HE Hongxi1,2, XIAO Jiafei1, LAN Qing1, YANG Haiying1, HUANG Mingliang1
（1. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Science, Guiyang 550081, China;
2. University of Chinese Academy of Science, Beijing 100049, China）
Abstract：Black mudstone was developed in the Lower Silurian Longmaxi Formation, which is distributed in northern Guizhou province. In order to clarify the sedimentary environment and shale gas accumulation conditions, the sedimentary and geochemical characteristics were studied and analyzed. The results showed that the mudstone has a silt structure or mud structure; the terranes are mostly thin layer and generally have horizontal bedding. Meanwhile, the mudstone contains the impregnated pyrite granule. These characteristics show that the hydrodynamic force was weak and the deposition rate was low when deposited. The enrichment coefficients of Mo, V and U of mudstone are all greater than 1. The mean Ce anomaly is -0.033. And the ratios of specific elements (V/Cr, Ni/Co, V/(V+Ni) and Ce/La) indicate that mudstone were formed in anoxic environment. On the whole, the sedimentary environment of black mudstone of Longmaxi Formation is characterized by relatively deep water, low deposition rate and hypoxic reduction state. TOC content of black mudstone of Longmaxi Formation varies from 2.78% to 4.36%, with an average of 3.79%. The vitrinite reflectance (Ro) varies from 1.88% to 2.16%, with an average of 1.99%, indicating that organic matter has reached the dry gas stage from mature to over-mature. The effective thickness is between 10m and 100m, the content of brittle minerals is generally between 20% and 30%, and the buried depth is about 0-3500m. The thickness of the strata increases gradually from south to north, and other parameters are similar in each profile. Therefore, the area to the north of Tongzi county and Suiyang County in Northern Guizhou has shale gas resource potential. These characteristics indicate that the black mudstone of Longmaxi Formation have good conditions for forming shale gas reservoir and have high shale gas resource potential.
Key words：Longmaxi formation; black mudstone; sedimentary environment; shale gas accumulation condition; Northern Guizhou Province
EARTH AND ENVIRONMENT Vol.50, No.6, Tot No.350, 2022, Page 898
