Upper Permian lacustrine organic facies evolution, southern Junggar basin, NW China

The Jingjingzigou, Lucaogou, and Hongyanchi Formations of the southern Junggar basin together comprise over 1000 m of organic-rich lacustrine facies. They record an overall evolution from relatively shallow, evaporative lakes to freshwater lakes associated with fluvial systems. The Jingjingzigou Formation is characterized by low TOC and HI, and biomarker features (such as abundant beta-carotane) consistent with a specialized saline or hypersaline biota. Biomarker distributions in Jingjingzigou Formation extracts most closely resemble oils from the giant Karamay oilfield. The Lucaogou Formation represents one of the richest and thickest lacustrine source rock intervals in the world, yet it contradicts conventional lacustrine source rock models in at least two important aspects. First, deposition occurred at middle paleolatitudes (39-43 degrees N) rather than in the tropics. Second, limited nutrient supply in a drainage basin dominated by intermediate volcanic rocks appears to have caused low to moderate primary productivities. Stable salinity stratification and low inorganic sedimentation rates in a deep lake nonetheless resulted in deposits with up to 20% TOC and HI near 800. The overlying Hongyanchi Formation has 1-5% TOC but low HI, and was deposited in freshwater oxic to sub-oxic lakes. (C) 1998 Elsevier Science Ltd. All rights reserved.