Palynostratigraphy and the assessment of gas and oil generation and accumulations in the Lower Paleozoic, Western Iraq

Palynological and organic geochemical analysis are performed in this study for 220 samples of cores and cuttings collected from the Ordovician Khabour, Silurian Akkas, and Upper Devonian Kaista Formations in wells Akkas/1-6, Khleisya/1, KH5/6, and KH5/1 of West Iraq. Their diagnostic organic matters are abundant acritarchs (134 species belonging to 54 genera, including marine algae of Tasmanites, Deflandstrum, and brazinophytes) and a few spores (21 species belonging to 16 genera) and Chitinozoa (43 species belonging to 12 genera) as well as scolecodonts, graptolite siculae, cuticles, and amorphous organic matters. On the basis of acritarchs with tentative selections of Chitinozoa and spores, this succession is subdivided into ten palynozones (PZ1–PZ10) within a stratigraphic framework and correlated with equivalent strata in Saudi Arabia and Libya. Beds of the Khabour and lower part of Akkas Formations were deposited in anoxic–dysoxic marine shelf environments northern Gondwana Continent with provincial acritarchs. These deposits were extending from outer to inner neritic with affects of local upwelling currents and lagoons, especially in boreholes Akkas/1, KH5/1, and KH5/6. Hydrocarbon generations potential are assessed by plotting organic matter types in palynofacies context of Bujaks (1970) graphical model with depths along with log of thermal maturation indices on the basis of the color changes of the acritarchs Diexallophasis denticulata–Orthosphaeridium ternatus and Baltisphaeridium constrictum as well as kerogen types and total organic carbon (TOC). These organic matters are up to 16% TOC, especially for the hot shale of the Lower Silurian Akkaz Formation, very low asphalting and sulfur, saturated and aromatic hydrocarbons of more than 96%, and high peaks of C2–C20 gas chromatography that could indicate predominant gas generation with some light oils. The associated gases are mainly methane and ethane of CH₄, C₂H₆, and C₃H₈. Accordingly, source potential for wet gas and condensates could be assessed for depth of 2,750–3,000 m and dry gas for depth of 3,570–3,650 m in well Akkas-1 only from the Ordovician Kabour Formation. Little oil might be generated from the lower Silurian Akkas formation in borehole Akkas-1 and KH5/6. These potential source rocks are extended toward Jordon, southwest Iraqi Desert and Syria. Accumulation sites of these generated gas and little oil could be within the sandstone porosities of 10–17% and permeability of 500 mD sealed by the non permeable shale's along closures of the structured anticline fold and fault of this field as well as along the unconformity boundary of the Upper Silurian Akkas Formation with the Upper Devonian Kaista Formation. Accordingly, Lower Paleozoic total petroleum system of generation, migration, and accumulations could be assessed for a basin includes West Iraq and their extensions in Jordon and Syria.