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Ordos Basin is a Mesozoic sedimentary basin that underwent long-term evolution on the North China Craton. Many scholars have confirmed that in the Late Triassic, the basin was surrounded by ancient continents, and there were multiple provenance supply directions. Combined with the nature of the basement of the basin and the characteristics of the present structure, it is believed that the Jiyuan area is located in the central and western parts of the basin, spanning two first-level structural units, the Tianhuan Depression and the Yishan Ramp. This special geographical location makes Jiyuan area affected by bidirectional provenance. Controlled by the northwest and northeast depositional systems in the basin, Jiyuan area has accepted complex sedimentation and diagenesis, forming a low-porosity ultralow-permeability reservoir. However, the understanding of bidirectional provenance has been neglected in many previous studies on reservoir characteristics in the Jiyuan area. Therefore, the differential evolution of sedimentation and diagenesis caused by bidirectional provenance will cause serious deviations in the original understanding of reservoir characteristics in the Jiyuan area, which will inevitably affect subsequent exploration and development research work. In this paper, the mineral composition, physical properties, diagenesis, and diagenetic evolution of the Jiyuan area are studied by combining a large number of tests such as core physical properties, casting thin sections, scanning electron microscopy, cathodoluminescence, and X-ray diffraction. Then, the origins of reservoir development in two areas dominated by bidirectional provenance are analyzed and compared. Furthermore, the diagenetic facies are characterized by a cluster analysis of logging data, and finally, the reasons for the differences in reservoir distribution and the genetic mechanism between the Yinshan provenance area (YPA) and Alxa provenance area (APA) are obtained. The results show that, first, due to the different provenance, compared with the YPA, the reservoir pore space in the APA is better developed and the physical properties are better. Second, the clay mineral content and diagenesis are more important causes of reservoir differentiation, and the reservoir pores in the YPA are more affected by kaolinite and chlorite filling than those in the APA. Although more dissolution improvements have been obtained, the damage to the reservoir caused by cementation in the middle and late stages is extremely fatal, while the chlorite film in the APA reservoir has a better protection effect on the primary intergranular pores. Third, after the evolution of pores in the APA reservoir, more intergranular pores are preserved, and the distribution range of high-quality diagenetic facies is wider than that in the YPA. Finally, sedimentation is the basis for high-quality reservoir development, and good mineral content composition and favorable diagenetic transformation cause reservoir dissimilarity.

The microscopic pore-throat structure of low-porosity and ultralow permeability sandstone reservoirs controls the seepage characteristics, which directly affects the water injection development efficiency of oilfields. Different from typical tight sandstone reservoirs, macropores and mesopores are more developed in the pore-throat structure of this type of reservoir, which changes the dominance of micropores over seepage capacity. Based on the full-range pore-throat structure characterization method and fractal theory, many experimental methods are used to study the influence of the microscopic pore-throat structure over the seepage characteristics in the Chang 9 reservoir in the Yanchang Formation of the Ordos Basin. The results of 12 typical samples show that the pore-throat structure has multifractal characteristics, and the occurrence degree of movable fluid and seepage capacity vary greatly, showing strong microscopic heterogeneity. Following characterization of the full-range pore-throat structure, the relative proportion of macropores and mesopores determines the physical properties of the reservoir. The pore-throat scale and structural heterogeneity have a significant impact on porosity, while the pore-throat structure connectivity has a crucial impact on permeability. Quartz provides resistance to compaction and preserves more primary pores. Additionally, the relationship between clay minerals and physical properties is not significant. Only illite and I/S mixed layers have a slight effect on permeability reduction. Furthermore, laumontite cementation is the key factor in the destruction of the pore-throat structure. Porosity has a significant effect on movable fluid occurrence and is more closely related to the two-phase seepage. Permeability controls the oil displacement efficiency in the anhydrous period, and porosity controls the oil displacement efficiency in the final period. The fractal dimension has some significant controls on the pore-throat structure, which are reflected in the fact that the higher the homogeneity of macropores is and the higher the heterogeneity of mesopores and micropores is, the better the reservoir development will be. In particular, the degree of macropore development guarantees reservoir quality. The control of the fractal dimension on the seepage capacity is complex, especially for mesopores and micropores; the higher the degree of heterogeneity is, the stronger the seepage capacity will be. The occurrence of movable fluid is significantly affected by the scale and heterogeneity of the pore-throat structure, which is reflected as stronger heterogeneity of the pore-throat structure and poorer relative seepage capacity.

The spatial and temporal evolution of the sandbody architecture of shallow-water deltas in open lacustrine basins is controlled by the classification of allocyclicity and autocyclicity. On the southwestern margin of the Ordos Basin, a braided river system deposited a shallow-water delta in the Late Triassic Period. Based on the principle of sequence stratigraphy and the hierarchical analysis of reservoir architecture, the spatial and temporal evolution of individual sandbodies in the Chang 81 member of the Yanchang Formation in the Zhenbei Oilfield is interpreted by utilizing data from cores, wells and outcrops. The research ideas are as follows: large deposition scale architectural elements (first- to third-order cycles, as defined by Miall) of different sequence levels are affected by allocyclicity associated with changes in tectonic activity, provenance, and sea level, and small deposition scale architectural elements (fourth- to fifth-order cycles, as defined by Miall) of different sedimentary facies mainly consist of individual sandbodies that are affected by autocyclicity associated with lake-level changes caused by various river processes. Based on previous studies, the results are as follows. The sedimentary characteristics of shallow-water deltas have been verified by core and outcrop data. In addition, three ultrashort-term cycles are identified on the basis of boundary sequences and lithofacies’ sequences in the outcrop section of the Rui River, and three sedimentary evolution stages of the delta front are defined. Finally, according to well data, five types of architectural elements at the level of single sandbodies are identified. The vertical superimposition and lateral contact relationships of different architectural elements indicate that during the three sedimentary evolution stages, the hydrodynamics weakened, strengthened slightly, and finally weakened substantially. Among the 20 kinds of architectural element spatial combination patterns formed by single sandbodies, primary and secondary sandbodies have great potential for hosting remaining oil. In the process of architectural spatiotemporal evolution, the geometry and connectivity of the underwater distributary channel gradually weakened, and the spatial relationship between the underwater distributary channel and other architectural elements increased. This article proposes a new method for researching shallow-water deltas and has some guiding significance for exploiting the remaining oil in oil fields.

The exploration and development of the dual-provenance lower assemblage of the Yanchang Formation in the Jiyuan area has progressed rapidly. At the intersection of this bidirectional provenance system, a complex and variable spatial combination of sand bodies formed, resulting in significant structural heterogeneity in the development and distribution of reservoirs. Based on previous studies, this paper combines core data and logging data with a large number of analytical tests and production performance data to carry out research on the Chang 82–Chang 9 reservoir group in the lower assemblage of the Yanchang Formation in the Shijiawan-Buziwan area. Based on the analysis of sedimentary conditions, the sand body development pattern at the intersection of the bidirectional sedimentary system in the study area was analysed by stepwise dissection of the sand body architecture. After the types and characteristics of the 4th- to 5th-level architectural elements were determined, the spatial distribution of the combinations of these elements was assessed and combined with logging discriminant analysis and geometric shape prediction methods to identify a ‘prism’ architectural distribution pattern. The architectural elements are connected with the distribution of diagenetic facies, the spatial distribution patterns of different types of diagenetic facies under the constraints of the architecture are summarized by region, and the locations of potential favourable reservoir development are discussed. The results show that the degree of superposition and combination of the eight skeletal architectural elements in the target layers gradually deteriorate from the bottom to the top. In addition, the development scale and degree of architectural elements in the braided river delta system in the west are better than those in the meandering river delta system in the east. In the different sedimentary areas, the spatial combination styles of the architectural elements are quite different, and the combination of these elements gradually changes from a combination of braided channels (FA1) and abandoned channels (FA2) to a combination of underwater distributary channels (FA4). Matching of the distribution of diagenetic facies with the distribution of architectural elements reveals that the diagenetic facies dominated by intergranular pores and dissolution pores (associated with good reservoir physical properties) are usually found at the bottom or in the lower to middle parts of the skeletal architectural elements.