Organic geochemical characteristics and accumulation of the organic matter in the Jurassic to Cretaceous sediments of the Saihantala Sag, Erlian Basin, China

The organic petrology and organic geochemistry of Jurassic to Cretaceous hydrocarbon source rocks from the Saihantala Sag, the Erlian Basin, have been analyzed to determine their geochemical characteristics, hydrocarbon potential and organic matter accumulation. The mudstones of the Alatanheli Formation (J1+2al), the Aershan Formation (K1ba), the lower Tengger member (K1bt1) and the upper Tengger member (K1bt2) and the Saihantala Formation (K1bs) contain variable total organic carbon concentrations and organic matter type. The observed macerals in the J1+2al, K1ba, K1bt1 and K1bt2 mudstones are sapropelinites (including lamalginites, mineral-bituminous groundmasses and rare telalginites), vitrinites and inertinites in order of abundance, whereas vitrinites and inertinites are the predominant macerals in the K1bs mudstones. The vitrinite reflectances are lower than 0.7% in the studied samples, suggestive of their low organic maturity, which is consistent with the strong yellow fluorescence of the lamalginites and telalginites, low Tmax values, the presence of 17β(H), 21β(H) hopanes and hop-17(21)-enes, and aliphatic and aromatic thermal maturity parameters. The studied samples were mainly deposited under anoxic saline lake environments. The major biological sources in the J1+2al, K1ba, K1bt1 and K1bt2 mudstones are bacteria and algae, whereas higher plants are the more important biological source in the K1bs mudstones as indicated by biomarkers and maceral contents. The J1+2al, K1ba, K1bt1 and K1bt2 mudstones mainly have more capacity to generate oil than gas, whereas the K1bs mudstones are mainly gas prone. Despite the low thermal maturity of the studied mudstones, abundant oil has been found in the southeast of the Saihantala Sag with mature source rocks. The nature of their generated products was due to their different formation mechanisms of organic matter accumulation. The organic matter accumulation in the J1+2al, K1ba, K1bt1 and K1bt2 mudstones was controlled by the high primary productivity and anoxic conditions, whereas the formation of the K1bs organic-rich sediments was mainly related with a high influx of higher plants.