*3.1. Samples*

In this study, a total of 137 samples of the P2l source rocks were collected from 14 wells in the Jimsar Depression, including 40 samples from the P2l 1 member and 97 samples from the P2l 2 member. The locations of the wells are shown in Figure 1c. Based on these samples, various analytical measurements were carried out on more than 700 samples, including the pyrolysis, extraction, and quantification of the organic carbon and soluble organic matter; the analysis of biomarker compounds; the determination of the whole-rock organic microscopic composition; and the determination of the vitrinite reflectance (Ro).

#### *3.2. Analytical Methods*

#### 3.2.1. Evaluation of Source Rocks

The source rock samples were cut into thin sections to observe the lithology of the source rocks and the characteristics of the organic detritus, and determine the Ro. The "Method for Isolation of Kerogen in Sedimentary Rocks", published and implemented in 2003, was used to isolate the kerogen, and the component identification, quantification, and the classification of the kerogen types of the isolated kerogen were carried out based on the "Methods for Microscopic Composition Identification and Type Classification by Transmitted Light-Fluorescence" industry standard (SY/T5125-2014).

The pyrolysis of the source rocks was conducted using a Rock-Eval VI standard pyrolysis instrument (France). The sample was crushed, and 100 mg of the sample was heated in helium gas. The hydrocarbons released during the pyrolysis were monitored using a hydrogen flame ionization monitor, and the CO and CO2 generated by the heating and oxidation of the residual organic matter after the pyrolysis were detected using a thermal conductivity detector. Following the rock pyrolysis analysis standard (GB/T18602- 2012), a constant temperature of 300 ◦C was set for 3 min to obtain S1; the temperature was increased from 300 ◦C to 650 ◦C at a rate of 25 ◦C for 1 min to obtain S2. Then the highest pyrolysis peak temperature (Tmax), Hydrocarbon Index (HCI), Hydrogen Index (HI), Oxygen Index (OI), Hydrocarbon Generation Potential (S1 + S2), and Production Index (PI) were obtained through equations for calculating the pyrolysis parameters.

#### 3.2.2. Simulation of Two-Dimensional Basin

The simulation of a two-dimensional basin was carried out using the PetroMod software. The data of heat flows of present and past, and the electrical conductivity of the source rocks, were obtained from previous studies that had carried out simulations [22,23]. The solution (EasyRo) given by Sweeney and Burnham (1990) [24] was used to convert the result to source rock maturity, and the applicable maturity interval was 0.3–4.6% Ro.

#### 3.2.3. Thermal Simulation Experiment in a Closed System

The source rock sample was sealed in a gold tube under the protection of an argon gas atmosphere. The gold tube was placed in an autoclave, and the autoclave was filled with water using a high-pressure pump. The high-pressure water caused the gold tube to be flexibly deformed, thereby exerting pressure on the sample. The sample was sealed under the protection of argon to ensure that there was no air contamination. The gold tube was sealed via arc welding. The samples were heated at rates of 20 ◦C/min and 2 ◦C/min. The temperature difference between each autoclave was less than 1 ◦C. The pressure was 50 MP, and the pressure fluctuation was less than 1 MP. The temperature range was 150–456 ◦C, and the temperature fluctuation was less than 1 ◦C. After heating, the gold tube was removed from the autoclave. The detected contents were gas (C1–C4), light hydrocarbons (C6–C14), and heavy hydrocarbons (C14+).

#### **4. Results and Discussion**

#### *4.1. Depositional Environment of Source Rocks*

#### 4.1.1. Normal Alkanes and Isoprenoid Alkanes

Isoprenoid alkane compounds are commonly used as indicators of the sedimentary environment [25,26]. The pristane/phytane (Pr/Ph) ratios of the P2l source rocks in the Jimsar Depression were 0.81–2.62, with an average of 1.37. Most of the values were in the range of 0.75–1.5, but quite a few samples had values of >2 (Table 1). Peters et al. (2005) [25] pointed out that when the Pr/Ph ratio is 0.8–3.0, there is grea<sup>t</sup> uncertainty in using this ratio to explain the depositional environment of the source rock. Therefore, the ratio of isoprenoid to normal alkanes [27] was used in this study. As is shown in Figure 2, the Pr/*n*-C17 and Ph/*n*-C18 values were 0.14–6.40 (average 1.54) and 0.09–4.33 (average 1.43), respectively, indicating a reducing environment overall, with bacteria and algae as the main sources of organic matter. The contribution of land-based organic matter was low, and the maturity was not high. It should be noted that the distribution characteristics of the isoprenoid alkanes and *n*-alkanes are also controlled by their maturity [25,28]. The maturity of the samples from the study area was relatively low, so the impact of the maturity could generally be ignored (Table 1, Figure 2).

**Figure 2.** Plot of Pr/*n*-C17 vs. Ph/*n*-C18 showing the depositional environment of the P2l source rocks in the Jimsar Depression (modified after Shanmugam, 1985) [27] (terrestrial organic matter/oxidation environment; maturity; marine organic matter/reducing environment).




**Table 1.** *Cont.*
