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2018 AAPG International Conference and Exhibition

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Decoding Charge History in a Complex Paleozoic Petroleum System by Combined Gas Geochemistry and Fluid Inclusions

Abstract

Determining the source of gas is important for gas show evaluation, petroleum system analysis, and prediction of fluid properties. Thermogenic and bacterial gases can be distinguished by their isotopic and chemical compositions. Determining whether thermogenic gas in question is derived from cracking of kerogen, oil, or wet gas, or otherwise represents a mixture of sources remains a challenge. Deciphering this can provide critical inputs to exploration and basin modeling for assessing the regional gas resource potential and configuring the timing and risk aspects of hydrocarbon charges. Hydrocarbon gases discovered in Paleozoic clastic reservoirs in Saudi Arabia are generally believed to be derived from Silurian Qusaiba hot shale. Gases in Ordovician sandstone reservoirs in the study area compare reasonably well chemically and isotopically with the gas generated from artificial maturation of Qusaiba kerogen, supporting their genetic relationship. The comparison further provides valuable information on the gas formation mechanism. Gas-condensates in the sandstones appears to be a product of primary cracking of kerogen (0.7~1.0% vitrinite reflectance equivalent, VRE), suggesting not only gas-condensate potential, but also oil potential that has not yet been realized by drilling. Fluid inclusions from sandstone reservoirs directly underlying the Qusaiba shale contain two main hydrocarbon populations with distinct API gravities of 20-38° and 45-55° (estimated from UV fluorescence) having homogenization temperatures (Th) for associated aqueous inclusions in the range 100-155 °C. This bimodal API distribution suggests an oil charge followed by a later gas-condensate accumulation, possibly because of exsolution from the early oil as a result of structural reactivation and uplift. Gas in a deeper extension of the same reservoirs 150 km to the north is dominantly dry and appears to be sourced primarily from late cracking of kerogen (> 1.4% VRE), supplemented by a small contribution from secondary cracking of wet gas-condensate within the source interval. The late charging and mixing model offers a plausible explanation for an observed isotope anomaly for ethane and propane in the northern area. Although oil inclusions were not observed optically here, the presence of methane in the vapor phase of some aqueous inclusions was evident by Raman spectroscopy. The methane, together with higher Th (150 to 170+ °C), agrees with dry gas trapping at a late overmature stage.