# Geochemical Analysis of a Multi-Layer Hydrocarbon Reservoir in the Wuerhe Area, Junggar Basin
## 4 Results and Discussion
### Reservoir Formation Model  
Based on the comprehensive analysis of geological tectonic movement and reservoir geochemical characteristics, two stages of tectonic movement occurred in the Wuerhe area, resulting in two stages of reservoir formation. During the Late Permian Jurassic, the thrust folds and large-scale faults formed by the Hercynian Indosinian movement in the Wuerhe area determined the accumulation of oil and gas in later stages. The folds provided certain conditions for the accumulation of oil and gas in later stages. The large fault connecting the Permian, Triassic, and Jurassic strata enabled the oil and gas produced by the source rocks of the Permian Fengcheng Formation to migrate upward form three sets of reservoirs. At the time of migration, the  
Figure 6: Pr/nC\({}_{17}\) vs. Ph/nC\({}_{18}\) diagram of source rocks and crude oil samples in Wuerhe area, showing most of the samples are distributed in the range of type I. (I: Organic matter produced by algae; II: Organic matter produced by algae and terrestrial; III: Organic matter produced by terrestrial). three reservoirs were all light oil reservoirs; then, their strata denuded. Being the uplift wall of the fault, the northern strata suffered from serious denudation. As a result, the original light oil reservoir was exposed to the surface and suffered from biodegradation, oxidation, and other secondary effects, forming an oil sand deposit. In total ion chromatograms (TIC) of Jurassic heavy oil, the content of n-alkanes is very small, and the baseline is higher (Figure 9 TIC). When a \"baseline bulge\" of biodegradation markers occurs, it forms certain compounds, the mixture of which is collectively termed as \"unknown complex mixture\" [15]. As shown in Figure 9, the peaks of pristine and phytane are similar to the main peaks of C\({}_{17}\) and C\({}_{18}\) n-alkanes, which indicates that the biological activity is strong. The nC\({}_{17}\) and nC\({}_{18}\) concentrations are greatly reduced by strong biodegradation of crude oil. Meanwhile, 25-nonbapane, considered as the biodegradation product of crude oil, was also detected in heavy oil (Figure 9 m/z191); 25-nonbapane is also known as 10-demethylhopane or demethylated propane or degraded propane. In the process of degradation, bacteria first consume light components such as n-alkanes, which lead to the enrichment of oil with heavy components and increases the oil density and viscosity. In the weak denudation nose bulge, the depth, temperature, and other conditions are suitable for the survival of microorganisms. The original light oil reservoir must have been seriously biodegradable and washed out to form a heavy oil reservoir. During the Yanshanian movement in the Cretaceous, the development of the main faults and folds in the Wuxia fault zone practically ceased, and only small normal faults and strike-slip fault systems developed in the shallow part of the uplifted nose. The fault system broke up to the surface and down to Jurassic and Triassic, forming a \"Y\" shape with large deep faults. In the Cretaceous solid bitumen within the fault, 25-nonbapane was not detected, and the distribution of saturated hydrocarbons was relatively even, indicating that the asphaltene was not formed by biodegradation. The crude oil probably reached near to the surface through the fault. Low temperatures and pressure might have led to the rapid loss of the light components in the oil; the remaining heavy components enabled the  
Figure 7: Diagram of C\({}_{27}\)-C\({}_{28}\)-C\({}_{29}\) regular sterane content of source rocks and crude oil samples in Wuerhe area, showing a high content of C\({}_{29}\) regular steranes. density and viscosity of the crude oil to increase continuously. Finally, the Cretaceous solid bitumen mine was formed, and the fault was closed [16].