Effect of pressure on maturation and expulsion of hydrocarbons: Zubair formation in North Rumaila Oilfield, Southern Iraq case study

This study focused on integrated petroleum maturation and expulsion as well as the formation of a trap before primary migration of the Zubair Formation in southern Iraq. Geochemical analysis showed that the shale layers in the formation contain the required amount of organic matter in the form of II-III kerogen, which has reached the maturity stage to generate and expel hydrocarbons. The study uses geological model to calculate sedimentation, tectonic subsidence, and the amount of eroded thickness resulting from tectonic processes in the sub-Hercynian, Laramide, and Alpine orogenies. The geothermal gradient is estimated based on decompacted formation thickness, revealing its influence on organic matter maturation in Zubair’s shales. The study also links the high rate of sedimentation and hydrocarbon generation to increasing pore fluid pressure in addition to the influence of the formation of acidic water on the dissolution of calcareous cementing material in the shales, causing fractures that led the generated hydrocarbons to migrate vertically up to Zubair sandstones, with the timing of migration was determined to be in the middle Paleocene. The study concludes that the pore fluid pressure decreased, approaching hydrostatic pressure due to the uplifting of Alpine orogeny. Finally, the study claims that the first structural closure was formed in the Zubair Formation during the Early Cenomanian and became structurally stable throughout the Late Cenomanian.