Diagenetic fluid evolution and its implication for hydrocarbon accumulation in Ordovician carbonate of the Tazhong area, Tarim Basin: Constraints from petrology, fluid inclusions, and geochemistry of calcite cements

Abstract

The Ordovician carbonate reservoirs in the Tarim Basin have undergone multiple tectonic events and fluid activities, complicating reservoir quality. Understanding these fluid-rock interaction processes is critical for unraveling reservoir heterogeneity evolution and hydrocarbon migration chronostratigraphy. Multiple generations of carbonate cements in fault-related reservoirs preserve important fluid activity signatures, providing constraints on fault reactivation, vertical hydrocarbon migration pathways, and accumulation preservation mechanisms. This investigation systematically examines fault-zone hosted carbonate cements and fracture-filling vein assemblages obtained from well cores, with particular emphasis on their fluid origins and the evolutionary processes. We utilized an integrated approach incorporating petrographic analysis, fluid inclusion microthermometry, and geochemical techniques to identify two distinct generations of fault-associated carbonate cements and three discrete phases of calcite veins, listed chronologically from oldest to youngest: fibrous calcite cements (C1), blocky calcite cements (C2), fracture-filling fine calcite cements (C3), coarser blocky calcite vein cements with zoned cathodoluminescence (C4), and last fracture-filling coarse calcite cements (C5).

The carbon and oxygen isotopic composition of C1, similar to well-preserved Ordovician carbonate rocks, along with its fibrous texture and near-micritic grain size, suggests formation during the early diagenetic stage under a marine environment. The lighter δ¹⁸O (av. = −7.12‰ ± 0.40‰ VPDB) and lower Sr (av. = 183.75 ± 32.30 ppm) content of C2 indicate precipitation during shallow burial diagenesis. Early vein cement (C3) containing single-phase liquid inclusions suggests precipitation in a near-surface environment. The estimated δ¹⁸O_fluid and REE_SN patterns, which parallel the seawater profile, further support the parent fluid of C3 originated from primary marine water. The slightly depleted δ¹³C values (av. = −0.97‰ ± 1.02‰) of C4 reflect external organic carbon input. Additionally, the δ¹⁸O_fluid and river-like REE_SN patterns, reflecting parent fluid of C4 was derived from a mixture of meteoric and marine water. The non-CL vein cement of C5 displays more depleted δ¹⁸O (av. = −10.98‰ ± 1.24‰) value and higher Fe (av. = 34 513.66 ± 269.3 ppm) and Mn (av. = 248.86 ± 104.85 ppm) concentrations, indicating its precipitation in an intermediate to deep burial reducing environment. The higher δ¹⁸O_fluid values, combined with higher temperatures and salinities, are consistent with a burial basin brine origin.

Consequently, we developed a spatial-temporal evolution model of fault-related fluid circulation. The driving mechanism of marine-meteoric mixed fluid circulation is associated with extensive exposure and erosion events induced by Caledonian tectonic uplift, as evidenced by ubiquitous breccias, abundant dissolution vugs and enlarged fractures. This stage of fluid activity significantly enhanced the permeability of fault zones, providing conduits for hydrocarbon migration, consistent with the observed bitumen and oil-bearing fluid inclusions within veins. Deep basin brine circulation, driven by late-stage fault reactivation, was facilitated by hot brines migrating upward along an array of en-echelon faults associated with the NEE-trending strike-slip fault system. We anticipate that our findings will advance the understanding of fault-controlled diagenetic processes and provide critical constraints for predicting hydrocarbon accumulation patterns in structurally complex reservoirs.