Sedimentary facies and oil seeps characterization of Fatha Formation in west of Iraq

Abstract

This study deals with the sedimentology of the middle Miocene Fatha (previously called Lower Fars) Formation in central Iraq (near Hit city west of Iraq and in select wells of the East Baghdad oil filed). It includes petrography, facies analysis, establish cyclicity and depositional environment. Crude oil characterization and affinity to the oil seeps near Hit city, west of Iraq are the main contest of this study. Detailed petrographic study is carried out on 98 thin sections from selected shallow wells near Hit area and East Baghdad field in addition to three outcrop sections. The effect of diagenetic processes is represented by neomorphism, cementation, dolomitization, dedolomitization and vanished evaporite. These processes cause decrease in porosity, while the dissolution process causes four kinds of secondary porosity: vugs, moldic pores, channel and rarely intercrystalline. The ultimate result of diagenesis has severely attended carbonate beds, than other lithologies and caused an increase in secondary porosity. Some views of oil appeared under the microscope as well as cracks in the field observations of this study. Facies analysis of Fatha Formation, along vertical and lateral distribution, helped in establishing its depositional environment. The data from outcrops, shallow wells and deep wells are used here but the main focus is on the data from the deep wells (EB-81, EB-82 and EB-92). Five major lithofacies are recognized in this study, namely, greenish gray marl, limestone, gypsum (and/or anhydrite), halite and red mudstone .The limestone lithofacies is divided into three microfacies. Lithofacies of the Fatha are nested in a rhythmic pattern or what is then known as a shallowing-upwards cycle. The Fatha Formation can be divided into three units named Unit 1, Unit 2 and Unit 3 (in ascending order). Different kinds of nesting of lithofacies are recognized in each unit. Only Unit 2 shows a complete rhythm. Six oil seep samples have been analyzed using Gas chromatography-Mass spectrometry (GC-MS) to determine their source, age and depositional environment. The results reveal heterogeneous oil sources of Jurassic as well as Triassic are the expected contribution to the accumulation of this oil. However, all the studied samples show heavy biodegradation due to their seeping nature that exposed this oil to the air and hence bacterial attack. These oils are heavy due to the loss of their light alkanes (C1-C20).