To clarify the impact of mixing intensity on reservoir pore-throat characteristics and its application in classifying mixed sandstone reservoirs, a case study of the Miocene Asmari Formation in the M Oilfield, Iraq, was conducted. Integrating core samples, thin sections, measured porosity-permeability data, well logs, and geochemical data, we analyzed how mixing intensity controls pore-throat structures and established a classification scheme for mixed sandstone reservoirs. Key findings include:①Diverse mixed rock types (e.g., sand-bearing grainstone, sandy dolomitic grainstone, dolomite-bearing sandstone, and dolomitic sandstone) were identified. Mixing intensity was quantified by the volumetric ratio of terrigenous clastics to carbonate components: <25% minor component = low mixing, 25%-50% = high mixing.②Mixing intensity significantly controls pore-throat structures. In sandstone reservoirs, when dolomitic mixing intensity (Hjy) <25%, primary intergranular pores with constricted throats dominate (Φ>15%, K>100mD). At Hjy>25%, pore systems evolve into intergranular + intercrystalline (dissolved) pores with constricted + intercrystalline throats, causing sharp declines in Φ and K. In carbonate reservoirs, siliciclastic mixing intensity (Hjs) >25% reduces pore-throat connectivity (K<10mD).③Based on pore-throat responses and mixing thresholds, mixed sandstone reservoirs are classified into four types (I, II, III, IV) with distinct characteristics.This study demonstrates that mixing intensity governs reservoir heterogeneity. The classification scheme integrating mixing intensity and pore-throat structures effectively predicts favorable reservoir distributions, providing a geological basis for efficient hydrocarbon exploration in mixed sandstone reservoirs.
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