Print this pageMesozoic Nonmarine Petroleum Source Rocks of the Southern Tarim Basin, China
RITTS, BRADLEY D.
Molecular organic geochemical studies of nonmarine Mesozoic source rocks and associated crude oils of western China have demonstrated lacustrine and terrestrial Jurassic sequences to be effective sources of liquid hydrocarbons in the Qaidam, northern Tarim, southern Junggar, and Turpan basins. Allied field studies document lithologic similarities between Mesozoic source rocks throughout northwestern China: black, laminated lacustrine shales and carbonaceous mudstones, as well as coals deposited in marginal lacustrine and meandering fluvial environments.
Organic-rich, nonmarine, potential source rocks also crop out on the southern and southeastern margin of the Tarim basin. Although no published subsurface data constrain the presence of source facies in the southern Tarim basin, outcrop-based basin analysis of Mesozoic strata strongly suggests that Jurassic source rocks occur in the subsurface of southern Tarim. Conventional organic geochemical analyses of marginal lacustrine to fluvial coal and carbonaceous mudstone, and lacustrine shale samples indicate that all three lithologies have significant oil-prone generative potential: petroleum potential is excellent (TOC ranges from less than 10% to almost 80%); and type II and III kerogen is indicated by hydrogen indices as high as 550 mg HC/g TOC, but more commonly between 150 and 350 mg HC/g TOC. The oil-prone nature of some of the coals is confirmed by visual maceral inspection, which reveals an abundance of type I and II macerals.
The level of thermal maturity throughout the south Tarim basin remains poorly known and, thus, an exploration risk. Samples from the margin of the basin are immature to late-mature based on vitrinite reflectance (Ro between 0.5 and 0.97%). Maturation modeling based on Mesozoic and Cenozoic burial history indicates that Jurassic source rocks in the subsurface of south Tarim also may be mature. Thus, available data constraining the Mesozoic and Cenozoic history of the south Tarim basin, and the quality and maturity of Jurassic source rocks indicate a speculative petroleum system involving a Jurassic source.