Sedimentary and reservoir characteristics of an Oligocene-Miocene mixed siliciclastic-carbonate succession in southeast Iraq

The Cenozoic mixed siliciclastic-carbonate succession in southeastern Iraq is an essential and unique reservoir in the region. However, the current understanding of its sedimentology and reservoir characteristics is insufficient. In this study, its sequence stratigraphy, depositional history, and reservoir characteristics are investigated by combining seismic and well log data, core observations, and porosity and permeability measurements from several oilfields in southeastern Iraq. Two types of shallow marine deposits in two third-order sequences are established. Ten facies associations are identified in the mixed siliciclastic-carbonate deposit: the carbonate-dominated type, including sabkha, lagoon, intertidal-subtidal mudflat sandflat, and shoal; the mixed type, including foreshore and shoreface, and; the siliciclastics-dominated type, including distributary channel, interdistributary bay, and crevasse. The lower third-order sequence formed in a siliciclastic delta-dominated shallow marine system, resulting in two patterns of reciprocal mixed deposition, one forming between different system tracts and the other forming between thin carbonate and sandstone layers in the highstand system tract; the factors controlling the deposition of this reciprocal mixed system include allocyclic processes such as sediment supply, sea-level change, and tectonic activities. In contrast, the upper third-order sequence was deposited in a carbonate tidal-dominated shallow marine system, mainly forming a coeval mixed deposition controlled by various hydrodynamic factors. The reservoir quality of the coeval mixed reservoir is facies dependent and diagenesis influenced. The shoreface and parts of the tidal mudflat and lagoon are the favorable reservoir facies; in this reservoir, the strong hydrodynamic depositional environment and the syn-depositional selective dissolution are the advantageous controlling factors. In the reciprocal mixed reservoir, the petrophysical properties of the sandstone layers are better than that of the carbonate layers, causing the carbonate layers to act as interlayers or baffles to some extent. Thus, horizontal well fracturing and vertical wells are recommended for exploiting the coeval and reciprocal mixed reservoirs, respectively. This study helps to better understand mixed siliciclastic-carbonate reservoirs and provides support for oil development in the Middle East region.