Paleogeographic Control on the Early Depositional Interval of the Yurtus Formation Constraining the Heterogeneous Development of Lower Cambrian Source Rocks in the Keping Area, Northwestern Tarim Basin

The Lower Cambrian Yurtus Formation along the northwestern margin of the Tarim Basin has commonly been regarded as a regionally developed marine source-rock interval. However, the formation features strong internal source-rock heterogeneity, and the paleogeographic mechanism governing its thin organic-poor intervals remains to be further clarified, which constrains a more comprehensive understanding and evaluation of the Yurtus Formation. This study integrated whole-well source-rock geochemical data with major- and trace-element records from Well K1 in the Keping area to evaluate the geochemical background of the Yurtus Formation, its contrast with adjacent strata, and the internal differentiation of a thin and organic-poor interval at 5269–5274 m. The whole-well dataset of analytical results shows that the deep Yurtus-related section occurs within an overall low-TOC and low-S2 background. The 5269–5274 m interval is represented by only approximately 5 m of source-rock-bearing strata and is compositionally distinct from the adjacent carbonate-dominated strata. Compared with the adjacent intervals, the target interval has higher mean Al, Si, K, Ti, Fe, V, Cr, Zr, Rb, Sr, and U values, but lower Ca and Mg values. Elemental profiles and ratio data further define three first-order meter-scale geochemical subunits: an upper carbonate-rich subunit with high Ca and Ca/Al values, a middle aluminosilicate-rich subunit with higher Al, Si, K, Ti, and Fe values, and a lower chemically enriched subunit marked by elevated V, Cr, U, Sr, P/Ti, Sr/Ca, V/Cr, and S/Fe values. Mo enrichment is weak, and the available Mo–U and Mo/Al evidence does not support a definitive interpretation of persistent euxinic conditions. Therefore, the lower subunit is interpreted as recording relatively stronger reducing and chemically reactive conditions rather than a stable euxinic water column. Elevated P/Ti in the lower subunit is treated as phosphorus enrichment associated with redox-sensitive chemical fixation and/or early diagenetic redistribution, rather than as direct evidence for increased primary productivity alone. Regional comparison with published Yurtus sections and wells indicates that the thin, low-TOC, and compositionally differentiated interval in Well K1 represents a local expression of source-rock heterogeneity. The results suggest that slope-break-related paleogeographic differentiation controlled accommodation, sediment supply, hydrodynamic disturbance, preservation efficiency, and early diagenetic modification, thereby governing the heterogeneous development of the Yurtus Formation along the northwestern Tarim margin.