Mechanisms controlling organic matter enrichment in the Lower Silurian Longmaxi Formation black shale unit, southwestern
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ORIGINAL PAPER
Mechanisms controlling organic matter enrichment in the Lower Silurian Longmaxi Formation black shale unit, southwestern margin of the Yangtze Platform, China Shaoze Zhao 1,2 & Yong Li 1,2 & Huajun Min 1,2 & Qian Yu 3 & Zhengjiang Wang 3 & Tao Deng 1,2 & Hao Liu 1,2 & Jian Chen 1,2 Received: 22 June 2018 / Accepted: 5 March 2019 # Saudi Society for Geosciences 2019
Abstract The Lower Silurian Longmaxi Formation black shale unit at the southwestern margin of the Yangtze Platform has a great potential to contain shale gas. Based on the geochemical and microscopic characteristics of the Longmaxi Formation black shale unit, the depositional environment and the characteristics of the silica in the black shale were analyzed. The mechanism whereby the Longmaxi Formation black shale unit in the study area was enriched in organic matter is discussed from the perspective of upwelling and sea-level change. The conclusions of the research are as follows: (1) The shale in the Gesala section with a total organic carbon (TOC) content greater than 2% is approximately 55 m thick, which is favorable for shale gas accumulation in the study area. (2) The Longmaxi Formation black shale unit was deposited in an anoxic marine environment. Marine primary productivity was high during the deposition of the Longmaxi Formation black shale unit. (3) The Longmaxi Formation black shale unit is rich in biogenic silica due to the development of ocean upwelling. (4) Upwelling and sea-level rise jointly controlled the enrichment of organic matter in the Longmaxi Formation black shale unit. The upwelling supplied nutrients and promoted biological reproduction, which promoted marine primary productivity. The higher productivity also contributed to the formation of an anoxic environment in the bottom seawater. The sea-level rise was favorable for the formation of a reducing deep-water environment and deposition of fine-grained sediments, which was beneficial to the preservation of organic matter. Keywords Southwestern margin of the Yangtze Platform . Lower Silurian Longmaxi Formation black shale unit . Depositional environments . Silica . Upwelling . Sea-level rise
Introduction The successful exploitation of shale gas in the USA has considerably increased the value of shale gas resources worldwide. Along with continuous advances in exploration for shale gas in China, major breakthroughs in shale gas exploration have been made in the Jiaoshiba area (the largest shale gas Editorial handling: S. Banerjee * Yong Li [email protected] 1
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China
2
College of Energy Resources, Chengdu University of Technology, Chengdu, China
3
Chengdu Center, China Geological Survey, Chengdu, China
field in China) of Chongqing and the Weiyuan and Changning areas of Sichuan (Zou et al. 2015). In the Yangtze Platform in southern China, the widely distributed Lower Silurian Longmaxi Formation black or dark mudstone/shale unit ranks among th
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