The Fushan Depression is one of the petroliferous depressions in the Beibuwan Basin, South China Sea. Previous studies have preliminarily explored the origin and source of crude oils in some areas of this depression. Nevertheless, no systematic investigations on the classification and origin of oils and hydrocarbon migration processes have been made for the entire petroleum system in this depression, which has significantly hindered the hydrocarbon exploration in the region. A total of 32 mudstone and 58 oil samples from the Fushan Depression were analyzed to definite the detailed oil-source correlation within the sequence and sedimentary framework. The organic matter of third member of Paleogene Liushagang Formation (Els₃) source rocks, both deltaic and lacustrine mudstone, are algal-dominated with high abundance of C₂₃ tricyclic terpane and C₃₀ 4-methylsteranes. The deltaic source rocks occurring in the first member (Els₁) and second member (Els₂) of the Paleogene Liushagang Formation are characterized by high abundance of C₁₉₊₂₀ tricyclic terpane and oleanane, reflecting a more terrestrial plants contribution. While lacustrine source rocks of Els₁ and Els₂ display the reduced input of terrigenous organic matter with relatively low abundance of C₁₉₊₂₀ tricyclic terpane and oleanane. Three types of oils were identified by their biomarker compositions in this study. Most of the oils discovered in the Huachang and Bailian Els₁ reservoir belong to group A and were derived from lacustrine source rocks of Els₁ and Els₂. Group B oils are found within the Els₁ and Els₂ reservoirs, showing a close relation to the deltaic source rocks of Els₁ and Els₂, respectively. Group C oils, occurring in the Els₃ reservoirs, have a good affinity with the Els₃ source rocks. The spatial distribution and accumulation of different groups of oils are mainly controlled by the sedimentary facies and specific structural conditions. The Els₂ reservoir in the Yong'an area belonging to Group B oil, are adjacent to the source kitchen and could be considered as the favorable exploration area in the future.

Due to limited data on the geochemical properties of natural gas, estimations are needed for the effective gas source rock in evaluating gas potential. However, the pronounced heterogeneity of mudstones in lacustrine successions complicates the prediction of the presence and geochemical characteristics of gas source rocks. In this paper, the Liaohe Subbasin of Northeast China is used as an example to construct a practical methodology for locating effective gas source rocks in typical lacustrine basins. Three types of gas source rocks, microbial, oil-type, and coal-type, were distinguished according to the different genetic types of their natural gas. A practical three-dimensional geological model was developed, refined, and applied to determine the spatial distribution of the mudstones in the Western Depression of the Liaohe Subbasin and to describe the geochemical characteristics (the abundance, type, and maturation levels of the organic matter). Application of the model in the subbasin indicates that the sedimentary facies have led to heterogeneity in the mudstones, particularly with respect to organic matter types. The effective gas source rock model constructed for the Western Depression shows that the upper sequence (SQ2) of the Fourth member (Mbr 4) of the Eocene Shahejie Formation (Fm) and the lower and middle sequences (SQ3 and SQ4) of the Third member (Mbr 3) form the principal gas-generating interval. The total volume of effective gas source rocks is estimated to be 586 km³. The effective microbial, oil-type, and coal-type gas source rocks are primarily found in the shallow western slope, the central sags, and the eastern slope of the Western Depression, respectively. This study provides a practical approach for more accurately identifying the occurrence and geochemical characteristics of effective natural gas source rocks, enabling a precise quantitative estimation of natural gas reserves.

Recently, significant oil discoveries have been made in the shallower pay zones of the Jurassic Badaowan Formation (J₁b) in the Mahu Sag, Junggar Basin, Northwest China. However, little work has been done on the geochemical characteristics and origins of the oil in the J₁b reservoir. This study analyzes 44 oil and 14 source rock samples from the area in order to reveal their organic geochemical characteristics and the origins of the oils. The J₁b oils are characterized by a low Pr/Ph ratio and high β-carotene and gammacerane indices, which indicate that they were mainly generated from source rocks deposited in a hypersaline environment. The oils are also extremely enhanced in C₂₉ regular steranes, possibly derived from halophilic algae. Oil-source correlation shows that the oils were derived from the Lower Permian Fengcheng Formation (P₁f) source rocks, which were deposited in a strongly stratified and highly saline water column with a predominance of algal/bacterial input in the organic matter. The source rocks of the Middle Permian lower-Wuerhe Formation (P₂w), which were deposited in fresh to slightly saline water conditions with a greater input of terrigenous organic matter, make only a minor contribution to the J₁b oils. The reconstruction of the oil accumulation process shows that the J₁b oil reservoir may have been twice charged during Late Jurassic–Early Cretaceous and the Paleogene–Neogene, respectively. A large amount volume of hydrocarbons generated in the P₁f source rock and leaked from T₁b oil reservoirs migrated along faults connecting source beds and shallow-buried secondary faults into Jurassic traps, resulting in large-scale accumulations in J₁b. These results are crucial for understanding the petroleum system of the Mahu Sag and will provide valuable guidance for petroleum exploration in the shallower formations in the slope area of the sag.