Characterization of Laminae and Lamina-Associated Fractures in the Tight Oil Reservoir of Xifeng Oilfield, Ordos Basin: CT Scanning of Full-Diameter Cores from Horizontal Well X119-19-39H

This study focuses on the Chang 8 Member tight sandstone reservoir in the Xifeng Oilfield. Integrated approaches including 3D reconstruction of CT digital cores, seepage simulation, and multi-parameter coupling analysis were employed to systematically classify lamina types, quantitatively characterize lamina-associated fractures, and elucidate the pore–lamina fracture coupling seepage mechanisms. The results show that laminae in the study area can be categorized into three types: banded, inclined, and cross-laminated. Lamina-associated fractures are dominated by bedding-parallel fractures, with a small number of cross-bedding fractures having developed in cross-laminated intervals. The development of lamina-associated fractures is jointly controlled by sedimentary heterogeneity and mechanical differences between interlaminar layers. A significant positive correlation exists between the volume proportion of interlaminar layers and fracture porosity (R2 = 0.8913). Cross-laminated intervals exhibit the optimal fracture parameters, with fracture porosity reaching up to 3.6% and maximum fracture volume exceeding 3500 mm3, facilitating the formation of three-dimensional interconnected fracture networks. Distinct pore–lamina fracture coupling patterns are observed in different lamina types: independent development with weak coupling in banded laminae, segmented synergy with heterogeneous coupling in inclined laminae, and strong synergy with network coupling in cross-laminated intervals. In cross-laminated sections, matrix porosity and fracture porosity evolve synchronously, yielding the highest permeability (up to 7.3 mD) and superior storage–permeability performance. Seepage simulations confirm that permeability in tight reservoirs is not directly controlled by matrix porosity; instead, lamina-associated fractures act as the dominant fluid migration pathways. Cross-laminated fractures form multi-directional high-efficiency seepage networks, while banded and inclined laminae exhibit unidirectional linear seepage and segmented dominant channeling characteristics, respectively. These findings provide critical geological insights for sweet spot evaluation, horizontal well trajectory optimization, and hydraulic fracturing design in tight oil reservoirs.