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Ultra‐deep Sinian carbonate rocks are rich in natural gas, and the Deng‐4 Member of the Central Sichuan Basin has been shown to have an extremely high degree of thermal evolution. They are characterized by dry gas produced due to the thermal cracking of palaeo‐oil reservoirs (PORs). The distribution of PORs as materials for the formation of cracked gas reservoirs remains unclear. The complex multistage evolution process and limited drilling data make it challenging to precisely and systematically recover a range of PORs. Based on basin and petroleum system modelling (BPSM), the bottom‐up migration and accumulation method was used to investigate the distribution of PORs. Research shows that the PORs in the Sinian Deng‐4 Member were mainly formed in the Early–Middle Triassic under the mixed contribution of Cambrian and Sinian source rocks, and the coupling of palaeostructural and trap settings controlled its accumulation. Generally, the distribution of PORs can be divided into two categories: one is located at the core of the uplift zone and is controlled by structural‐stratigraphic composite traps, whereas the other is controlled by a series of lithologic traps formed in the slope area. Sensitivity analysis showed that the distribution patterns of the PORs were mostly related to the facies conditions (physical characteristics) of the reservoirs. The widely distributed PORs provide necessary and sufficient material conditions for the formation of cracked Sinian gas reservoirs in the Central Sichuan Basin, and the North Slope area has great potential for natural gas exploration in the future. [ABSTRACT FROM AUTHOR]

• A path model of trap oil cracking was established. • Oil-bearing inclusions indicate the existence of paleo-oil reservoirs. • Oil cracking is followed by unstable, major components, deasphalting and coking. • Escape of wet gas and TSR alteration form the dry gas reservoir. Deep strata are extremely rich in hydrocarbon resources, and trap oil has undergone a long-term and multistage geological evolution that results in thermal cracking. However, compared with controlled experimental conditions, studies on the thermal cracking process of trapped oil under subsurface geological conditions are relatively scarce. Therefore, the major aim of this study is to reconstruct the oil reservoir cracking process based on an evolutionary study of a typical Ediacaran gas reservoir in China. By evaluating the detailed reservoir petrology, natural gas composition, isotopes, residual solid bitumen (SB) characteristics, fluid inclusion analysis, and in situ U–Pb dating of dolomite, this study combined an oil cracking kinetic model with actual geological elements and evolution to recover the four stages of trap oil cracking. Mutual verification of the forward model and inversion demonstrated that a suitable oil cracking kinetic model can be extrapolated to geological conditions. With an increase in the thermal evolution of the reservoir, the most unstable component in the oil first cracks, and a small amount of gaseous products are preferentially dissolved in the liquid oil. The major components of petroleum undergo thermal cracking and conversion to produce short-chain liquid hydrocarbons and wet gases. A large amount of wet gas generates an abnormally high fluid pressure. Wet gas usually escapes from potential channels (i.e., caprock microfractures and unconformity surfaces), thus dynamically maintaining the energy balance of the trap system and weakening the impact of high pressure on oil cracking. The wet gas products precipitate asphaltene in the oil, thus adjusting the composition and properties of the trap oil. The escape of the initial wet gas products, accompanied by a thermochemical sulfate reduction (TSR) reaction with residual oil, C 2 H 6 , and other wet gases as reactants, led to the formation of CH 4 -rich natural gas with a high drying coefficient in the trap of the highly over-mature evolution stage. Precipitated asphalt and the asphalt directly produced by oil cracking, coke rapidly under high temperatures and pressures to form SB with abnormally high reflectivity. Under geological conditions, oil cracking behavior is generally controlled by a combination of temperature history, tectonic history, fluid pressure background, trap preservation conditions, secondary alteration (e.g., TSR), and oil type, among which the reservoir temperature history remains the most important. Other factors should not be underestimated in a specific region. This study revealed a pathway model for the thermal cracking of trapped oil under real geological conditions, providing references for similar studies and other pathway models in different regions worldwide. [ABSTRACT FROM AUTHOR]

The hydrocarbon-source correlation in highly-mature and over-mature stages is a challenging task in natural gas exploration. The Ediacaran-Lower Cambrian natural gas in the central Sichuan Basin is typical oil-cracking gas. With a burial depth of more than 4650 m, the gas is now in the highly-mature to the over-mature stage (equivalent Ro >1.8%) with apparent alteration by thermochemical sulfate reduction (TSR), leading to difficulties for gas-source correlation. Conventional hydrocarbon-source correlation mainly relies on organic geochemical methods, taking media such as biomarkers and carbon isotope for correlation. However, in the highly-mature and over-mature stages, the characteristics of conventional biomarkers tend to be homogenised. Furthermore, the gas components and carbon isotope of natural gas are frequently altered by TSR in the deep burial stage. Thus, conventional organic geochemical methods are significantly restricted. This paper explored the source of highly-mature natural gas from the DY and LWM Formations in the central Sichuan Basin by studying the inorganic geochemical information (trace elements and rare earth elements) of the reservoir bitumen and potential source rocks. The results show that the depositional environment of the potential source rocks (the QZS and the D-3 source rocks) in the central Sichuan Basin were different. The redox-sensitive parameters (V/(V + Ni), Mo/Ni, δCe and Ce/Y) and the provenance-sensitive parameters (La/Co and La/Sc) of the source rocks have apparent differences, which can be used as the basis for gas-source correlation. The correlation results show that the LWM gas was derived from the QZS source rock, while the D-2 and D-4 gases were mainly derived from the QZS source rock, with a partial contribution of the D-3 source rock. During geologic evolution, the crude oil generated from the two sets of source rocks were thoroughly mixed in the paleo-oil reservoir of the D-4 Member. As a result, the characteristics of the D-3 source rock were obscured by the QZS. In contrast, the solid bitumen in the D-2 paleo-oil-reservoir recorded the contribution of the D-3 source rock. However, this contribution was limited within the range of 50 m adjacent to the D-3 Member by a more significant oil generation from the QZS source rock located in Deyang-Anyue Rift Trough. This study clarified the gas source differences between the D-2 Member, D-4 Member and LWM Formation, providing a new insight for in-depth study on the gas accumulation of the Ediacaran-Lower Cambrian in the central Sichuan Basin. • The obstacle of gas-source correlation by organic geochemistry in ultra-deeply buried strata was overcome. • Trace and rare earth elements are effective in highly- to over-mature gas-bitumen-source correlation. • Redox and provenance-sensitive parameters can effectively distinguish different source rocks and reservoir bitumens. • Two paleo-oil-reservoir accumulation modes have been established for ultra-deeply buried gas reservoirs. [ABSTRACT FROM AUTHOR]