Controls of oil family distribution and composition in nonmarine petroleum systems: A case study from the Turpan-Hami basin, northwestern China

Controls on oil family distribution in tectonically complicated, nonmarine, petroliferous basins are commonly difficult to isolate because of the varying ages of potential source rocks, the complex assemblage of organic-rich sedimentary facies, and the geographic variability of burial histories. The Turpan-Hami basin of northwestern China is an oil-bearing intermontane basin where stratigraphic, sedimentologic, and geochemical controls are sufficient to address each of these issues independently and to determine how they influence the current distribution and composition of liquid hydrocarbons. Source rock age is one of three major statistically significant discriminators affecting oil family composition. Both Lower/Middle (Lower or Middle) Jurassic and Upper Permian rocks are important source rocks for the basin. A newly developed diterpane biomarker parameter can distinguish Permian rocks and their correlative oils from Jurassic coals and mudrocks and their derivative oils. Source facies is a second key control on petroleum occurrence and character. A variety of biomarker parameters that reflect source rock depositional conditions are indexed to rock samples from interpreted depositional environments. By erecting rock-to-oil correlation models, the biomarker parameters separate oil families into endmember groups: group 1 oils Lower/Middle Jurassic peatland/ swamp facies (high land-plant input, less reducing conditions), group 2 oils = Lower/Middle Jurassic profundal lacustrine facies (high algal input, more reducing conditions), and group 3 oils Upper Permian lacustrine facies (high algal, stratified, anoxic conditions). Burial history exercises a third major control on petroleum distribution. Source rock maturation modeling can demonstrate that relatively uninterrupted burial in the asymmetrically subsiding northern Turpan-Hami area (Taibei depression) exhausted Upper Permian-sourced rocks by the Late Cretaceous, which led to southward migration of Upper Permian-sourced oils (group 3) into Triassic reservoirs of southern and southwestern Turpan-Hami (Tainan and Tokesun depressions). Subsequent to uplift of the central basin thrust that currently partitions Taibei from Tainan, Lower/Middle Jurassic-sourced oils were expelled in the Taibei depression by Paleocene-Eocene time, which locally charged Jurassic and Cretaceous reservoirs (groups 1 and 2), forming Turpan-Hami's largest oil accumulations in the basin.