Developmental characteristics and geological significance of the bentonite in the Upper Ordovician Wufeng - Lower Silurian Longmaxi Formation in eastern Sichuan Basin, SW China

WANG Yuman,LI Xinjing,WANG Hao,JIANG Shan,CHEN Bo,MA Jie,DAI Bing

Table 1 Development characteristics of bentonite in the Wufeng Formation-Longmaxi Formation on the Qizhu section in Shizhu.

Sub- layer Thick- ness/m Geological parameters of black shale Development characteristics of bentonite Lithofacies GR/cps TOC/% 5 0.98 Thin-bedded siliceous shale 160-164 2.70-2.88 2 bentonite beds occur at the top, 0.5-1.0 cm thick each, weathered to grey white clay beds 6-1 1.10 Thin interbedded carbonaceous shale and siliceous shale 154-172 1.92-2.31 3 bentonite beds occur in the middle and upper parts, 4-5 cm thick each in the upper part, 2-3 cm thick each in the middle part and 1-2 cm thick each in the lower part, 10-30 cm apart from each other, and 209-218 cps in GR. Mineral composition: 6.9% quartz, 1.8% feldspar, 1.4% calcite, 18.9% pyrite and 71.0% clay 6-3 0.71 Thin-bedded siliceous shale 198-227 1 bentonite bed at the top is lead grey and 3-4 cm thick 7 1.16 Thin-bedded siliceous shale 224-239 2.68-5.55 1 bentonite bed at the top, 2-3 cm thick 11 2.27 Thin- to moderate- bedded siliceous shale 226-298 3.21-6.25 1 bentonite bed 0.5-1.0 cm thick, at 50 cm from the bottom Top of 12 1.22 Moderate-bedded siliceous shale, with high clay content 215-223 3.20 2 bentonite beds at the top, 25-30 cm from each other, the lower bed is 1-2 cm thick, and the upper bed is 2-3 cm thick 14 2.61 Thick-bedded argillaceous and siliceous mixed shale 207-231 3.17-3.39 1 bentonite bed at the top, 0.5-1.0 cm thick 15 2.83 Thick-bedded argillaceous and siliceous mixed shale 213-240 2.69-4.21 2 bentonite beds in the middle and top parts, 0.5 cm thick each, and about 1.4 m from each other 16 4.76 Thick-bedded argillaceous and siliceous mixed shale 197-214 2.30-2.69 2 bentonite beds at the middle and top parts, over 1.0 m apart, and 0.5 to 1.5 cm thick each 17 2.08 Thick-bedded argillaceous and siliceous mixed shale 196-209 1.93-2.10 1 bentonite bed at the top, 3-4 cm thick 18 2.68 Massive argillaceous and siliceous mixed shale 199-226 1.63-1.96 6 bentonite beds in the 1.5 m thick lower part, 15-30 cm apart from each other, and 0.5-2.0 cm thick each. 1 bentonite bed at the top, 0.5-1.0 cm thick 19 3.26 Thick-bedded argillaceous and siliceous mixed shale 178-200 1.48-1.52 1 bentonite bed at the top, 1 cm thick 22 2.08 Massive argillaceous and siliceous mixed shale 195-202 2.03 1 bentonite bed at the top, 2-3 cm thick 24 0.08- 0.10 Demirastrite belt thick-bedded bentonite, lead grey, with GR of 216- 220 cps, and mineral composition of 1.8% quartz, 1.0% feldspar, 67.5% pyrite, 1.9% barite and 27.8% clay 29 4.68 Argillaceous shale 198-216 1.79-1.91 1 bentonite bed at the bottom, 1-3 cm thick. 1 bentonite bed at the top, 0.5-2.0 cm thick 30 4.14 Argillaceous shale, massive 205-244 1.66-1.98 1 bentonite bed at the top, 0.5-1.0 cm thick 32 8.01 Carbonaceous shale, massive 211-261 1.76-2.06 1 bentonite bed at the top, 0.5-1.0 cm thick 34 7.65 Carbonaceous shale, massive 193-237 1.84-1.87 1 bentonite bed at the top, 10 cm thick, and 237 cps in GR