Genesis of Dolomite and Diagenetic-Pore Evolution Model of High-Quality Reservoirs in the Third Member of the Middle Triassic Leikoupo Formation, Northwestern Sichuan Basin

Abstract

Recent exploration has achieved significant breakthroughs in dolomite reservoirs within the third member of the Middle Triassic Leikoupo Formation (T2l3) in the northwestern Sichuan Basin. However, limited understanding persists about the sources of dolomitizing fluids, dolomite genesis, and diagenetic-porosity evolution, hindering research on high-quality reservoir formation. This study integrates petrographic, geochemical (C–O–Sr isotopes, major and trace elements, rare earth elements), and cathodoluminescence analyses to clarify the origins of diagenetic fluids and formation mechanisms of various dolomite types in T2l3 and establishes a diagenetic-porosity evolution model for high-quality reservoirs. The results indicate that (1) the T2l3 dolostones in the northwestern Sichuan Basin can be classified into four types: Mud-microcrystalline dolostone (D1), algal dolostone (D2), grain-dominated dolostone (D3), and crystalline dolostone (D4). (2) The δ13C values and 87Sr/86Sr ratios of the four dolostone types generally fall within the ranges of coeval seawater, and their Z values are approximately 130. Their rare-earth element distribution patterns are also broadly comparable to those of contemporaneous seawater, showing relative enrichment in light rare-earth elements and depletion in heavy rare-earth elements. These characteristics suggest that the dolomitizing fluids of the four dolostone types were derived predominantly from highly saline, concentrated coeval seawater. (3) D1 formed at temperatures of approximately 33–41 °C. Its relatively high Sr, Na, and Fe contents and δ18O values, together with relatively low Mn contents and dolomite ordering, suggest that D1 was formed by evaporative dolomitization in a near-surface, oxidizing environment during the penecontemporaneous stage. D2 and D3 formed at approximately 43–56 °C and are characterized by moderate dolomite ordering, negatively shifted δ18O values, and generally low Fe and Mn contents. These features indicate that they were formed mainly through reflux dolomitization under shallow-burial conditions. D4 formed at approximately 61–76 °C. Its elevated Fe and Mn contents and higher degree of dolomite ordering are consistent with increasing burial depth, whereas its relatively low Sr and Na contents and δ18O values collectively suggest formation or substantial modification through burial dolomitization under intermediate- to deep-burial conditions. (4) The diagenetic-pore evolution of the high-quality reservoirs was controlled by multiple constructive and destructive diagenetic processes. Penecontemporaneous dissolution generated the initial pore system, whereas cementation and compaction during shallow burial caused substantial reservoir densification. During intermediate to deep burial, burial-related dissolution represented the principal constructive process responsible for secondary pore regeneration. The ultimate development of high-quality reservoirs depended on late-stage dissolution exceeding the destructive effects of earlier diagenesis, together with the pore-preserving effect of hydrocarbon charging. These findings provide new insights into the genesis of dolostones and the formation mechanisms of high-quality carbonate reservoirs and offer a geological basis for future hydrocarbon exploration and development in the T2l3 of the northwestern Sichuan Basin.