Comprehensive evaluation of geology-engineering “sweet spot” of tight oil reservoir in Maling Area, Ordos Basin, China

With the continuous growth of global demand for energy, unconventional oil and gas resources such as tight oil, has become a key task for exploration and development targets in China. The Ordos Basin’s Chang 7 reservoir stands as China’s largest repository of unconventional tight oil and gas resources. It is characterized by a unique coexistence of source and reservoir rocks within tight sandstone formations. The area boasts an abundance of high-quality hydrocarbon source rocks, a complex pore structure, pronounced heterogeneity in oil saturation, and a significant variation in compressibility. These features pose considerable challenges to the efficient and extensive development of the reservoir scale. This research focuses on the Chang 7 tight oil reservoir within the Maling district of the Ordos Basin, aiming to systematically investigate the quality of hydrocarbon source rocks, the oil content of the reservoir, and the engineering properties. By selecting key geological and engineering parameters that influence the reservoir quality, a “sweet spot” classification and evaluation framework for tight oil is developed using fuzzy mathematical approaches. This framework is then applied to assess the tight oil reservoirs in the study area and to predict the presence of sweet spots of tight oil. The findings of this research indicate that the tight oil sweet spot evaluation scheme is comprised of three categories with a total of nine indicators. The tight oil in the study area is further categorized into four distinct classes through the introduction of a multivariate comprehensive evaluation coefficient. Specifically, the “sweet spots” are characterized by a thickness of high-quality hydrocarbon source rock exceeding 15 m and a reservoir oil content index surpassing 15. Additionally, the brittleness index in these areas is found to be more than 45%. It is observed that in Class I and Class II “sweet spots,” the reservoirs are more developed, leading to higher initial production rates and better long-term stable production outcomes. The study underscores that the efficient development of tight oil is contingent upon an integrated assessment of the quality of hydrocarbon source rock (as the foundation), reservoir oil content (as an intrinsic factor), and engineering quality (as an extrinsic factor). This comprehensive approach is essential for optimizing the extraction and utilization of tight oil resources.