Dissolution pore types of the Wufeng Formation and the Longmaxi Formation in the Sichuan Basin, south China: Implications for shale gas enrichment

摘要

In shale gas reservoirs, scientific consensus indicates that the main pore types are organic matter pores, mineral pores and micro-fractures. The organic matter pores are crucial to porosity, and in many cases they form the main pore type favorable for shale gas enrichment. Dissolution pores can influence shale gas enrichment and preservation. Although the various types of dissolution pores have been reported, few studies have identified the characteristics of mineral dissolution pores, let alone the development of dissolution pores and their effect on shale gas enrichment. Based on observations from shale gas wells in the Sichuan Basin, this paper studied the types of dissolution pores in the Wufeng Formation and the Longmaxi Formation. The study shows that the types of dissolution pores are carbonate dissolution pores, feldspar dissolution pores, and quartz dissolution pores and that carbonate dissolution pores are the main type. The formation time of the dissolution pores can be identified through analyzing the time of the mineral generation and the time at which the solid bitumen and/or minerals filled in the dissolution pores. We found that the dissolution pores in the secondary enlargement of minerals formed later than those in the original minerals. The dissolution pores without solid bitumen may have formed later than the dissolution pores with solid bitumen filling. Given that the dissolution pores had formed before the peak stage of oil generation, the dissolution pores should be well connected through the dissolved trench, which would largely improve the reservoir volume for hydrocarbon storage and matrix permeability. If there is no solid bitumen filling in the dissolution pores and the formation of the dissolution pores began after the crude oil was charged but before the peak stage of gas generation, these conditions will have some destructive effects on shale gas enrichment. In the worst-case scenario, if the dissolution pores formed after the peak stage of gas generation, the development of the pores will likely destroy the shale gas enrichment and can convert reservoir pressure to a normal pressure. The new findings in this paper reveal that the formation time of the dissolution pores has a controlling effect on shale gas enrichment in the Wufeng Formation and the Longmaxi Formation in the Sichuan Basin; these findings also provide valuable guidance for future research on shale gas reservoirs around the world.