Effects of diagenesis on the acoustic velocity of the Triassic oolitic shoals in the Yudongzi outcrop of Erlangmiao area, Northwest Sichuan Basin

Abstract

The oolitic shoals of the Triassic carbonate platform margin in the Yudongzi (鱼洞子) outcrop of Erlangmiao (二郎庙) area in the northwestern Sichuan (四川) basin present a scarce oppor- tunity to quantitatively describe their diagenesis and its effects on the acoustic velocity. Using a detailed field geologic survey, profiles illustration of typical depositional system, and systematic testing, five types of diagenesis have been identified in the oolitic shoals: micritization, cementation, compaction and pressolution, dissolution, and dolomitisation. The cementation is composed of four subtypes (micrite cements, fibrous calcite cements, granular calcite cements, and blocky calcite cements). The dissolution is formed from three subtypes (freshwater selective dissolution, burial non-selective dissolution, and burial selective dissolution). The dolomitisation is composed of three subtypes (fine-crystalline dolo- mites, microcrystalline dolomites, and medium-crystalline dolomites). In order to quantitatively de- scribe the diagenetic fabric of oolitic shoals, the micritic grain content, calcite cement content, mean pore diameter, pore types, dolomite content, and dolomite types have been evaluated. Based on these data, the relationship between the acoustic velocity and diagenesis of oolitic shoals has