Origin of Pyrite Morphotypes: Implications for Albian-Cenomanian Carbonate Reservoirs, Northern Kuwait

Summary The Late Cretaceous (Albian-Cenomanian) Tuba Carbonate Formation in the Sabriyah Oil Field contains diverse pyrite (FeS2) morphotypes that record complex diagenetic and thermal evolution. This study integrates morphological analysis with stable sulfur isotope geochemistry (δ34S‰ vs. VCDT) to constrain pyrite genesis and its impact on reservoir. Three principal morphotypes were identified: (1) framboidal pyrites (biogenic pyrite) (50 μm), and (3) Anhedral crystals irregular forms with overgrowth textures (thermal pyrite). Framboidal varieties formed via Microbial Sulfate Reduction (MSR) under anoxic-euxinic conditions during early diagenesis, yielding strongly depleted δ34S‰ values (−10‰). Intermediate morphotypes, subhedral crystals reflect episodic fluid migration and redox fluctuations, with transitional δ34S‰ isotopic signatures (−20.0‰ to −10.1‰). Analysis of 19 samples classified ten as biogenic (52.6%), six as mixed biothermal (31.6%), and three as thermal (15.8%). These genetic distinctions control reservoir architecture: biogenic pyrites preserve intergranular porosity, while thermal pyrites occlude pore throats and reduce permeability. The δ34S‰ signature provides both a diagenetic tracer for burial history reconstruction and a predictive tool for reservoir quality assessment in heterogeneous carbonate systems.