Experimental Study On Water Absorption Of Lower Paleozoic Marine Mudstones In Sichuan Basin And Adjacent Areas

The shale gas conditions of the Cambrian Niutitang Formation in southern China are superior,and the proven reserves are abundant,but only in a few areas,breakthroughs have been made.In most areas,there is almost no commercial shale gas production,and the gas content varies greatly in the area.This is a clear contrast with the Longmaxi Formation,and the reason is not clear.This paper compares the Yichang area of Hubei with the breakthrough of the Cambrian shale gas and the Weiyuan area of Sichuan with the unfavorable Chongqing and Guiyang areas,45 samples of Cambrian organic shale samples from the Yichang WPZK001 drilling section,Hubei Changyang outcrop section,Sichuan Weiyuan 001-4 drilling section,Chongqing Youyang outcrop section,and Kaiyang County drilling section of Guizhou were selected,In addition,12 samples of organic-rich shale samples from the Longmaxi Formation outcrop section of Shuanghe Town,Changning County,Sichuan Province,and core samples from the QQ-1 well of the southeastern Guizhou Province were selected.The 57 samples were subjected to small size rock cylinder water absorption test,helium porosity measurement and XRD mineral composition analysis,and some samples were selected for SEM pore imaging observation.The effects of organic matter abundance and mineralogy composition changes on the saturated water absorption of the samples were analyzed,and the main controlling factors affecting the pore development of these two sets of organic-rich shales were discussed.Based on SEM pore imaging and helium porosity data,the differences in pore development between the Cambrian and Silurian organic-rich shales were compared.(1)Similar water absorption characteristics on organic-rich shales were observed between the Silurian Longmaxi Formation shales and the Cambrian shales in Yichang and Weiyuan areas where the breakthroughs in Cambrian shale gas production have been achieved.In contrast,there is no correlation between water uptake and total organic carbon content of the Niutitang Formation shales in the Youyang area of Chongqing and the Kaiyang area of Guizhou where the Cambrian shale gas production hasn’t been made any success yet.These observations indicate that the development of organic matter hosted-pores is one of critical factors to evaluate the Cambrian shale gas.(2)The difference in mineralogical compositions also plays an important role to the saturated water absorption of mudstone.There is a good positive correlation between the water uptake of the Silurian Longmaxi Formation shale and the quartz content,however,there isn’t any correlation between the water uptake and clay mineral content and the carbonate content.In contrast,a good positive correlation exists between water uptake and clay mineral content and carbonate mineral content of the Cambrian-aged Niutangtang Formation shales in Kaiyang County.These observations clearly suggest that the development of minerals pores in the Cambrian shales plays some roles to pore network and shale gas storage.The order of Helium porosity is: Longmaxi Formation(6.13%)> Changyang outcrop profile of Shuijingtuo Formation(4.19%)> Weiyuan 001-4 drilling core profile of Qiongzhusi Formation(3.47%)> Kaiyang Drilling core section of Niutitang Formation(2.83%),and the porosity of the Cambrian shale(Niutitang Formation/Qianzhusi Formation/Shuijingtuo Formation)is generally smaller than that of the Silurian Longmaxi Formation shale.SEM pore imaging observation shows that the dissolution pores of carbonate minerals are abundantly developed in the Cambrian shale(Niutitang Formation/Qiongzhusi Formation/Shuijingtuo Formation),while the mineral pores of the Silurian Longmaxi Formation shale are mainly intergranular pores and fracture pores.Compared with the organic matter pores of the Cambrian shale in the Yangye 1 core well of the Yichang area,the Silurian Longmaoxi Formation shale is superior in terms of pore number,pore size and pore connectivity.(3)The results of water absorption experiments show that one of the key factors affecting the difference in gas-bearing content of the Cambrian organic-rich mudstone is the development of organic matter pores in the shale.Our results provide some insights and an important research idea for studying the difference in gas-bearing content of the Cambrian shale.