The multistage genesis of dolomites in the Feixianguan Formation on the eastern flank of the Kaijiang–Liangping trough, northeastern Sichuan Basin

The Feixianguan Formation along the eastern and western margins of the Kaijiang–Liangping trough in northeastern Sichuan is dominated by dolostone reservoirs that retain porosities up to 28% even at burial depths of 5,500–7,000 m, making them major targets for gas exploration. However, the vertical distribution of dolostone types on the eastern flank varies markedly, and the origins and diagenetic environments of dolomitizing fluids remain controversial. This study integrates core, thin-section, cathodoluminescence (CL), and geochemical data (δ¹³C, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, rare-earth and trace elements) to constrain dolomitization processes in the Feixianguan Formation. Four dolostone types are identified: micritic (D1), oolitic (D2), crystalline (D3), and residual oolitic dolostone (D4). All show weak CL; D1 exhibits dark-red luminescence due to near-surface oxidation where Fe and Mn occur in high-valence states and cannot enter the dolomite lattice. Geochemically, D1 shows relatively high δ¹³C, reflecting evaporative seawater involvement; D2 has lower δ¹⁸O and the highest Ce/Ce*, indicating meteoric dilution and stronger oxidation; D3 and D4 display lower Ce/Ce* and higher U, suggesting more reducing, closed diagenetic systems. Their ⁸⁷Sr/⁸⁶Sr ratios approximate coeval seawater, implying residual trapped seawater as the main fluid. D1 formed during syndepositional evaporative dolomitization, D2 through seepage-reflux under meteoric influence, and D3–D4 during burial, with D4 undergoing partial recrystallization under shallow- to moderate-temperature conditions, producing heterogeneous margin-to-center replacement and preserving residual oolitic textures. These results reveal an evolutionary transition from shallow evaporative to deep closed diagenetic systems, offering insights into reservoir development and exploration potential.