Accurate determination of the oil–water interface and transition zone thickness is crucial in bottom water reservoirs. Currently, engineers primarily rely on regional geological data and experimental data to ascertain complex oil–water interfaces. However, incomplete geological static data and core data can potentially lead to misjudgment of the oil–water relationship, adversely impacting reservoir development. This study employs numerical simulation methods combined with sedimentary microfacies characteristics to analyze factors influencing water production from oil wells, block oil–water interface elevation, and recovery degree in the T1-11H unit of the T1 oil reservoir. Our findings elucidate that variations in capillary pressure resulting from differences in reservoir properties contribute to the absence of a uniform oil–water interface within this unit. Based on this understanding, we propose an approach for determining the thickness of the oil–water transition zone within different areas using a divided area numerical simulation model. Multiple modeling iterations and interactive verifications are conducted with actual drilling results serving as validation standards for establishing appropriate transition zone thicknesses in the T1-11H unit. The discrepancy between our determined thickness through numerical simulation method and actual drilling results is less than 5%. These research outcomes provide a foundation for subsequent recalculation of reservoir reserves and adjustment of well networks.