Evidence for Mg-rich fluids in the diagenesis and porosity evolution of the Sirban Dolomite, Hazara Basin, Pakistan

The Sirban Dolomite of the Hazara Basin, Pakistan, records a complex diagenetic evolution that strongly controls its reservoir quality. This study integrates field observations, petrography, stable isotope geochemistry (δ¹⁸O, δ¹³C), X-ray diffraction (XRD), and petrophysical analysis (JMicrovision) to reconstruct dolomitization history and porosity evolution. Data are derived from measured stratigraphic sections of 250 m and 240 m, 40 thin sections, and multiple isotope samples. Field observations reveal bedding-parallel stylolites indicative of burial compaction, varied chert associated with silica-rich conditions and syn-sedimentary exposure, algal stromatolites reflecting biological activity, brecciation linked to tectonic episodes, hematitic beds and quartzites suggesting subaerial exposure, and upwelled phosphatic horizons redeposited on shallow marine shelves. Petrographic analysis identifies five matrix dolomite phases (MD-1 to MD-5), ranging from early cryptocrystalline dolomicrite (MD-1), typically associated with stromatolites, chert, and phosphates in intertidal to supratidal settings, to coarse crystalline and anhedral dolomite (MD-5) formed during deep burial. Intermediate phases include fabric-retentive dolomites (MD-2) and transitional recrystallized dolomites (MD-3), preserving ghost textures such as peloids. In addition, five generations of dolomite cement (including zoned and saddle dolomites), along with detrital quartz, syntaxial calcite, and telogenetic calcite, are recognized. XRD analysis confirms dolomite as the dominant phase with minor calcite, quartz, and gypsum, showing variations in stoichiometry and ordering that reflect early to shallow burial dolomitization. Fabric-retentive and coarse dolomites indicate a progression from near-surface to intermediate burial diagenetic environments. Stable isotope data indicate progressive fluid evolution. Early dolomicrite (MD-1; δ¹⁸O: − 5.71 to − 3.23‰ VPDB; mean − 3.85‰) preserves a Cambrian marine signature, suggesting formation in restricted lagoonal to tidal-flat environments via reflux-seepage processes. Fabric-retentive dolomite (MD-2; δ¹⁸O: − 6.84 to − 5.65‰ VPDB) overlaps with marine values, indicating shallow burial diagenesis. Transitional dolomite (MD-3; δ¹⁸O ≈ − 5.46‰ VPDB) reflects recrystallization of earlier phases by Mg-rich pore fluids. Later dolomite phases and cements exhibit more depleted values (δ¹⁸O down to − 9.37‰; δ¹³C: − 3.87 to − 0.39‰ VPDB), consistent with burial-modified Mg-rich basinal brines. XRD analysis indicates dolomite stoichiometry ranging from 48.43 to 54.66% CaCO₃ and ordering indices between 0.37 and 0.75, suggesting finely crystalline Ca-rich to near-stoichiometric dolomite formed under near-surface to shallow burial diagenetic conditions. Porosity analysis based on 2-D thin-section data shows that compaction and cementation significantly reduced reservoir quality; however, residual porosity types including vuggy, fenestral, fracture, and dissolution porosity persist, with values ranging from 0.2 to 14.8%. These results provide insights into porosity distribution, although 3-D reservoir properties, particularly permeability, remain unconstrained. A multiphase diagenetic model is proposed: (1) early reflux-seepage dolomitization in restricted lagoonal–tidal flat environments, and (2) subsequent burial-related recrystallization and cementation driven by Mg-rich fluids, possibly sourced from arenaceous and argillaceous units of the underlying Mirpur Formation. However, constraints on fluid temperature and origin remain limited due to the absence of trace element geochemistry, Sr-isotope data, and fluid inclusion microthermometry.