Research paperModeling free gas content of the Lower Paleozoic shales in the Weiyuan area of the Sichuan Basin, China

- Two sets of Lower Paleozoic shales are characterized by a strong uplifting and a very high maturity.
- Geological, geochemical and porosity models of the two sets of shales were established.
- Evolution history of free gas contents of the shales were modeled using the PVTSim software.
- Current free gas content of the shales is 1.52-3.42 m3/t, similar with their measured result.

Two sets of Lower Paleozoic organic-rich shales develop well in the Weiyuan area of the Sichuan Basin: the Lower Cambrian Jiulaodong shale and the Lower Silurian Longmaxi shale. The Weiyuan area underwent a strong subsidence during the Triassic to Early Cretaceous and followed by an extensive uplifting and erosion after the Late Cretaceous. This has brought about great changes to the temperature and pressure conditions of the shales, which is vitally important for the accumulation and preservation of shale gas. Based on the burial and thermal history, averaged TOC and porosity data, geological and geochemical models for the two sets of shales were established. Within each of the shale units, gas generation was modeled and the evolution of the free gas content was calculated using the PVTSim software. Results show that the free gas content in the Lower Cambrian and Lower Silurian shales in the studied area reached the maxima of 1.98-2.93 m3/t and 3.29-4.91 m3/t, respectively (under a pressure coefficient of 1.0-2.0) at their maximum burial. Subsequently, the free gas content continuously decreased as the shale was uplifted. At present, the free gas content in the two sets of shales is 1.52-2.43 m3/t and 1.94-3.42 m3/t, respectively (under a current pressure coefficient of 1.0-2.0). The results are roughly coincident with the gas content data obtained from in situ measurements in the Weiyuan area. We proposed that the Lower Cambrian and Lower Silurian shales have a shale gas potential, even though they have experienced a strong uplifting.