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 SiO 2 , Al 2 O 3 and Fe 2 O 3 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 riseand-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.

Characteristics of the Shale Gas Reservoir Rocks in the Lower Silurian Longmaxi Formation, East Sichuan Basin, China

“…The unconventional gas pools represented by shale gas are characterized by selfgeneration and self-storage and accumulation with relatively short hydrocarbon migration distances (Curtis, 2002;Zhang et al, 2004;Boyer et al,2006;Hill et al, 2007;Jarvie et al, 2004Jarvie et al, , 2007Jarvie et al, , 2008Liu et al, 2009;Guo et al, 2011). The USA and Canada are the only countries which currently exploit shale gas on industrial scale.…”

Shale reservoir characterization and control factors on gas accumulation of the Lower Cambrian Niutitang shale, Sichuan Basin, South China

“…The Sichuan Basin evolved from a Sinian (Z1)‐Middle Triassic (T2) passive continental margin, during which thick marine carbonate and clastics interbedded with volcanic beds were deposited (Chen et al, ; Liang et al, ; Liu, Algeo, Guo, Fan, & Lu, ). The strata thickness is about 4,100–7,000 m. In the Late Triassic, the basin was affected by closure of the Palaeo‐Tethys and the subduction of oceanic crust of the Yangtze Plate (Guo et al, ; Liang et al, ). The Jurassice‐Quaternary was essentially a foreland basin stage with intense folding, uplift, and erosion.…”

“…Shale gas exploration and development has been successful in the United States, whereas the study of shale gas in China is still in its infancy (Bowker, 2007;Guo, Jiang, Zhang, & Li, 2011;Liang et al, 2016). The black shale is widely distributed in many strata in various basins in China, and previous researches have shown that shale gas resources in China may total some 36 trillion cubic metres (data from EIA), which suggest the great potential for exploration and exploitation of shale gas (Liu, Ma, Huang, Zeng, & Zhang, 2011;Zou et al, 2010).…”

The Potential of China's Lacustrine Shale Gas Resources

“…Since 2008, various domestic research institutions have obtained shale gas recoverable resource predictions ranging from 10×10 12 m 3 to 32×10 12 m 3 in China (Zhang et al, 2008;Zhu et al, 2009;Dong et al, 2009;Zou et al, 2009;Zhang et al, 2011;Guo et al, 2011); among them, the continental shale gas resource is about one third of the total Chinese shale gas resource (Zhang et al, 2008; …”