South Torgay - Petroleum Systems Revisited

Abstract

The South Torgay Basin, located in central Kazakhstan, comprises several pull-apart basins largely formed by Early Jurassic transtensional forces along the westernmost parts of the continental-scale Main Karatau Fault (MKF) system. Despite confirmed recoverable reserves of roughly 110,000 kilotonnes (~810 MMBO) of oil and gas from over fifty producing fields, the basins’ petroleum system is poorly understood. Most of the studies are focused on individual fields or disciplines, while literature lacks a coherent petroleum system overview. This study integrates and uses modern tools and principles to reprocess published geological, geochemical, geophysical, and reservoir temperature data to reappraise the basin’s petroleum system. Specifically, tectono-stratigraphic analysis, produced oil and source rock geochemistry, basin modelling, and source - rock expulsion potential calculations were done and integrated to propose locations and quality of source rocks, as well as reservoirs, followed by interpretation of migration pathways, petroleum charge, remigration, and unconventional potential. The results suggest the presence of multiple prolific Jurassic source rocks, such as >15 m thick shales within the Karagansky Formation, characterized by up to 13.5% TOC of Type I kerogen. The biomarkers of the produced oil suggest charge from lacustrine source rocks characterized by Type II and Type III kerogen and/or their mixtures. These results are in line with interpreted depositional environments of buried successions comprised of stacked algal-rich deep lacustrine and plant-rich deltaic and fluvial systems. Source rock kinetics estimates that the ultimate expulsion potential from source rocks in the Aryskum, Akshabulak, and Bozingen Grabens is up to 136.1 billion barrels of oil. Integrated tectono-stratigraphy and burial history suggest that oil migrated in at least two stages, driven by burial and inversion tectonic events in the Late Jurassic and Late Cretaceous. The updated model points to significant potential for new conventional oil discoveries along migration routes in a range of Lower and Middle Jurassic layers as well as within pre-Mesozoic buried hills plays, and in underexplored Bozingen and Sarylan Grabens. Both Karagansky and Kumkol black shales host unconventional shale oil and gas potential. Compared to other ‘pull-apart’ basins such as the Bohai Sea, Vienna, and Dead Sea basins, the South Torgay basin is characterized by the most classic tectono-stratigraphic framework and may serve as a global analogue for pull-apart petroleum system studies. Thus, the proposed workflow should be universally applicable.