Abstract
The geological characteristics of shale and the gas composition and carbon isotopic variation from the shale are very important indicators for the shale gas study. This article analyzed the geological characteristics of Cambrian Niutitang shale and focused on the gas composition and carbon isotopic characteristics of the shale gas from the Cambrian Niutitang Formation. We can see that the total organic carbon content ranges from 0.41 to 4.29%, with an average of 1.65%, which is mainly type I and Ⅱ1. The thermal maturity of sample ranges from 1.98 to 2.53%, with an average of 2.30%, which is in the postmature stage of thermal evolution. The Cambrian Niutitang Formation shale gas in the southern margin of the Hannan ancient uplift is the high-quality hydrocarbon gas methane that takes an absolute majority with a content ranging from 89.60 to 99.19% with an average of 96.46%. The wetness of the gas from the study area ranges from 0.06 to 0.42%, with only 0.25% on an average, which is defined as typical dry gas. The distribution of carbon isotope values has partial “reverse” characteristics that are δ13C1 > δ13C2 and δ13C2 < δ13C3. The natural gas of Niutitang Formation in the study area belongs to thermogenic gas. At the same time, it is found that some data points are beyond the range of thermogenic gas, which is mainly due to the low-humidity and heavy carbon isotope value of methane caused by the high maturity of source rock. In the study area, the Niutitang Formation natural gas carbon isotope values are located in Zone III with the characteristics of mixed reversal gas in the δ13C1-δ13C2 part.
Highlights
Cambrian Niutitang Formation organic-rich shale is a set of main shale strata in southern China, and shale gas has been found and broken through in many areas
The distribution of carbon isotope values of methane, ethane, and propane has partial “reverse” characteristics that are δ13C1 > δ13C2 and δ13C2 < δ13C3 (Figure 2), due to the δ13C1 is far less than -30‰ and the CO2 content is far less than 60%, which shows the characteristics of the alkane gas in Niutitang Formation shale in the study area with organic genetic gas
Put the δ13C1 value and the calculated C1/ C2+3 value from the Table 2 into the chart, and it is judged that the natural gas of Niutitang Formation in the study area belongs to thermogenic gas
Summary
Cambrian Niutitang Formation organic-rich shale is a set of main shale strata in southern China, and shale gas has been found and broken through in many areas. This set of shale strata is widely developed in the southern margin of Hannan ancient uplift and is an important field of shale gas survey and exploration in the future. One of the reasons is that most of the gas wells with carbon isotope inversion are medium- and highproduction wells (Han et al, 2013)
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