Abstract
<jats:title>Abstract</jats:title> <jats:p> The temperature field fundamentally governs the process and total volume of hydrocarbon generation from source rocks, as well as the resultant phase state of fluids within basin reservoirs. A lack of clarity regarding the thermal regime of ultradeep strata in China’s Tarim and Sichuan Basins currently impedes hydrocarbon exploration efforts. This research addresses this deficiency by summarizing and contrasting the current thermal and pressure characteristics in the two basins, with a specific analysis of the hydrocarbon phases in the Tarim Basin’s Ordovician strata. The modern thermal settings show a distinct contrast: the average heat flux for the Tarim Basin is 42.5 ± 7.6 mW/m <jats:sup>2</jats:sup> , compared to 53.8 ± 7.6 mW/m <jats:sup>2</jats:sup> for the Sichuan Basin, thereby affirming their characterization as “cold” and “warm” basins, respectively. The thermal contrast is sustained across depth: the geothermal gradients in the Tarim Basin decrease slightly with depth (e.g., 21.6 ± 2.9 °C/km for 0–5000 m to 19.6 ± 2.8 °C/km for 0–7000 m), while those in the Sichuan Basin show a slight increase (e.g., 21.9 ± 2.3 °C/km for 0–5000 m to 23.3 ± 2.4 °C/km for 0–7000 m). Ultradeep formation temperatures are spatially controlled primarily by basement architecture, fault systems, and hydrothermal convection. Furthermore, thermal modeling established distinct paleoheat flow histories for the basins: the Tarim Basin’s internal heat flux generally declined after the Early Cambrian, punctuated solely by an abrupt Early Permian peak. Conversely, the Sichuan Basin’s thermal history unfolded in three stages—a steady state from the Cambrian to Early Permian, followed by a rapid rise at the close of the Early Permian, and a subsequent cooling phase after the Late Permian. The consistently higher thermal regime in the Sichuan Basin resulted in differential hydrocarbon generation and conservation in their ultradeep strata. The pressure evolution path of the main reservoirs was reconstructed using numerical simulation constrained by fluid inclusions. The Upper Ordovician reservoir in the Tarim Basin largely maintained normal pressure or mild overpressure throughout its geological history. Conversely, the Cambrian Formation in the Sichuan Basin experienced an abnormal high-pressure stage, peaking during the Late Cretaceous when the strata reached its maximum burial depth. Furthermore, the maturity evolution of the main Paleozoic source rocks were reconstructed utilizing the Easy%R <jats:sub>o</jats:sub> model, constrained by the thermal history data. The final analysis included a discussion of the hydrocarbon preservation phase state in the deep Tarim Basin reservoirs, informed by the established temperature and pressure data. This comprehensive study significantly advances the theoretical understanding of ultradeep geothermal and pressure fields, providing a vital framework for accurately assessing ultradeep source rock maturation and the resultant hydrocarbon preservation state in both the Tarim and Sichuan Basins. </jats:p>