Productive Potential of Upper Ordovician and Lower Silurian Shale Gas Plays in the Sichuan Basin

摘要

The Wufeng-Longmaxi Shale, Upper Ordovician and Lower Silurian marked by a widespread transgression in Yangtze Platform region are unique with highly thermal maturity, tectonically complex setting and significantly variable in-situ stress field. The Sichuan Basin with comparatively favorable Lower Paleozoic shale geology is multi-tectonic event stacked basin located in the northwest of Yangtze Platform. Abnormally large fluid pressure has been encountered in widely separated anticlines in the Eastern and Southern Sichuan Basin. At the same time, the Central Uplift zone provides a case in which the system evolved from overpressure to normal pressure due to the effects of uplift and erosion. However, the mechanisms responsible for over-pressuring maintenance and productive potential have not been well conducted and there are even many uncertainties to identify productive potential of the shale plays under considerably structural complexity and present-day active compression mainly influenced by the subduction and collision between the Indian Plate and the Eurasian Plate. In this paper, the impact of the long-term and the short term cycles variations in the distribution and quality of organic matter rich shale reservoirs through a completed sequential analysis are discussed and also the characteristics of fractures related to the detachment anticline and fault for the thrusting under the background of regional tectonics and burial history, especially bedding-parallel fibrous calcite layers situated between strong layer and week layer within relatively brittle siliceous and carbonaceous shale of the lower Longmaxi interval, are integrated with fluid inclusion microthermometry and documentary geologic data to interpret key deformation events, hydrocarbon generation and variation in fluid pressure and temperature through time. Further consideration of tectonic compression in different geologic setting is taken in the study to shed light on the dynamic processes involved in the preservation potential of subsurface high fluid overpressures and the great effect on shale gas production. Evidences suggest that considerable complexity of structure and in-situ stress is not simply negative for the Lower Paleozoic shale in Southern China. That the drilling and completion practice matches the geological condition is significantly helpful to face fracturing and production challenges of shale gas reservoirs in Sichuan Basin and surrounding.