Geochemistry and Genesis of Zubair Shales (Lower Cretaceous) in Rumaila Oilfield, Southern Iraq

Exploration and development of shale oil and gas resources depend on understanding the mechanisms that influence the buildup of organic-rich shale. In shallow marine facies, the development of organic-rich sediments is largely dependent on the sedimentary environment. The black shale is the main lithology forming the reservoir in the Zubair Formation. The discovery of silica origin, and the differentiation between euxinic sulphuretted and anoxic ferruginous conditions, continue to require further study. First, the geochemical features of Zubair oil shales are examined using geochemical data collected from two wells, R167 and Ru4, in the northern and southern Rumaila oilfields in Basrah, Southern Iraq. Next, the provenance, depositional basin, and evolution of the paleoenvironment are addressed. Lastly, the primary controlling factor of organic matter enrichment is identified. The findings demonstrate that Zubair shales are able to be separated into two units: organic-rich and organic-low shales, with average total organic carbon (TOC) contents of 13.5, 9.04, and 2.39% for R167 and Ru4, respectively. High concentrations of SiO2, V, Ni, Zn, Sr, Zr, Ba, and the distribution of various rare earth elements also distinguish the organic-rich shales. Compared to the organic-low shales, the organic-rich shales have a significantly higher proportion of biological quartz and a lower terrigenous input. In the organic-rich shales, the paleoclimate is warm, humid, and of low salinity, while the organic-low shales are cold, dry, and of high salinity. The organic- low shales exhibit an elevated stagnant level with suboxic to oxic conditions and lower paleoproductivity. In contrast, the organic-rich shales exhibit a comparatively lower stagnant level with euxinic to anoxic conditions. Oil shales accumulate in a semi-stagnant basin. The three-stage environmental evolution model demonstrates that paleo-productivity, redox conditions, and sea level fluctuation have a strong causal connection.