Print this pageLower-Middle Jurassic Nonmarine Petroleum Source Rocks and Related Oils of the Qaidam Basin, NW China
RITTS, BRADLEY D.; ANDREW D. HANSON, and J. M. MOLDOWAN
Lower and Middle Jurassic nonmarine strata of the Mesozoic Qaidam basin in northwestern China contain abundant coals, carbonaceous mudstones, and organic-rich lacustrine shales. Rock-Eval pyrolysis and visual maceral description indicate that these potential petroleum source rocks have significant oil-prone generative potential. Total organic carbon rarely falls below 1% in source horizons and is commonly 10-20%, ranging to more than 80% in coals. Rock-Eval pyrolysis yields hydrogen indices up to 680 mg HC/g TOC, and many samples, including coals, plot between type III and type I maturation pathways on modified van Krevelen diagrams. The oil-prone nature of some Qaidam coals is supported by kerogen compositions exceeding 50% type I and II macerals.
Biomarker data define two distinct groups of Qaidam crude oils, generated and expelled from different source rocks. One group of oils, interpreted to be derived from Tertiary source rocks, has biomarker characteristics typical of hypersaline, anoxic lacustrine source rocks: low pristane:phytane ratio, high gammacerane and C[34] and C[35] homohopane concentrations, and beta-carotane. The second group of oils possesses a freshwater lacustrine to terrestrial biomarker signature: high pristane:phytane ratios (up to 4.6), odd n-alkane preference, dominance of C[29] steranes, dominance of diasteranes over steranes, and lack of saline lacustrine indicators. This second group of oils is demonstrably derived from Lower-Middle Jurassic lacustrine-terrestrial source rocks.
The best Jurassic source rocks occur along the northeastern margin of the basin, coincident with the area of the basin from which the Jurassic oils were sampled. The demonstrated Jurassic petroleum source units on the northeastern margin of the basin may suggest a previously unrecognized petroleum system and a new play type, not only for other parts of the Qaidam basin, but in other basins of northwestern China (e.g.: southern Tarim basin), where similar Mesozoic source rocks are widespread.