2019 AAPG Annual Convention and Exhibition:

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Tectonic Origin of Major Salt Accumulations in the Precaspian Basin

Abstract

Over several years, Chevron conducted a basin-wide regional assessment in the Precaspian Basin using a holistic approach to integrate diverse data types and vintages. This allowed a comprehensive understanding of the evolution of the basin that differs in some important ways from publications and vendor products. This presentation focuses on the Late Paleozoic, a key moment for all risk elements and a time of fundamental change for the entire basin. A major stress reorientation throughout Europe at the Middle/Late Carboniferous boundary was contemporaneous to the drowning of isolated carbonate platforms in the Precaspian. Deepwater shales that later became source rocks as well as seals were deposited on top and in between the drowned platforms. Along the eastern edge of the Eastern European Craton, the collision with the Uralian blocks caused a north-south trending seaway in the very Early Permian (Asselian-Sakmarian). In the south, multiple lines of evidence indicate that the Middle Caspian blocks collided with the Precaspian only in the later Early Permian (Artinskian-Kungurian), as opposed to the generally assumed timing of Middle Carboniferous. Thrusting and erosion associated with this collision cut off the seaway in the south, forcing marine waters to flow into the Precaspian basin from the Barents Sea, which is more than 2000km farther north. Due to evaporation, the marine waters in the north became more and more concentrated on their way south, with carbonate sediments accumulating across choke points in the north, followed by an anhydrite dominated series further south, and finally an about 6km thick, halite-dominated sequence in the Precaspian bowl. Complete drawdown must have occurred in the Kungurian, when potash salts were deposited throughout the Precaspian and in some local depressions farther north. The regional work impacts all risk elements. Some areas became more attractive, e.g., salt encased mini-basins, or the fold belt caused by the Early Permian collision in the south. There, an expansive carbonate platform has subsequently been folded into anticlines, as opposed to the old model that assumed smaller carbonate platforms growing on top of active anticlines. A much larger area appears prospective now and the leads themselves are aerially much more expansive than those in the previous regional model.