The Nigerian oil and gas sector, long regarded as the backbone of the national economy, is undergoing a major transformation following downstream deregulation and the enactment of the Petroleum Industry Act (PIA) 2021. These reforms have opened the industry to increased competition, foreign participation, and indigenous involvement, particularly by Small and Medium Enterprises (SMEs). While SMEs are increasingly significant contributors to local content development, technology transfer, and employment generation, they face unique challenges in a liberalised market, including exposure to oil price volatility, foreign exchange instability, and competition from well-capitalised multinationals. This study investigates strategic investment models tailored to indigenous oil and gas SMEs in Nigeria’s liberalised market. Drawing on theoretical frameworks and sector-specific evidence, the paper examines models such as capital budgeting, portfolio diversification, vertical integration, public–private partnerships, risk management, sustainable investment, innovation-driven models, and asset optimisation models. Particular emphasis is placed on the sustainable investment model, which integrates profitability, environmental responsibility, social engagement, and governance transparency, aligning SME strategies with both national content mandates and global ESG standards. Findings reveal that many SMEs still rely on reactive, ad hoc investment decisions, leading to inefficiencies, vulnerability to shocks, and missed opportunities in high-potential areas such as gas commercialisation, downstream distribution, and renewable integration. The study concludes that structured strategic investment models are indispensable for ensuring SME competitiveness, resilience, and long-term growth in Nigeria’s evolving energy landscape. It recommends a dual approach: SMEs should adopt hybrid investment strategies combining risk management, sustainability, and diversification, while policymakers should provide financing support, policy stability, and incentives for green investments.

 Keywords: Small and Medium Enterprises (SMEs), Liberalised Market, Petroleum Industry Act (PIA), Capital budgeting.