Tectonostratigraphic Framework and Depositional History Pattern of the Cretaceous Successions Period in Southern Iraq

Abstract

Twenty oil wells were selected to study the tectonostratigraphic of the Cretaceous in southern Iraq, in order to develop a comprehensive description of the petroleum system in the region. That was conducted through an interpretation of the technical reports and the available information of the wells, which include sedimentary, stratigraphy, tectonic reports, and oil reservoir studies of the Cretaceous. Stratigraphically, a third order cycle was identified in the Cretaceous succession in southern Iraq, which also comprises seven and half cycles of the fourth order. Eight genetic stratigraphic sequences were also identified, as well as eight maximum flooding surfaces. The concept of the tectonostratigraphic boundary (TSB) and the tectonostratigraphic unit (TSU) has been adapted in this study. In the present study, the Cretaceous period in southern Iraq considers one tectonostratigraphic system (TSS) consisting of four main tectonostratigraphic categories. Each category consists of a set or group of secondary tectonostratigraphic units; these are TSU1A, TSU1B, TSU1C, TSU1D-TSU2A, TSU2B-TSU3A, TSU3B, TSU3C-TSU4A, TSU4B, and TSU4. These units are separated by five tectonostratigraphic boundaries presented from TSB1 to TSB5 by Sulaiy, Shuaiba, Mauddud, Khasib, lower part of Tanuma, and Shiranish. The lateral extensions of the TSUs that are close to the passive margin (northeast part of the study area) are hydrocarbon reservoirs. The lateral extensions TSUs that are far from the passive margin (southwest part of the study area) are hydrocarbon generator source. The intermediate unite is characterized as both a generator source and reservoir hydrocarbon. Vertically, the TSUs are characterized by improved reservoir properties with reduced depth due to the lack of compressional tectonic force, which leads to forming a good primary porosity. The transfer from north to south of the study area represents a trend of improvement in reservoir characteristics for the same reason as mentioned previously. Finally, the TSB represents a source generator hydrocarbon more than a reservoir.