Paleoenvironment of Lower Silurian Black Shale and its Significance to the Potential of Shale Gas, Southeast of Chongqing, China

Abstract

Black shale is studied because of its economic value, paleoclimatic and paleogeographic significance. Due intensive development southeast of Chongqing, Longmaxi black shale, characterized by thickness in excess of 100 m, is becoming a location of concentrated research interest for shale gas exploration in China. The paleoenvironment, mineral composition, elements and reservoir characteristics of black shale of the Lower Silurian Longmaxi formation southeast of Chongqing were obtained by cores, outcrops, slices analysis, X-ray diffraction and X-ray fluorescence spectrum. The results show that Longmaxi black shale, including turbidite and suspended sediment, is deposited primarily in deep water. The sum of SiO2, Al2O3 and Fe2O3 content ranges from 73.82% to 90.54% by weight; quartz, feldspar and clay minerals are the dominant minerals in the rocks, ranging from 56% to 100%. Trace element contents of Mo (7.71 μg/g) and U (5.97 μg/g) are higher than that of earth crust and mudstone, the indexes of the anoxic reducing environment in the Longmaxi stage in the study area. The relative sea level of the Lower Silurian Longmaxi period, which correlates to total organic carbon content, experienced two rise-and-fall cycles; meanwhile, the reducibility of the ancient sea waters experienced a similar period of rise-and-fall cycles. Total organic carbon content is quite higher near the maximum flooding surface in study area. After mineral analysis, the reservoir property was found to be similar to the Ohio shale that provided the first commercial gas production in the United States, thereby inspiring and instructing shale gas exploration and exploitation southeast of Chongqing.