Natural Gas Exploration Using Carbon Isotopic Fractionation Effect: A Case Study of Shanxi Formation, Upper Palaeozoic Group in the Center of Ordos Basin, China

Abstract

Abstrac t—Carbon isotopic fractionation would occur during the process of natural gas generation and migration. As for 'self-generating and self-preserving' tight sandstone gas reservoir, carbon isotope better keeps the current maturity and migration results. In this paper, three local hydrocarbon generation centers and two relatively lower permeability zones were determined from the study area and it is suggested that natural gas migrated roughly from south to north direction through the analysis of carbon isotope isoline. Thus, we predict three favorable exploration areas using natural gas storage index combined with above isotopic results. It is a common geochemistry phenomenon that carbon isotopic fractionation would occur during the process of natural gas generation and migration. Carbon isotope becomes heavier as organic matter of the gas source rock becomes more mature; meanwhile, carbon isotope appears lighter along the direction of natural gas migration. Therefore natural gas carbon isotope records parent material source, and information of geology and geochemistry action that parent material and its products experience. Researchers in China and other countries have found and confirmed fractionation effect of natural gas carbon isotope, and built quantitative relations of δ13C1 and Ro that suggested specific mathematical relationship for the different type of source rocks. It is verified by indoor research that carbon isotopic fractionation range is wide under low temperature, and hydrocarbons that organic matter discharges contain lighter carbon isotope; as the temperature rises, fractionation effect is reduced, and carbon isotope of generated compounds become heavier(1),(2). It is also concluded by indoor experiments that natural gas always displays carbon isotopic fractionation effect to a different extent when it is injected into and diffuses through several types of lower permeability rocks respectively, in which montmorillonitic clay is the most obvious, kaolinitic clay takes the second and limestone fractionation effect is the least(3)-(6). Moreover, gas isotopic fractionation is influenced by migration dimension and channel. When natural gas flows in bulk through reservoir rocks, carbon isotopic fractionation is not obvious as the gas-rock medium contract area is small and the dwelling time is short; whereas the gas is seeping through the rocks with a smaller scale and slower flow rate, the fractionation is strong because of the long dwelling time and the adequate gas-rock contract, that leads carbon isotope along front of migration direction becomes light. As a result, an application of carbon isotope information is to infer the direction of natural gas migration and the possible enrichment zone. According to the geochemistry analysis and some other geologic evidence of gas reservoir in the eastern-central part of the Ordos basin, this paper just discusses how to predict natural gas migration and favourable exploration areas using carbon isotopic fractionation information.