Paleoenvironmental Reconstruction and Organic-Matter Enrichment Mechanisms of Marine Shale from the Ordovician Wulalike Formation, Northwestern Ordos Basin

Abstract

The marine shale of the Ordovician Wulalike Formation in the northwestern Ordos Basin is an important target for shale gas exploration in northern China. The shale is thermally mature to locally highly mature, and generally has relatively low total organic carbon (TOC) and gas contents. Consequently, the paleoenvironmental controls on organic matter enrichment and the criteria for identifying favorable exploration intervals remain poorly constrained. This study integrates newly generated data from Wells LY1 and NBY1 with compiled data from seven representative wells (LY1, NBY1, QT9, ZP1, YT1, HT1, and L105). Organic geochemical, biomarker, major- and trace-element, rare earth element, and mineralogical data are used to reconstruct paleowater depth, bottom-water redox conditions, water-mass restriction, paleoclimate, paleosalinity, and paleoproductivity. The results reveal an upward transition from comparatively deeper water, weaker ventilation, and more reducing conditions in the Wu-3 Member to shallower, better-circulated, and more oxygenated conditions in the Wu-1 Member. The Wu-3 Member records the strongest bottom-water reduction and water-mass restriction, the greatest nutrient availability and paleoproductivity, and the most favorable preservation conditions. The Wu-2 Member records intermediate conditions, whereas the Wu-1 Member accumulated in relatively shallow, oxic–dysoxic water. Organic matter was derived predominantly from marine algae, phytoplankton, and acritarchs, with limited terrigenous input, and the silica was mainly biogenic. Organic matter enrichment was jointly controlled by primary productivity, reducing preservation conditions, rapid burial, and dilution by carbonate and terrigenous material. The lower Wu-3 and middle Wu-2 members are identified as the most favorable shale gas exploration intervals. These findings clarify the coupling between paleoceanographic evolution and organic matter accumulation and provide a geological basis for predicting shale gas sweet spots in the Wulalike Formation of the northwestern Ordos Basin.

Bibliographic Information