Spatial and temporal variability of Paleocene–Miocene organofacies of the Kura Basin, eastern Azerbaijan, and implications for basin evolution and petroleum generation

Abstract

Fine-grained Paleocene–Miocene strata of eastern Azerbaijan record the evolution of the Kura Basin from an open marine system connecting the South Caspian Basin to the Paratethys of eastern Europe to an isolated epicontinental sea. Tectonic uplift associated with the Greater Caucasus mountains in Azerbaijan resulted in exposure of these strata over much of the northeastern part of the basin, allowing detailed observation and sampling over a broad area. Four hundred Paleocene–Miocene samples collected in eastern Azerbaijan demonstrate spatial and temporal geochemical heterogeneity within the predominately mudstone succession, indicating a complex evolution of this elongate basin. Through detailed organic geochemical analysis of samples, trends can be seen in the distribution of total organic carbon (TOC), pyrolysis characteristics, and biomarker results from a subset of twenty-four samples. In addition to confirming the well-documented regional basin restriction during the Oligocene to early Miocene deposition of the Maikop Series, heterogeneity of these geochemical indicators across the study area shows spatial variability in oxicity and terrestrial organic input within the basin on a sub-regional scale.Observed geochemical variability suggests a complex evolutionary history of the Kura Basin during the Paleocene–Miocene, and this has important implications for hydrocarbon prospectivity in the region. Within the 400 collected samples, total organic carbon (TOC) values range from 0.3 to 6.3%, with Oligocene–Miocene samples showing average values of 1.4%, compared to lower TOC values found in Paleocene–Eocene strata (average=0.3%). Rock-Eval pyrolysis shows that the majority of strata are organic lean, immature, and mixed oil/gas to gas prone (Type II/III to Type III), with a smaller group of latest Eocene–Early Miocene samples having better oil prone source potential. Gas chromatography and biomarker analysis of 24 samples reinforces the immaturity of most samples, shows varying levels of terrestrial input in all samples, suggests well-oxygenated waters prevailed with periodic suboxic to dysoxic/anoxic events, highlights gross differences between the Paleocene–Eocene and Oligocene–Miocene stratigraphic intervals, and shows generally good lateral correlation between samples. These interpretations reinforce what is seen through inorganic geochemical evaluation of trace metals, bulk composition, and stable isotopic ratios, and offer more detail as to the evolution of this basin and the implications for oil and gas prospectivity of the region.