Multiple controllers on organic matter accumulation at the Mesopotamia Basin of the Chia Gara Formation shale, Central Iraq Dejavniki, ki nadzorujejo kopičenje organske snovi v skrilavcih formacije Chia Gara v Mezopotamskem bazenu v osrednjem Iraku The Tithonian–Berriasian Chia Gara Formation in Central Iraq represents a major breakthrough in shale oil exploration. This study investigates two continuous core sections (140m and 137m) from the East Baghdad (EB1) and Balad (Ba1) oilfields, located within the intrashelf basin along the passive margin of the Arabian Plate. Comprehensive geochemical analyses, including major and trace elements, Mo isotopes, and total organic carbon (TOC), were conducted to evaluate the mechanisms controlling organic matter enrichment. The EB1 and Ba1 cores are subdivided into five and four organic sedimentary facies (OSF), respectively. In EB1, TOC values in the lower OSF 1 and OSF2 range from 3.55 to 8.0 wt.% and 2.31 to 4.42wt.% and are higher than those of the upper OSF 3, OSF 4, and OSF 5 that range from 0.5 to 3.77wt. %. In Ba1, OSF 3 and OSF 5, which range from 0.39 to 2.7wt.%. Redox indicators such as U/Th, Ni/Co, EF(U), and EF(Mo) show a progressive shift towards more oxic bottom-water conditions. The δ98/95Mo data (–0.3 to 2.5‰) confirm the presence of euxinic conditions, particularly within OSF2 in both EB1 and Ba1, thereby resolving previous uncertainties regarding bottom-water redox conditions. Primary productivity indices (Cu/Al and Ni/Al ratios) suggest relatively low productivity in deep-water slope and basinal facies. Furthermore, the upwelling indicators (EF(Co) × EF(Mn)), varying from 0 to 0.48 for EB1 and 0.05 to 0.18 for Ba1, suggest that preservation conditions played a more significant role than productivity conditions within the deep-water slope and basin. Differential subsidence, with local uplifts and erosion within the Mesopotamian Foredeep Basin likely contributed to spatial variability in organic enrichment. Overall, this study highlights the importance of the redox environment in the development of high-quality hydrocarbon source rocks in unrestricted environments that present upwelling. Titonijsko–berriasijska formacija Chia Gara v osrednjem Iraku predstavlja pomemben preboj pri raziskovanju nafte iz skrilavcev. Študija obravnava dva neprekinjena jedrna odseka (140m in 137m) z naftnih polj East Baghdad (EB1) in Balad (Ba1), ki se nahajata v znotrajšelfnem bazenu ob pasivnem robu Arabske plošče. Opravljene so bile obsežne geokemične analize, vključno z določitvijo glavnih in slednih elementov, izotopov Mo ter skupnega organskega ogljika (TOC), z namenom opredelitve mehanizmov, ki nadzorujejo obogatitev z organsko snovjo. Jedri EB1 in Ba1 sta razdeljeni v pet oziroma štiri organske sedimentne faciese (OSF). V jedru EB1 se vrednosti TOC v spodnjih OSF1 in OSF2 gibljejo med 3,55 in 8,0ut.% ter 2,31 in 4,42ut.%, kar je višje kot v zgornjih OSF3, OSF4 in OSF5 (0,5 do 3,77ut.%). V jedru Ba1 so vrednosti TOC v OSF3 in OSF5 med 0,39 in 2,7ut. %. Redoks kazalniki, kot so U/Th, Ni/Co, EF(U) in EF(Mo), kažejo postopen premik proti bolj oksidacijskim razmeram na dnu vodnega stolpca. Podatki δ98/95Mo (–0.3 do 2.5‰) p o t r j u j e j o prisotnost evksiničnih razmer, zlasti v OSF2, kar odpravlja prejšnje nejasnosti glede redoks pogojev pri dnu. Kazalnika primarne produktivnosti (razmerji Cu/Al in Ni/Al) nakazujeta razmeroma nizko produktivnost v globokovodnih pobočnih in bazenskih faciesih, medtem ko kazalniki dviganja globokovodnih vod (EF(Co) × EF(Mn)), ki se gibljejo med 0 in 0,48 za EB1 ter med 0,05 in 0,18 za Ba1, kažejo, da so pogoji ohranjanja imeli večji vpliv kot produktivnost. Diferencialno pogrezanje, skupaj z lokalnimi dvigi in erozijo v Mezopotamskem predgorskem bazenu, je verjetno vplivalo na prostorsko variabilnost obogatitve z organsko snovjo. Študija tako poudarja ključno vlogo redoks okolja pri nastanku kakovostnih izvornih kamnin za ogljikovodike v odprtih okoljih z značilnim dviganjem globokovodnih vod. Mo isotopes redox conditions organic matter primary productivity geochemical methods Upper Jurassic–Early Cretaceous Mesopotamian Basin Izotopi Mo redoks pogoji organska snov primarna produktivnost geokemične metode zgornja jura–spodnja kreda Mezopotamski bazen true false true Angleški jezik Slovenski jezik Neznano 2026-07-06 10:41:38 2026-07-06 10:41:38 2026-07-07 03:51:14 0000-00-00 00:00:00 2026 0 0 str. 55-76 no. 1 vol. 69 2026 0000-00-00 Zaloznikova Objavljeno NiDoloceno 0000-00-00 0000-00-00 2026-04-07 55 0016-7789 10.5474/geologija.2026.003 283526915 1949-Article_Text-12342745-1-10-20260630.pdf 1949-Article_Text-12342745-1-10-20260630.pdf 1 D94B0BC26D04EFC19F4E9F4448790A13 4b987159bd41dc24a52ed150423ed86ffa28481c34258a8b0c86aa40279d6586 e6c0563c-7916-11f1-b02e-001a4af901a5 https://dirros.openscience.si/Dokument.php?lang=slv&id=47107 Geološki zavod Slovenije 0 0 0