Thermal Evolution of the Paleozoic Hydrocarbon Source Rocks in the Sichuan Basin: A Joint Inversion Result of Ro Data and Thermochronological Modeling

Zhu, Chuanqing; Hu, Shengbiao; Rao, Song

The Sichuan Basin is one of the most important oil-gas bearing basins in SW China. A recently discovered large gas field, Puguang gas field, shed some lights on the petroleum exploration prospect in this basin. The thermal history of the Sichuan Basin has been reconstructed based on a large paleo-thermal indicators (Ro, AFT) dataset. The Ro profiles of some deep boreholes in the basin show ‘dog-leg' shapes at the unconformity between upper and middle Permian. The inversion result based on the Ro data suggests two historic different thermal regimes since the Paleozoic in the Sichuan Basin, the earlier one ending at the beginning of late Permian, the later one began since the last denudation. The low-T thermochronological modelling result suggests that the last denudation in different regions started at almost same time (~100-80Ma), except for the western Sichuan Basin, in which the denudation started much later (~60-50Ma). This provides significant information supplementary to the thermal history reconstruction that based on the Ro data: the cooling started at ~60-50 Ma (Paleocene) in the west, and at ~100-80 Ma (late Cretaceous) in the other regions.

The maturation history of the hydrocarbon source rocks of some deep boreholes in the Sichuan basin have been modeled based on the thermal evolution modeling results and the burial history of the source rocks, and the effect of the geological events on the evolution of the source rocks, especially the Paleozoic source rocks are also evaluated. In the southwestern part of the Sichuan Basin which closed to the Emeishan Large Igneous Province (ELIP), the organic matter of the most Paleozoic source rocks became mature in middle Late Permian, the source rocks of lower Permian were heated to its peak maturity (maximum Ro > 3.0%) at around 260 Ma and there was no secondary hydrocarbon generation in the later time. This indicates that the thermal evolution of the Paleozoic source rocks was strongly affected by the Emeishan Mantle Plume (EMP) in this area. However, in the regions that far away from the ELIP, the Puguang gas field included, secondary hydrocarbon generation of the source rocks existed, the thermal evolution of the source rocks was related to the Mesozoic foreland basin evolution. Therefore, the latter appears to be more prospective to hydrocarbon reservoir accumulation.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013