The Mishrif, Yamama, and Nahr Umr reservoirs petroleum system analysis, Nasiriya oilfield, Southern Iraq

Abstract

Forty rock samples were taken from cores and cuttings, and five oil samples were taken from Mishrif, Nahr Umr, and Yamama formations in the Nasiriya oilfield wells, of longitudinal, asymmetrical anticline structure, of about 34 km length and 13 km width located at the unstable plat form, Mesopotamian basin zone. Three major oil reservoirs in the Nasiriya oilfield: (1) Late Cenomanian–Early Turonian Mishrif Formation with reserved oil approximately 1,009 million tons show measured porosities up to 40 % and the oils are aromatic, medium gravity, high sulfur content, and interpreted as being sourced from: type II/III carbonate rocks interbedded and deposited in a reducing marine environment with high salinity based on biomarkers and isotopic analysis; Middle–Upper Jurassic age is based on sterane ratios analysis of isoprenoids and isotopes, and thermally mature source rocks. (2) The reservoirs of the Albian Nahr Umr of approximately 68 million tons reserved oil show measured porosities up to 23 %. (3) Late Berriasian–Early Valanginian Yamama formations of 275 million tons reserved oil measured porosities up to 28.8 %. The Nahr Umr and Yamama reservoirs oils are saturated, light to medium API gravity, low to medium sulfur content which are usually derived from marine shale and carbonate source rocks (containing type II kerogen). The upper seal is the Middle Miocene anhydrites of the Lower Fars Formation and the lower regional seal is Upper Jurassic Gotnia anhydrites. Source rock geochemical analysis suggests for all Cretaceous source rocks samples, immature to mature, and shows that the Sulaiy and Yamama formations to be of good quality source rock with high total organic carbon (up to 8 wt% TOC). The Lower Cretaceous source rocks were deposited in a suboxic–anoxic basin and show good hydrogen indices, and are likely to have charged the reservoirs with low amounts of hydrocarbons, during the Miocene. The most likely source rock for the Mishrif-, Yamama-, and Nahr Umr-reservoired oils are the Middle–Upper Jurassic source rock. The migration from the source rock is likely to be largely vertical and possibly along faults before reaching highly permeable reservoirs of the reservoir formations. Structural traps in the Mishrif, Nahr Umr, and Yamama formation reservoirs are likely to have formed in the Late Cretaceous.