Deep and ultra-deep natural gas exploration in the Sichuan Basin: Progress and prospect

Abstract

In recent years, major breakthroughs have been made in natural gas exploration in deep and ultra-deep strata in the Sichuan Basin, but the overall successful rate is low. To further clarify the prospects there, it is necessary to make an in-depth analysis of the previously discovered large-scale reef-shoal gas fields such as Puguang, Yuanba, Anyue and Longgang and deep shale gas discovery in Dingshan and Dongxi, southern Sichuan Basin. On one hand, large high-energy facies are the basis for controlling the development of large-scale reef-shoal reservoirs in conventional reef-shoal areas. The reservoir original porosity is high. The atmospheric freshwater dissolution in the early diagenetic stage, dolomitization, unconformity karst, and “pore–fracture coupling” mainly control the development of secondary pores. The contribution of hydrothermal fluids to reservoir is double-sided, and such early pores can be preserved till present due to those retention processes such as early hydrocarbon charging. Apart from continuous preservation as the key factor, most gas reservoirs are featured by “near-source enrichment, phase transformation, and dynamic adjustment”. On the other hand, deep shale gas generally has the characteristics of “high pressure, high porosity, and high gas content”, that is, “overpressure and rich gas”. The key to the development of high-quality deep shales with high pores are “quartz compression retaining pore" and “reservoir fluid overpressure”. The weak tectonic effect in the late stage is the main reason for deep shale gas to maintain the “high pressure and high gas content”. In conclusion, technological advances like geological target identification and “sweet spot” prediction, as well as deep, high-temperature and high-pressure engineering processes, are the guarantee for efficient exploration of conventional and unconventional deep and ultra-deep natural gas, which has great potential in the Sichuan Basin.