Origins and Geochemistry of Dolomites and Their Dissolution in the Middle Triassic Leikoupo Formation, Western Sichuan Basin, China

摘要

Triassic dolomites occur pervasively in the Western Sichuan Basin. Although these strata have been deeply buried and affected by multiple phases of dolomitization and dissolution, some intervals in the upper part of the Leikoupo Formation have developed high porosity. Based on their petrographic and geochemical features, three major types of dolomite fabrics are recognized, namely, dolomicrite, fabric-retentive dolomite, and fabric-destructive dolomite. Geochemical evidence indicates that the dolomicrite formed following the Sabkha model in a low-temperature hypersaline environment, as these rocks exhibit abnormally high Sr and Na contents, lower Fe and Mn contents, δ 18 O values generally ranging from ?1.70‰ to ?1.67‰ (with an average value of ?1.69‰), and higher Mg/Ca ratios. The fabric-retentive dolomite formed following the seepage-reflux model in a shallow burial environment, and these rocks exhibit the highest 87 Sr/ 86 Sr ratios, δ 18 O values generally ranging from ?6.10‰ to ?2.50‰ (with an average value of ?3.98‰), and a wide range of Fe and Mn contents, indicating that they may have been altered by meteoric water. The fabric-destructive dolomite formed following the burial model at elevated temperatures; these rocks exhibit the lowest Sr and Na contents, δ 18 O values generally ranging from ?7.01‰ to ?6.62‰ (with an average value of ?6.79‰), relatively higher Mg/Ca values, and lower 87 Sr/ 86 Sr ratios. The early Sabkha, seepage-reflux dolomitization and penecontemporaneous periodic meteoric freshwater selective dissolution processes formed multi-period, overlapping moldic pores, algal framework pores, and intragranular dissolution pores. The superposition of organic acid dissolution during the burial period is the main controlling factor of the formation of deeply buried, high-quality dolomite reservoirs in the Leikoupo Formation.