Alkali-based chemical enhanced oil recovery is used to increase the incremental oil production and relies on the interaction of alkali lye with crude oil. Optimizing the chemical formulation depends on screening multiple alkali concentration ranges to maximize the generated emulsion volume. However, the strong chemical variation of the crude oil, caused by biodegradation within the reservoirs, and its influence on generating in-situ soaps has never been studied in detail. In this paper, organic geochemical oil characterization was combined with phase screening data. Thereby, differences in terms of initial crude oil composition (slightly degraded, moderate degraded, and heavy degraded oils), but also regarding the excess alkali-equilibrated oil phase, and the hydrocarbons which are included in the in-situ soap (emulsion phase) were worked out. Different alkalis, concentration ranges, and water-oil-ratios were studied, to understand the control mechanism for the in-situ soap generation. The formed emulsions were observed over time (emulsion stability) and classified into possible emulsion types. Alterations caused by the alkali-oil interaction were quantified by the use of organic geochemistry. The degree of biodegradation seems to be an important control mechanism. Slightly degraded oils do not interact with the alkali lye due to a high content of non-polar compounds such as alkanes and isoprenoids. Moderately and heavily degraded oils show varying interactions with the alkali solution. The degree of interaction depends on the used degradation type, the alkali type as well as on the used concentration. Moderately degraded oils interact in a lower concentration range compared to heavily degraded oils, which show an increased non-soap forming acid content. Phase experiments conducted over a long period of time (100 days) were used to determine which emulsion types (micro or macro) were formed. Stable micro emulsions were preferably generated over time in formulations prepared with potassium carbonate (K2CO3). In contrast, formulations containing sodium carbonate (Na2CO3) hardly did not generate stable micro emulsions. The composition of alkali-equilibrated oil differs from the initial oil composition and shows a strong dependency of the used alkali type. Potassium carbonate caused mild alterations of the moderately and heavily degraded oils, whereas the impact of sodium carbonate was more severe. In addition, dissolution of naphthalenes, steranes, hopanes, and parts of the generated in-situ soap into the aqueous phase, was observed during the performed phase experiments of the moderately degraded oils (at the elevated pH values).
