Chapter 10 - Gas geochemistry indicates Ordovician marine micrites as the main source rock of natural gas in a weathered limestone reservoir (Jingbian Gas Field, Ordos Basin, China)

Abstract

The tectonic stability of the Ordos Basin is favorable for the generation and preservation of natural gas. One of the five giant gas fields in the basin is the Jingbian Gas Field. Ordovician weathered limestone constitutes the reservoir, but it has long been discussed whether the source rocks are Carboniferous-Permian coal measures or Ordovician marine carbonates. Abundant geochemical evidence indicates now that the gas originated from a mature to overmature source. The carbon isotopic signature suggests that this source was Ordovician limestone containing sapropelic organic matter. The thermal maturity of the gas indicates a high- to overmature stage, suggesting that it originated from secondary H-T cracking of oil and gas. Thermochemical sulfate reduction may have played a role in the alteration of gas dryness and δ13C1 where gypsum forms an effective cap rock. Mixing of the gas from the Ordovician source with coal-type gas from Carboniferous-Permian coal is minor in the Jingbian Gas Field.