Characteristics of Hydrocarbon Accumulation in Paleozoic Marine Carbonates in the World

Abstract

The discovered original proved plus probable (2P) reserves in the world-wide Paleozoic marine carbonates amount to 418.9 billion barrels of oil equivalent, accounting for a quarter of the total reserves in the marine carbonates in the world. Stratigraphically, the reserves are concentrated in three intervals: the Permian (hosting 68.9% of the total), Carboniferous (21.3%), and Devonian (6.0%). Such a distribution is similar to the one characteristic of the reserves in the Paleozoic dolomite reservoirs. The Cambrian, Ordovician and Silurian host a small percentage. Petroleum reserves in the Permian are largely confined to the Arabian and Zagros basins, and those of Carboniferous and Devonian are concentrated in the Former Soviet Union. In contrast, reserves in the Cambrian and Ordovician are mainly distributed in the North America and China. Oil and gas in Paleozoic marine carbonate reservoirs are largely derived from Paleozoic source rocks. The most important source intervals are Silurian (contributing 58.4% of the total), Carboniferous (20.5%), Devonian (10.7%), Permian (6.2%), Ordovician (2.0%) and Cambrian (0.9%). Shale, bituminous shale and bituminous carbonate are main source rock lithologies, whereas calcareous shale, pure carbonate and argillaceous carbonate made little contribution. Cambrian and Ordovician source rocks are dominated by carbonates. In the Chinese Tarim Basin, oil and gas in Ordovician reservoirs were derived from Cambrian and Ordovician source rocks, indicating that the carbonate source rocks do have a considerable source potential. The three most important seal intervals are the Permian (sealing 51.5% of the total), Triassic (32.8%) and Carboniferous (9.0%). Evaporites are the dominant seal rock, followed by tight carbonates and fine clastics. In China, the principal seal rocks are Carboniferous and Ordovician fine clastics. Areas with development of effective evaporite seals and source rocks are the favorable play fairways in the exploration of Paleozoic marine carbonates in China. Over 85% of the total are distributed in reservoirs at burial depths of 1,500–4,500 m, whereas only 3.0% are reservoired in deep reservoirs with burial depths of more than 4,500 m. Deep reservoirs discovered so far are mainly distributed in China where significant breakthroughs have been made in exploration of deep reservoirs in the Tarim and Sichuan basins. Additional new deep discoveries are expected to be made in the two basins in the future.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015