Dissolution mechanism of siliciclastic particles in Xujiahe Formation, West Sichuan Depression, Sichuan Basin

The siliceous debris dissolution event in the diagenetic series of clastic rocks of the Upper Triassic Xujiahe Formation in the West Sichuan Depression were studied to establish a theoretical dissolution model. The model may be used for recognizing dissolution by means of macroscopic observation of drill cores, profiles and testing data. Results show that the dissolution of siliciclastic particles happens under acidic formation water conditions with F participation. The F component detected in inclusions should be the sum of F^-, HF₂^-, un-dissociated HF molecule, (HF)₂, among which the un-dissociated HF molecule and (HF)₂ are the main elements. The active component (HF)₂ or HF₂^-were adsorbed on the surface of silicon particles during the dissolution under the catalysis of H⁺. Material exchange and transformation of other mineral assemblages occurred at the same time, including felspar and rock debris dissolution as well as siliceous cementation, etc. The keys to recognize siliceous dissolution mainly include geochemistry characteristics, partly dissolved siliceous particles, open contact fractures between siliceous grains in tight sandstones and mineral assemblages.