Formation Mechanism, Reservoir Characteristics, and Identification of Low‐Efficiency Oil Layers: A Case Study of the Chang 6 Reservoir, Ordos Basin

ABSTRACT This study presents a comprehensive characterization of the low‐efficiency reservoirs within the Chang 6 member of the Ordos Basin, elucidating their formation mechanisms and establishing quantitative identification standards. These reservoirs typically develop in low‐energy sedimentary microfacies, specifically the margins of underwater distributary channels and inter‐distributary bays. Petrological analysis reveals that the reservoirs consist of fine‐grained feldspar sandstones rich in plastic interstitial materials and carbonate cements (>5%), exhibiting ultralow physical properties (average porosity 7.12%, average permeability 0.27 mD) and a restricted pore structure (average displacement pressure 1.79 MPa, median throat radius 0.03 µm). The genesis of these low‐efficiency zones is governed by a sedimentary‐diagenetic coupling mechanism: Fine‐grained sedimentation facilitated intense mechanical compaction, whereas late‐stage carbonate cementation caused substantial porosity loss, solidifying the tight nature of the formation. Hydrocarbon accumulation follows a “densification before accumulation” model, driven by an overpressure difference of 6–7 MPa generated by the underlying Chang 7 source rocks, which facilitated migration through microfractures into the tight sand bodies. To address the challenge of identifying thin, low‐resistivity oil layers, Walsh inversion and resolution matching techniques were employed to enhance logging signal precision. On the basis of core‐log calibration, a quantitative identification standard was established, defining low‐efficiency oil layers by an acoustic transit time and deep resistivity . Despite their marginal petrophysical properties, the reservoirs exhibit a high brittleness index (>60%) and low horizontal stress difference, indicating that large‐scale volume fracturing is a viable strategy for commercial development.