Log-Based Identification and Distribution Characteristics of Diagenetic Facies in Low-Permeability Dolomite Reservoirs

Low-permeability dolomite reservoirs undergo multiple-stage diagnostic transformations, resulting in complex pore networks and pronounced heterogeneity that challenge reservoir prediction. Focusing on the Ordovician Majiagou Formation dolomite reservoir in the Yan'an Gas Field, China, this study integrates cathodoluminescence, scanning electron microscopy, and statistical logging crossplots to establish a quantitative framework for diagenetic facies classification. The results indicate that the diagenetic environment of the Majiagou Formation primarily comprises a seawater sedimentary setting, an atmospheric leaching setting, and a burial diagenetic setting. Five principal diagenetic facies were identified within the dolomite reservoir: quasi-syngenetic dolomitization facies, atmospheric freshwater dissolution facies, weathered karst breccia facies, argillic filling facies, and buried dissolution facies. The PE-DEN and PE-RLLD crossplot techniques achieve 82.8% identification accuracy against thin-section analysis. Additionally, alternative approaches such as RLLD-AC and CNL-RLLD crossplots provide supplementary discrimination. Influenced by paleogeomorphic uplift and denudation, the atmospheric freshwater dissolution facies and weathered karst breccia facies are predominantly developed at the formation's upper strata, whereas the base of the formation is more susceptible to overlying acid fluid dissolution during the burial phase, leading to the predominance of the buried dissolution facies. The dissolution intensity of atmospheric freshwater dissolution facies and quasi-syngenetic dolomitization facies is significant, resulting in favorable physical properties. Integrating the paleogeomorphic context, regions such as the gentle hills on the karst slopes and both sides of grooves are recommended as prime targets for future gas reservoir exploration. This research is applicable to carbonate basins with analogous tectonic settings, enabling systematic evaluation of diagenetic facies in low-permeability dolomite reservoirs.