Dense brine refluxing: A new genetic interpretation of widespread anhydrite lumps in the Oligocene-Lower Miocene Asmari Formation of the Zagros foreland basin, NE Iraq

The Asmari Formation in the Zagros Basin of NE Iraq contains a thick sequence of Oligocene-Lower Miocene carbonates, and it forms one of the best-known carbonate reservoirs in the world. The formation mainly consists of dolostones, limestones, sandstones, and mudstones, and there is a general upward increase in the abundance of evaporites (generally in the form of anhydrite lumps) in the dolostones of the upper part. The anhydrite types and textures provide evidence on the salinity, depth, temperature, environment, and sea-level fluctuations which can be used for paleoenvironmental and climatic interpretations. And they also exert a significant control on reservoir quality, which has attracted the attention of many petroleum industries. Previous studies suggested that the anhydrite lumps in the Upper Asmari Formation (Aquitanian-Burdigalian) were synsedimentary deposits in a hypersaline setting of a sabkha (salt flat). However, this paper shows that 1) the Upper Asmari Formation was deposited in an inner-to mid-ramp setting, based on the sedimentological, petrographical, and paleontological characteristics; 2) the anhydrite lumps are irregular and variable in size, and are generally found within or associated with karst systems; and 3) fossil assemblages indicate that the synsedimentary salinity was normal or only slightly hypersaline, which means that the anhydrite was not directly deposited from the synsedimentary marine water. After comprehensive analysis of the anhydrite lumps, it is proposed in this paper that the anhydrite lumps were formed within the Upper Asmari Formation (Aquitanian-Burdigalian) due to downwards seepage of the overlying hypersaline water, which is termed as dense brine refluxing, a new genetic model. It is suggested 1) that the Upper Asmari Formation (Aquitanian-Burdigalian) was deposited in an inner-to mid-ramp setting and subjected to eogenetic karstification generating karst systems in local area with relative high landscape due to sea level change; 2) that during the middle Miocene tectonic compression resulted in the Zagros Basin isolated and the seawater becoming hypersaline with higher salt concentrations and a high density; and 3) the hypersaline water seeped downwards through the underlying porous sediments, forming anhydrite lumps and resulting in a decrease in the porosity of the Upper Asmari Formation (Aquitanian-Burdigalian). This paper provides new perspectives on the origin of anhydrite lumps and the resulting effects on reservoir quality in the Zagros foreland basin.