Accumulation Conditions and Enrichment Mechanism of Shale Gas in Nangao Section, Yangtze Platform, S. China
Chao Luo, Shugen Liu, Wei Sun, Bo Ran, Di Yang, Shiyu Wang, Yuehao Ye, and Xuan Zhang
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (CDUT), Chengdu, Sichuan, China
It is characterized with a sequence of Early Cambrian black shale in Nangao section located at the Qiannan sag in southeastern margin of Yangtze platform. The black shale with thickness of 118.2 m is composed of mudstone deposits of deep-water shale facies in the lower part of the section, and with silty shale or calcareous siltstone of shallow-water shelf facies in the upper part. Along the strata the value of V/Sc, Ni/Co, V/(Ni + V) and δU are 11.3,138.7, 21.2, 0.79 and 1.86, respectively. They indicated that the black shale was deposited in the oxygen environment. Furthermore, the higher values of U/Th (>1) compared with negative anomalies in Ce, positive anomalies in Eu and Y, suggest that there is hydrotherm-related sedimentation (Yang, 2007, 2008). The Lower Cambrian black shale is characterized with type-I kerogen, high abundance and high maturity. Of which the sapropelinite accounts for 70.91%, δ13Corg is -33.7‰ ~ -29.1‰ (Teng, 2008), the TOC values range from 0.38% to 15.31%, with an average value of 5.2% from 46 samples, partly with values >2.0%. In general, the thickness of the black shale with TOC>2% is 114.3m, of which the Re values range from 1.97% to 3.87%, with an average value of 2.64% (Tan, 2011). The mineral composition of the black shale predominantly includes clay (e.g. illite and chlorite), quartz, feldspar, barite, and gypsum, etc. Quartz content is relatively high and reduced upwards, with an average of 53%, Quartz content is too high or too low are not conducive to the enrichment of organic matter, when the quartz content between 20% and 70%, and a positive correlation with TOC values. Clay content is relatively low and increased upwards, with an average of 34% but was negatively correlated with TOC values which as opposed to the quartz’s (Figure 1). There are few carbonate minerals, only partly at the top. The porosity and density of the black shale ranges from 0.99% to 4.66% with an average of 2.39%, and from 1.99 g/cm³ to 2.62 g/cm³ with an average of 2.47 g/cm³, respectively. There are four types of pores, such as interparticle pores, intraparticle pores, fracture pores and organic-matter pores, of which the first two are more common in minerals, the latter two less. Furthermore, the adsorption isotherm experiments suggest that adsorbed gas in the black shale is 1.71cm³/g~2.62cm³/g with an average of 2.33cm³/g, indicating a strong adsorption capacity. Breakup of Rodinia supercontinent and seafloor spreading acceleration, on the one hand, frequent hydrothermal activity and submarine activities provide a lot of multi-metal and siliceous sources; On the other hand, the rise in ocean currents carry large amounts of nutrients, plankton and algae blooms to create favorable conditions for organic matter enrichment.
AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013