Controls of Structural Evolution and Complex Lithologic Architecture on the Identification and Accumulation Mechanisms of Low-Contrast Reservoirs: A Case Study from the Chang 3 Member, Zhenbei Area, Ordos Basin

Low-resistivity reservoirs characterized by weak log contrasts are highly concealed and therefore difficult to detect using conventional oil–water discrimination methods. Recent exploration and development indicate that low-resistivity reservoirs are widely developed in the Triassic Chang 3 Member of the Zhenbei area, Ordos Basin. However, contrasting tectonic evolution associated with the Tianhuan Depression and complex lithologic assemblages in the western and eastern sectors have resulted in complicated hydrocarbon migration and accumulation processes. In this study, integrated well-log and geochemical data were used to systematically investigate the genesis of low-resistivity reservoirs in the Chang 3 Member and to establish oil–water discrimination charts. Three-dimensional seismic flattening was applied to restore the Late Jurassic paleostructure of the western Chang 3 Member and to analyze its tectonic evolution. Reservoir petrology and pore–throat architecture in the western and eastern areas were comparatively examined using thin-section petrography, field-emission scanning electron microscopy (FESEM), and high-pressure mercury intrusion. Results indicate that the development of low-resistivity reservoirs in the Chang 3 Member is primarily controlled by highly saline formation water and elevated bound-water saturation. Based on these controls, the invasion factor–acoustic transit time cross-plot and the apparent spontaneous potential difference (ΔSP) method effectively discriminate oil- and water-bearing intervals in a total of 25 wells within the study area. Paleostructural restoration reveals that the western Chang 3 Member has undergone a tectonic inversion from a west-high–east-low configuration since the Late Jurassic to the present-day east-high–west-low geometry. Oil–source correlation indicates that hydrocarbons in the Chang 3 reservoirs were mainly derived from the underlying Chang 7 source rocks, whereas the bimodal distribution of fluid-inclusion homogenization temperatures suggests that the reservoirs experienced two distinct charging episodes. Integrated analysis suggests that tectonic inversion during the Yanshanian movement, combined with multistage hydrocarbon charging, led to secondary migration and partial destruction of early-formed reservoirs in the western area, resulting in predominantly scattered accumulations. In contrast, the eastern area experienced relatively limited tectonic modification, and laterally extensive accumulations are controlled by Type I–III lithologic–structural traps formed by the Chang 3 reservoir interval and its overlying strata. These findings provide an important geological basis for the identification of low-contrast reservoirs and for the exploration and development of hydrocarbon accumulations that are jointly controlled by tectonic evolution and lithologic heterogeneity.