Late Paleozoic tectonics, sedimentation, and petroleum potential of the Junggar and Tarim basins, northwest China

Sedimentary rocks exposed on the flanks of the modern Tian Shan record the final retreat of marine waters from the region during the Late Carboniferous to Early Permian, and erosion of a paleo-Tian Shan range during the middle to Late Permian. The Junggar ocean was filled by a regressive sedimentary sequence of Upper Carboniferous to Lower Permian volcanogenic turbidites to shelf sandstones. In contrast, the northwest Tarim area received marine limestones and quartz sandstones derived from the interior of the Tarim block during the Carboniferous through late Early Permian. These rocks represent deposition either on the northwest passive margin of Tarim, or, alternatively, in a marine gulf covering parts of the combined Tarim/Central Tian Shan block. Collision of this block with a volcanic arc or arcs preserved in the north Tian Shan uplifted a paleo-Tian Shan range in the late Early Permian, which shed non-marine sediments into flexurally-subsiding foreland basins on either side of the range. Upper Permian sandstones in each basin bear the distinctive provenance imprint of volcanic and plutonic lithologies presently exposed within the range.; Uplift of the paleo-Tian Shan closed drainage patterns in the Junggar basin during the Late Permian, allowing deposition of up to 1000 m of laminated lacustrine mudstones and siltstones in a deep temperate-zone lake (or lakes). These "oil shales" commonly exceed 20% TOC and 100 mgHC/g, ranking them among the richest petroleum source rocks in the world; bulk organic matter compositions range from Type I to Type III. Sterane and carotane distributions correlate well between different areas of the basin, strongly suggesting that Upper Permian oil shales are the principal source of oils in the basin. Biomarker distributions evince a complex system of tectono-hydrologic controls on lake salinity during the Late Permian, however. Hypersaline depositional conditions prevailed in the northwest part of the basin, while orographically-induced precipitation on the paleo-Tian Shan provided freshwater input to the southern areas of the basin. Vitrinite reflectances, biomarker maturity ratios, and MATOIL{dollar}sp{lcub}rm TM{rcub}{dollar} computer modelling of thermal maturity all indicate that southeast Junggar oil shales have achieved sufficient thermal maturity for oil generation.