Dominant Controls on Upper Triassic Organic-Rich Shale Deposition: Geochemical Evidences From the Chang 7 Shale in the Ordos Basin, China

Abstract

The Upper Triassic organic-rich Chang 7 shale is an unconventional shale-gas reservoir that has been a major hydrocarbon target of gas production in the Ordos basin, northern China. This study uses major, trace and rare earth element (REEs) abundances documented through from a 115-m core obtained from the western part of the Ordos basin, to assess dominant controls on temporal variations in total organic carbon (TOC) in the Chang 7 Shale. Our results suggest that TOC trends in the Chang 7 Shale can be directly linked with the tectonic and magmatic evolution of Qinling orogen. The Chang 7₃ and Chang 7₂ sub-members of the lower part of the Chang 7 were likely deposited in association with heightened tectonic and magmatic activity of the Qinling orogeny. Sediments derived from these sub-members display LREE and trace element compositions similar to the Tianshui rhyolites suggesting deposition contemporaneous with magmatic and/or volcanic activity of the Qinling Orogeny. Global warming caused by volcanic outgassing of greenhouse gas enhanced continental weathering, and the subsequent increase in riverine input of nutrients (e.g., phosphorous and iron) to the lake. This scenario may explain the collapse of nutrient balance (i.e., enriched in phosphorous and iron, but depleted in nitrogen in sediments deposited during the Chang 7₃ and Chang 7₂ sub-members). Higher nutrient concentrations would have stimulated primary productivity in the surface water, enhancing higher organic carbon flux to the lake bottom. In contrast, lower LREE and trace element abundances in the Chang 7₁ sub-member at the top of the Chang 7 suggest deposition during a tectonic and magmatic quiescent period of the Qinling orogeny. The diminished volcanic ashes could have lowered the rate of primary productivity, which reflected by enriched nitrogen in sediments. Moreover, the reduced mountain building rate would have decreased the subsidence rate in the basin’s foredeep, favoring shallower water and oxic/suboxic bottom-water conditions. Thus, the hydrography of the lake during accumulation of the Chang 7₁ was not favorable for organic carbon accumulation. Together, the correlation of Qinling mountain building and magmatism and deposition of the Chang 7 Shale provides evidence for the role of tectonism in the organic carbon enrichment in the Chang 7 Shale.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018