Progradational Stratigraphic Architecture and Its Implications for the Chang 6–Chang 7 Members of Yanchang Formation in Hujianshan Oilfield, Ordos Basin

The traditional correlation and division scheme of the Yanchang Formation in the Ordos Basin, based on quasi-isopach stratigraphy, exhibits diachronous characteristics. To reconstruct the original depositional framework of the Chang 6–Chang 7 Members in the Hujianshan Oilfield, located in the northwestern part of the basin, a chronostratigraphic correlation and framework construction was conducted. Using densely distributed well data and abandoning the conventional concept of isopach stratigraphy, an isochronous stratigraphic framework was established through well-by-well correlation, guided by marker beds and aided by sedimentary cycle characteristics. The results reveal the development of progradational stratigraphic architectures with different provenance directions within the Chang 6–Chang 7 interval. During the depositional period of the Chang 7 to Chang 63, the study area was a mixed-source region where southwestern and northeastern provenances exhibited interfingering and compensatory variation in a vertical sequence. During the Chang 72+3, Chang 631, Chang 62, and Chang 61 deposition stages, the southwestern source was dominant, while the northeastern source prevailed during the Chang 71 and Chang 632 stages. These findings differ significantly from previous interpretations based on the traditional stratigraphic division of the Yanchang Formation, which generally assumed northeastern provenance control over the Hujianshan Oilfield. This study expands the recognized extent of the southwestern provenance and provides direct stratigraphic evidence for mixed-source depositional settings within the Yanchang Formation. The findings offer important insights for transitioning from traditional diachronous isopach stratigraphy to chronostratigraphic frameworks in geological characterization of related hydrocarbon reservoirs in the Ordos Basin.