The Naiman Sag is located in the southwestern part of the Songliao Basin. It remains underexplored, and the conditions for hydrocarbon accumulation and the main controlling factors are not yet well understood. This study based on a comprehensive review of previous studies and integrates data from drilling, seismic profiles, reservoir lithology and petrophysics. It analyzes the petroleum geological characteristics, controlling factors for hydrocarbon accumulation, and resource potential of the Naiman Sag. The results indicate the primary hydrocarbon source rocks in the region are the Shahai, lower Jiufotang, and Yixian formations. These are mainly concentrated in the northern part of the sag and are generally at a low-to-mature stage (R_O<1.0%). Among them, the lower Jiufotang Formation, developed in semi-deep lake to deep lake facies, contains high-quality source rocks with high organic matter abundance (average TOC of 2.79%) and predominantly of Type I and Type II₁ kerogen. These source rocks are the primary contributors to hydrocarbons in the region. Biomarker characteristics indicate that the source rocks formed in a high-salinity, strongly reducing lacustrine environment, with mixed contributions from lower aquatic organisms and higher terrestrial plants. The hydrocarbons in the region are primarily heavy oil and wet gas, with heavy oils not undergoing significant biodegradation and mainly controlled by low maturity. The natural gas is characterized as sapropelic kerogen-derived gas in the low-to-mature stage, directly resulting from kerogen cracking. Hydrocarbon accumulation is primarily controlled by favorable source-reservoir configurations, dominant depositional facies, and advantageous lithology, demonstrating a trinity of “facies-lithology-structure” coupling. Additionally, deep fluid activities may play an important role in the accumulation process by not only promoting secondary hydrocarbon generation but also contributing to the enrichment of associated noble gases such as hydrogen and helium. Overall, the central and deeper “sweet-spot” zones of the sag, with abundant hydrocarbon supply, proximal traps, and active fluid movement, show significant exploration potential. Future studies should focus on the unconventional accumulation mechanisms under multi-factor coupling. This will deepen the understanding of the petroleum system in the Naiman Sag.