting: Different simulation depths were set to realize the results of simulating the different thermal evolution stages. As a result, the simulated depths were 2.8, 3.9, 4.45, 5.2, 5.9, 6.2, and 6.4 km, and the corresponding simulated temperatures were 335 °C, 360 °C, 400 °C, 455 °C, 480 °C, 525 °C, and 575 °C, respectively. The heating rate of the experiment was 1 °C/min, and after reaching the target temperature, the temperature was retained at the target temperature for 48 h. 2) Operation of the experimental instrument: After pressing the start button, the experiment commenced. 3) Acquisition and quantification of the pyrolysis products: The gaseous hydrocarbons were collected first, followed by separation and quantification of the expelled oil and residual water. Finally, the solid residues were extracted using chloroform to obtain the content of chloroform asphalt “A,” which was defined as residual bitumen (Ma et al., 2017).
Stable carbon isotopes of kerogen, expelled oil, residual bitumen, and gaseous hydrocarbons at every simulated temperature point were determined using a stable isotope ratio mass spectrometer (Delta Plus V and Mat-253; Thermo Fisher Scientific, Waltham, MA, USA). The carbon isotope values were reported as δ-values, which are expressed as δsample= (Rsample/Rstandard-1) × 1,000 (‰) (where R denotes the ratio of 13C/12C). The δ13C values were normalized to the Pee Dee Belemnite (PDB) (Sun et al., 2015; Wu et al., 2019).