Geochemistry of Saudi Arabian Natural Gas

Abstract

Saudi Aramco has measured carbon isotope compositions of C1-C5 hydrocarbons and CO2 on hundreds of gases from exploration tests and producing wells. The isotope data group the gases into thermogenic families with different sources. For example, δ13C of gases associated with Jurassic Ghawar-type crudes increases from -54.6 3.0 ‰ for C1 to -39.6 1.6 ‰ for C2 and -27.6 0.9 ‰ for n-C5. In contrast, δ13C of Paleozoic gases increases from -42.1 6.1 ‰ for C1 to -31.9 4.5 ‰ for C2 and -29.5 3.0 ‰ for n-C5. The isotopic characteristics of the Jurassic-Ghawar and Paleozoic gas families appear heavily influenced by their source kerogen, Type IIS for the former (Upper Jurassic Hanifa and Tuwaiq Mountain carbonates) and Type II (Silurian Qusaiba hot shale) for the latter. C1/C1-C5 (mol/mol) of Paleozoic hydrocarbons ranges from 0.65 to 1.00 and is correlated with an increase in methane δ13C from -47 to -37 ‰. Within this range, we judge that liquids abundance and isotopes are controlled primarily by source rock maturation. Khuff, Unayzah, Devonian and Silurian- Ordovician gases are indistinguishable on common interpretive plots such as C1/C1-C5 hydrocarbons versus methane δ13C, providing evidence for a single dominant Paleozoic source. Most Paleozoic gases contain less than 10% CO2. δ13C of CO2 in unaltered gases from Khuff carbonate reservoirs typically fall between -3 and 3 ‰ whereas those from deeper clastic reservoirs fall between -20 and -5 ‰. Gases containing more than ppm levels of H2S are restricted to the Khuff. Increasing H2S is accompanied by a decrease in CO2 δ13C to been altered by thermochemical sulphate reduction (TSR). Paleozoic gas-condensates around the Ghawar structure commonly contain less than 15% N2. Nitrogen abundance here appears to be controlled by mixing between wet thermogenic gas and a high-N2 gas component of uncertain origin. Khuff gases in coastal and offshore fields contain less than 10% to as much as 40% N2 and often far less than 1% C2 hydrocarbons. In one field with incontrovertible petrographic evidence for TSR, C2 hydrocarbons are below detection limits and methane and carbon dioxide both have δ13C of approximately -22 ‰. N2, accounting for 25% of the gas, appears to have been enriched due to destruction of methane by TSR.