Variation history and dynamic evolution of the Mesozoic marine petroleum in the Papuan Fold Belt

The Papuan Basin, positioned in the dynamic boundary where the Australian Plate shifts northward and the Pacific Plate moves westward, is a Mesozoic and Cenozoic foreland basin. Within this basin, the Papuan Fold Belt stands out as a secondary structural unit rich in Mesozoic oil and gas reserves, harboring immense exploration potential. However, frequent tectonic activity in the area complicated the aggregation and adjustment of oil and gas, posing significant challenges to hydrocarbon exploration. By analyzing the geochemical characteristics and sources of oil and gas, the joint inversion of multiple ancient temperature scales was used to simulate the burial history and thermal history of a single well. Additionally, by analyzing reservoir fluid inclusions, the dynamic evolution of hydrocarbon formation was restored. The results indicate that: (1) the Mesozoic oil and gas in the Papuan Fold Belt situated predominantly in the western and central sections, following a "gas in the west and oil in the east" distribution pattern. (2) The oil and gas filling epochs could be categorized into "two phases and four episodes." During the first three episodes, spanning 105~85 Ma, 78~53 Ma, and 47~32 Ma, the filling process progressed from low-maturity oil to high-maturity oil and then to natural gas. The crude oil trapped in inclusions during these periods exhibited higher maturity, containing an abundant amount of regular steranes ααα(20R)-C₂₇, pointing to a Jurassic source rock. The later filling episode during 4～0 Ma, showed a mixed filling of highly mature oil and natural gas, with traces of low-maturity oil. The crude oil currently stored in the reservoir displayed lower maturity than its predecessors and is enriched with oleanane, suggesting the Jurassic and Cretaceous source rocks as its origin. (3) The formation of oil and gas reservoirs was influenced by multiple tectonic movements, including the types and evolutionary degrees of source rocks, the development of large-scale traps, efficient vertical lateral transport systems, and preservation conditions. This study introduced a dynamic evolution model called "dynamic equilibrium of dual-source vertical and lateral short-distance hydrocarbon supply accumulation adjustment", based on clues from structural evolution. This study proposed an understanding that the Cretaceous source rocks located in the lower plate of the compression thrust belt also have hydrocarbon source contributions to the Mesozoic oil and gas system, and provided insight to the future oil and gas exploration in the region.