Chemostratigraphic Insights Into Fluvio-Lacustrine Deposition, Yangchang Formation, Upper Triassic, Ordos Basin, China

Abstract

The Yanchang Formation of North-Central China represents a large (370,000 km²), long-lived (>20 million years) fluvio-lacustrine system of late Triassic age. The potential exists for the Yanchang Fm to be used as a depositional and hydrocarbon reservoir analog to overlying Jurassic-aged strata in the Ordos Basin, and to similarly-deposited strata in other non-marine, hydrocarbon-bearing basins in China (e.g, Sichuan and Songliao basins). The current understanding of the tectonic and climatic history during Yanchang deposition indicates a complex syn- and post-depositional compressional tectonic setting, with a temperate-humid climate. X-ray fluorescence and x-ray diffraction analyses constrain stratigraphic changes in mineralogy and provide insights into the conditions under which organic matter accumulated. Measurements of total organic carbon (TOC) reveal strong mineralogical controls on organic matter accumulation. The chemostratigraphic study of a long drill core from near the central axis of the Ordos depocenter reveals thick (<10 to 65 m) intervals of fine-grained lacustrine strata, interpreted to represent deep water lacustrine deposition, interspersed with thick (10–30 m) intervals of arkosic sandstones, reflective of deltaic deposition during lake lowstands. Along with elevated concentrations of %Al, a proxy for clay content, maximum TOC values in the deep water lacustrine facies reach 8%, with average values of ~3%. The fine-grained, organic-rich facies is also characterized by elevated sulfur concentrations (up to 6%), a proxy for pyrite, and an indicator of more reducing conditions in the deep water mass of the lake during highstand intervals. Concentrations of %P are also elevated in the deep water facies, indicating enhanced preservation of phosphorus, probably from fish bones, teeth, and scales. Most redox-sensitive trace element (RSTE) concentrations are only mildly elevated in the deep water lacustrine facies, especially zinc (Zn), vanadium (V), and uranium (U); however, U concentrations are punctuated in thin intervals within the deep lacustrine facies, suggesting episodes of enhanced suboxia/anoxia during lacustrine highstands. While dramatic shifts between delta and deep lacustrine deposition are recorded in the Yanchang strata, no evidence indicates that the lacustrine system ever built up a significant salt concentration, suggesting that an outlet was maintained throughout the depositional history.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015