Water-Soluble Organic Acids at Deep-Ultradeep Depth in Chinese Sedimentary Basins : Experimental Results and Geological Significances

Li, P. and Liu, Q. and Zhou, Z. and Li, J. and Zhou, S. (2025) Water-Soluble Organic Acids at Deep-Ultradeep Depth in Chinese Sedimentary Basins : Experimental Results and Geological Significances. Diqiu Kexue - Zhongguo Dizhi Daxue Xuebao/Earth Science - Journal of China University of Geosciences, 36 (6). pp. 2546-2560.

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Abstract

Water-soluble organic acid anions (WSOAA) in subsurface water have been intensively studied during past several decades. They are used as natural gas precursor, tracer for the movement of underground fluid, indicator for porosity improvement, and detecter of deep subsurface life on the Earth. However, little is known about the distributions and origins of organic acids at deep-ultradeep depth underground. Herein, we collected twenty-nine source rock samples covering a wide maturity range from the Ordos, Qinshui, Junggar, Minhe, and Southern North China basins, as well as six subsurface water samples with depth between 6 544 and 8 396 m from industrial gas producing wells in the Tarim Basin, China. We carried out pyrolysis experiments at various temperatures (250–450 °C) to investigate the role of water on the generation of organic acids. Results show that there are considerable amounts of WSOAA detected in both high-over mature source rocks and deep-ultradeep subsurface water. WSOAA mainly consists of monocarboxylates, predominately formate and acetate. High-TOC oil-generating source rock has low production rate of organic acids due to lack of hydrogen. Different source rocks have distinct ratios of formate to acetate concentration, expressed as c(formate)/c(acetate), which is due to significant differences in both initial molecular structure and metabolite. This indicates that c(formate)/c(acetate) can be used to distinguish types of organic matters (OMs). Concentrations of WSOAA show a “sharp decrease-slight increase-slow decrease” evolution trend with progressive maturity. Moreover, there are higher production rates of organic acids under hydrous pyrolysis experiments at ⩾400 °C. All geochemical signatures indicate that at both deep-ultradeep depth and high-over mature stages, the formation of organic acids is attributed to the thermochemical oxidation of organic components by mainly hydroxyl radicals, challenging the traditional model of organic acid evolution. This work suggests that petroleum exploration can be extended to great depths in sedimentary basins, when formation temperatures are not exceeding 230 °C.