Lower Permian Fluvial Sediments, Ga’ara Depression, Western Iraq: Depositional Environment and Hydrocarbon Potential

Abstract

     Synthesis of sedimentologic, paleocurrent, and organic geochemistry data of the Lower Permian Ga’ara Formation from the Western Desert, western Iraq, shows good hydrocarbon potentiality and deposition by high sinuosity and mixed-load channels, likely by a meandering river system. The Ga’ara Formation includes kaolinitic mudstone beds of various colors and channelized quartzitic sandstone beds. Based on the lithofacies identification, five lithofacies associations have been recognized: channel-floor, point-bar, abandoned channel plug, crevasse splay, and interchannel flood basin. In addition, the paleocurrent analysis and sandstone percentage map indicate a variation of the paleoflow spatially and temporally with a general direction range between NE and Sw, and the depositional environment has been interpreted as a meandering river system.

To unravel the hydrocarbon potentiality of the Ga’ara Formation deposits, Pyrolysis and TOC% analyses were conducted on selected samples. The studies indicated that the Ga’ara Formation in western Iraq could be a moderate to excellent rock source. The PCI agrees with TOC content, suggesting that the generation potentiality of the mudstones as a source rock is poor and poor to excellent. All the analyzed samples show that their hydrocarbons are indigenous. The Ga’ara mudstones of the Nijili and West Tayyarah have very good generative potential, whereas the Ubairan samples have excellent generation potentiality. The Nijili and Ubairan samples are at an early stage of oil generation, i.e., immature. In contrast, the Tayyarah sample represents a postmature phase at the end of oil generation. Collectively, these new data on spatial distribution, geologic characters, and organic geochemistry propose that the Ga’ara Formation in western Iraq could be a valuable economic asset with good hydrocarbon potentiality.