The Late Pleistocene and Holocene Evolution of The North Caspian Plain

Abstract

The rapid rates of sea level change in the Caspian Sea since its formation (Proterozoic) have resulted in unique sedimentary sequences, including many oil and gas reservoirs. An important hydrocarbon province is the fine grained-sandstone of the Productive Series in the South Caspian Basin that consist for at least some part of sheetflood sandstones. Their sedimentology and reservoir architecture remain uncertain. This study reconstructs the chrono-stratigraphic framework of the low-gradient North-East Caspian plain between the Ural and Emba Rivers, and its potential as a reservoir analogue for Productive Series. Because of the similarities between the sedimentary systems, like rapid sea level changes and the absence of a shelf break. The data for this study consist of 26 shallow core- and 10 outcrop-descriptions and include the analyses of selected samples for Optically Stimulated Luminescence dating, radiocarbon dating and grain-size distribution analyses. Three sea-level cycles (I-III) were identified depositing four different sedimentary sequences (a-d): (I-a) marine clays (41 ka BP) followed by a regression resulting in an erosional surface, (II-b) transgressive shallow marine silty clays and sandy barrier deposits (16-17 ka BP) (locally identified as ‘Khvalynian chocolate clays’), (II-c) Terminal Splay Complex consisting of clayey sands (9-11 ka BP) related to a major regressive event, that are eroded into a deflation landscape (e.g. yardangs), locally known as ‘Baer-hills’ and Sors’ and (III-d) present day inter-yardang deposits lagoonal- and playa- like deposits consisting of fine silty clays. Preservation of these fines is closely related to sea-level oscillations. Possibly a fourth sea-level cycle occurred before 41 ka BP depositing marine clays. The identification of a Holocene terminal splay complex and yardang-formation gives new insight in the regional sedimentary processes. More importantly this complex could serve as a new analogue for the Productive Series, giving alternative insights in reservoir architecture and internal connectivity, e.g. the Productive Series could be more terrestrial of origin despite the absence of evidence for sub-aerial exposure.Applied GeologyApplied Earth SciencesCivil Engineering and Geoscience