Comparative Reservoir Characterization of Marine–Continental Transitional and Lacustrine Shales: Insights from the Shan 2 and Chang 7 Members in the Yan’an Area, Ordos Basin

Differences in reservoir properties between marine–continental transitional and lacustrine shales continue to obscure the mechanisms that control hydrocarbon enrichment, complicating the design of targeted exploration strategies. Here, we present a systematic, multi-scale comparison of the Permian Shan 2 Member (marine–continental transitional) and the Triassic Chang 7 Member (lacustrine) in the Yan’an area, Ordos Basin. In this study, the whole-rock and clay mineral X-ray diffraction (XRD), organic geochemistry (TOC and Ro), high-pressure mercury intrusion, low-temperature N2 and CO2 adsorption, argon ion polishing scanning electron microscopy (SEM) and rock mechanics tests were carried out on 78 subset core samples, and the micro-CT, three-dimensional micro-CT reconstruction was carried out on one shale sample selected from the Shan 2 Member and the Chang 7 Member, respectively. The average TOC of the marine–continental transitional shale in the Shan 2 Member is 2.74%, the average Ro is 2.78%, and the kerogen type is type III, which is at the over-mature stage of oil and gas. The main mineral composition is clay minerals, and the reservoir space is dominated by inorganic pores. It was observed in the local volume of the sample scanned by micro-CT that the three-dimensional porosity was 5.08%, and the pore connectivity was poor. The mechanical parameters of shale rock in Shan 2 section show that the in situ stress difference is small and the compressive strength is large. The mechanical properties of the rock hinder crack initiation and are not conducive to crack propagation, so the fracability of Shan 2 shale is poor. In contrast, the average TOC of Chang 7 lacustrine shale is 3.12%, and the average Ro is 1.16%. The type of kerogen is mainly type I, which is in the stage of mature oil and gas symbiosis. Mineral composition: the quartz and feldspar-based brittle mineral content is higher, the clay mineral content is lower, and rich organic pores are developed. Micro-CT scanning of a local volume of the sample showed that the three-dimensional porosity was 7.26%, and there was a continuous and flaky pore connectivity network. Due to its low Poisson’s ratio and low compressive strength, Chang 7 shale is conducive to crack initiation and shear fracturing; in the process of hydraulic fracturing, complex network cracks can still form, so the fracability of Chang 7 shale is good. Integrating reservoir characteristics, including present-day burial depth, Ro, TOC, depositional setting and fracability, the Chang 7 lacustrine shale may represent a promising primary exploration target for shale oil and gas in the Yan’an area. In contrast, the Shan 2 transitional shale appears better suited for co-development with its interbedded coal seams and tight sandstones. This comparative analysis offers insights into the formulation of differentiated exploration and development strategies for transitional and lacustrine shale systems. Notably, the dataset of this study has limited representativeness, and any such strategy would require validation through engineering design, production testing and economic assessment prior to field implementation.