Despite the obvious link between hydrocarbon seepage at the surface and the activity of petroleum systems at depth, the majority of studies on seep carbonates concentrate on shallow aspects, whereas they are rarely oriented toward a source-to-sink perspective. Here, seep carbonates are conceptualized as a key element for analyzing petroleum systems at basin scale. Mapping of a 150-m-thick, 200-m-wide outcrop of the Lower Cretaceous slope wedge of the southeastern France Basin reveals two distribution patterns of seep carbonate concretions: (1) aligned in a continuous bed secant to the stratigraphy and (2) clustered in patches vertically stacked over 35 m. The δ¹³C signatures of the seep carbonates as light as −40‰ Peedee belemnite point toward a biogenic methane–dominated seepage. A set of turbidite channels pinching out right below the seep carbonates is seen as a potential gas trap, whereas biostratigraphic analysis indicates that the channels are coeval with a prominent black shale in the center of the basin. Any biogenic methane generated by the black shale could have used the channels as drains up to their pinch-out. The switch from a continuous seep carbonate level to stacked patches suggests initial widespread capillary leakage, followed by the opening of a preferential migration pathway. The distribution of seep carbonates thus appears dominated by breaching mechanisms of reservoir–cap-rock seals. This case study illustrates how seep carbonate mapping, in combination with the tectono-sedimentary context of each basin, may be a valuable tool for petroleum geologists.