Hartha Formation (late Campanian–Maastrichtian) is one of the most important carbonate hydrocarbon reservoirs in Iraq. This study focuses on the reconstruction of Hartha Formation facies architecture in the Majnoon oilfield in southern Iraq. Sixty thin sections were made from several wells (MJ-29, MJ-47, MJ-88, MJ-91, MJ-92, and MJ-93). Thin section analyses display the dominance of nine microfacies, represented by Mudstone, Wackestone, Wackestone–Packstone, Packstone, Packstone–Grainstone, Grainstone, Boundstone, Rudstone, and Crystalline dolomite microfacies. Hartha Formation deposited on a shallow carbonate platform with a ramp setting. These include supratidal environment, restricted marine, shallow open marine, shoal, and deep outer ramp. Most of the carbonate grains in the Hartha Formation are represented by benthic foraminifera, rudist, echinoderms, corals, algae, and peloids with the appearance of some planktonic foraminifera at the bottom of the formation. The relative sea-level fluctuation has a great effect on the digenetic development of carbonate succession.