Abstract

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.

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