The Fengcheng Formation is a crucial source rock and the primary reservoir for oil accumulation in the Mahu Sag. Crude oils are distributed throughout the Fengcheng Formation, ranging from the edge to the interior of the sag in the southern Mahu Sag. These crude oils originate from in-situ source rocks in shallowly buried areas and the inner deep sag. During migration, the crude oil from the inner deep sag affects the source rocks close to carrier beds, leading to changes in the organic geochemical characteristics of the source rocks. These changes might alter source rock evaluations and oil-source correlation. Based on data such as total organic carbon (TOC), Rock-Eval pyrolysis of source rocks, and gas chromatography-mass spectrometry (GC-MS) of the saturated fraction, and considering the geological characteristics of the study area, we define the identification characteristics of source rock affected by migrated hydrocarbons and establish the various patterns of influence that migrated hydrocarbons have on the source rock of the Fengcheng Formation in the southern Mahu Sag. The source rocks of the Fengcheng Formation are mostly fair to good, containing mainly Type Ⅱ organic matter and being thermally mature enough to generate oil. Source rocks affected by migrated hydrocarbons exhibit relatively high hydrocarbon contents (S₁/TOC > 110 mg HC/g TOC, Extract/TOC > 30 %, HC: hydrocarbon), relatively low Rock-Eval T_max values, and relatively high tricyclic terpane contents with a descending and mountain-shaped distribution. Furthermore, biomarker composition parameters indicate a higher thermal maturity than in-situ source rocks. Through a comparison of the extract biomarker fingerprints of adjacent reservoirs and mudstones in different boreholes, three types of influence patterns of migrated hydrocarbons are identified: the edge-influence of thin sandstone-thick mudstone, the mixed-influence of sandstone-mudstone interbedded, and the full-influence of thick sandstone-thin mudstone. This finding reminds us that the influence of migrated hydrocarbons must be considered when evaluating source rocks and conducting oil-source correlation.

The Mesozoic and Cenozoic strata in the Junggar basin developed two sets of shallow to semi-deep lacustrine shale, namely, the Cretaceous Qingshuihe Formation (K_1q) and the Paleogene Anjihaihe Formation (E_2-3a). Through organic petrology and scanning electron microscope (SEM) observation, it is found that the primary hydrocarbon-generating organic matter (OM) in the two sets of strata is different. The biological precursor of the E_2-3a OM is mainly green algae (Pediastrum), while the precursor of K_1q kerogen is mainly cyanobacteria (Oscillatoria). Then, the E_2-3a green algae-rich shale and K_1q cyanobacteria-rich shale were subjected to hydrous pyrolysis and kinetic analysis, respectively. The results show that the evolution modes of hydrocarbon generation of the typical shales are very different. Green algae have the characteristics of a low oil generation threshold, heavy oil quality, and no prominent oil peak, while cyanobacteria have the characteristics of late oil generation, concentrated hydrocarbon generation, and relatively light oil quality. The characteristics of oil generation can also be well reflected in the composition evolution of the crude oil components. The carbon isotope of gas, kerogen, and extracts of the E_2-3a green algae-rich shale are significantly heavier than the K₁q cyanobacteria-rich shale, which may be related to the living habits of their biological precursors, carbon source usage, photosynthesis efficiency, and carbon fixation efficiency.

The Middle Permian Lucaogou Formation in Northwestern China mainly contains typical saline lacustrine oil shale. To study the differences in geochemistry and hydrocarbon generation of source-rock samples in Lucaogou Formation in Jimusaer Sag, 57 core samples from two boreholes were analyzed herein by performing total organic carbon (TOC) analysis, Rock–Eval pyrolysis, and gas chromatographic–mass spectrometry experiments on saturated hydrocarbons. The kinetics of oil generation were studied using two samples comprising typical maceral components. The results showed that the hydrocarbons produced by telalginite are relatively rich in pristane (Pr), phytane (Ph), β-carotane, high-carbon normal alkanes, and C₂₉ regular steranes. Hydrocarbons produced by lamalginite contain a significantly higher content of C₂₀ tricyclic terpanes (TT), C₂₁TT, C₂₄ tetracyclic terpanes (TeT), C₂₉ norhopane, and C₂₈ regular sterane. Based on the pyrolysis and biomarker compound parameters, telalginite has a higher conversion rate for hydrocarbons than lamalginite in the low-mature to mature stage, which is consistent with their kinetic analysis. Lamalginite source rock displays a much narrower distribution of activation energies than telalginite source rocks. Such narrower activation energy distribution effectively narrows the main stage of hydrocarbon generation. In addition, the activation energy distribution of lamalginite concentrated in the high-value interval, indicating the characteristics of the relatively lagging hydrocarbon generation of lamalginite.