Which of the following describes 'Not sweet' natural gas?

The term 'Not sweet' natural gas specifically refers to gas that contains high levels of hydrogen sulfide (H₂S). This type of gas is often described as 'sour' gas due to the presence of this compound, which is toxic and can be hazardous if not handled properly. The presence of hydrogen sulfide in natural gas is significant because it necessitates special processing and safety measures during extraction and use.

Conversely, a gas that is rich in natural gas liquids relates to its composition and richness, but does not specifically address the presence of impurities like hydrogen sulfide. A gas that contains no impurities would actually qualify as 'sweet' gas, as it lacks the contaminants associated with hydrogen sulfide. While natural gas can be used for cooking, this characteristic does not inherently define whether the gas is sweet or not. Thus, the defining feature of 'Not sweet' natural gas is the high levels of hydrogen sulfide present.