Geochemical controls on colloidal stability and polar compounds deposition in Cretaceous –Tertiary carbonate reservoirs using core extracts, SW Iran

Asphaltene instability and precipitation represent major flow-assurance challenges in carbonate reservoirs and are strongly influenced by fluid composition, thermal maturity, and hydrocarbon charge history. Predicting these processes remains difficult because conventional compositional indices often fail to capture the combined effects of active polar compounds and refractory organic matter under heterogeneous reservoir conditions. In this study, we investigate the geochemical controls on asphaltene stability in Cretaceous–Tertiary carbonate reservoirs using an integrated analysis of bulk oil composition, biomarker distributions, and Rock-Eval pyrolysis data derived from reservoir rock bitumen. Rock-Eval pyrolysis was performed on 127 core samples, complemented by biomarker (GC–MS) and SARA compositional analyses (Iatroscan–FID) on 21 core extracts from multiple carbonate reservoirs. The results show that variations in the balance between saturates, aromatics, resins, and asphaltenes exert a primary control on oil colloidal behavior. Depletion of resin and aromatic fractions combined with saturate enrichment promotes colloidal instability and increases the likelihood of asphaltene aggregation. Shallow reservoir intervals display relatively stable colloidal systems, whereas deeper intervals exhibit pronounced instability reflected by elevated colloidal instability indices and reduced polarity indices. Geochemical results indicate that the free oils and bitumen are non-biodegraded and are predominantly derived from marine Type II organic matter with varying levels of thermal maturity. To better characterize polar compound behavior, a composite polar index (CPI) is introduced by integrating generative polar fractions (S2b), refractory organic matter (residual carbon), and reactive organic content derived from Rock-Eval parameters. The CPI effectively discriminates stable, transitional, and critical intervals and identifies zones prone to polar compound accumulation and asphaltene precipitation, including conditions where conventional SARA-based indices show limitations. The results indicate that asphaltene instability is primarily governed by compositional evolution and thermal maturity rather than pressure-related effects alone. This integrated geochemical framework provides a quantitative and transferable approach for predicting asphaltene precipitation risk in heterogeneous carbonate reservoirs.