Effects of hydrothermal fluids on carbonate reservoirs: Case from the lower Permian Sichuan Basin (Qixia Formation), China

ABSTRACT The effects of hydrothermal fluids on carbonate reservoirs are a critical issue in petroleum geology. Here we present a comprehensive study of the lower Permian Qixia Formation in the Sichuan Basin, southwestern China, based on petrology, geochemistry, and geochronology. We identified universal effects of hydrothermal fluids on sucrosic dolomites of the Qixia Formation. These dolomites are classified on the basis of petrologic characteristics as anhedral medium- to coarse-crystalline dolomites (Dol-1), euhedral medium- to coarse-crystalline dolomites (Dol-2), and saddle dolomites (SD). The Dol-2 is further subclassified as dark-red Dol-2a and bright-rimmed Dol-2b according to the cathodoluminescence. The Dol-1 and Dol-2a, which, respectively, originated from remnant-grain-bearing matrix limestones and those modified by eogenetic karstification, are the products of horizontal thermal convective dolomitization by marine-related fluids during the shallow burial just after deposition (from 274.5 ± 9.9 to 269.1 ± 12.0 Ma). They are both geochemically characterized by left-leaning rare earth elements (REE) patterns with positive Y anomalies and relatively low fluid-inclusion homogenization temperatures (70°C–150°C). The Dol-2a have flatter REE patterns and lower Y/Ho values than Dol-1. The Dol-2b are the products of Dol-2a superimposed by fault-controlled thermal convective dolomitization by marine-related fluids during the Emeishan Large Igneous province (from 263.2 ± 10.3 to 258.7 ± 8.1 Ma). The SD are the cements of the dolomitizing fluids. The Dol-2b and SD are both geochemically characterized by left-leaning REE patterns with positive Eu and Y anomalies and relatively high fluid-inclusion homogenization temperatures (120°C–220°C). The SD have more positive Eu anomalies than Dol-2b. Hydrothermal dolomitization in the Qixia Formation made a limited contribution to reservoirs, which were inherited and modified during the early stage of hydrothermal dolomitization and infilled and destroyed during the late stage. Our study highlights that hydrothermal effects on reservoirs cannot be generalized and that the origin and properties of hydrothermal fluids and modified lithologic associations should be comprehensively considered, as they produce diverse hydrothermal effects. Whether or not high-quality carbonate reservoirs develop in deep to ultradeep zones is highly dependent on the occurrence of original favorable sedimentary facies and early constructive diagenetic modification. Burial diagenesis during the late stage may improve reservoirs conditionally.