Deposition and Chemical Diagenesis of Tertiary Carbonates, Kirkuk Oil Field, Iraq

The Tertiary carbonate rocks of the Kirkuk oil field in Iraq have been subdivided into 18 major lithologic facies. Their spatial relationships suggest that carbonate deposition proceeded in alternating carbonate ramp and carbonate rimmed shelf settings. Geochemically, these 18 facies can be grouped into three populations: namely, the nearshore (mud flat and bioherm), foreslope, and basinal populations. The nearshore population consists of mudstone, wackestone, packstone, and grainstone with bioclasts of miliolids, peneroplids, rotalids, red algae, corals, Numulites, Lepidocyclina, and crinoids. The basinal population encompasses mudstone and wackestone with Globigerina, radiolarians, and tintinids. The foreslope population consists of packstone and grainstone with a mixtu e of Nummulites, Lepidocyclina, and traces of corals and red algae. The nearshore and foreslope groups are characterized by severe recrystallization and contain abundant cements. The porosity is of primary (intergranular and intraskeletal) and secondary (dissolution and fracture) types and is associated mainly with the bioherm and foreslope facies. The nearshore population is characterized by low sodium and strontium contents and light ^dgr18O and ^dgr13C, whereas the basinal one has the opposite attributes. The foreslope group represents a mixture of these two end members, with nearshore components being dominant. Comparison of the approximate composition of diagenetic solutions responsible for deposition and/or mineralogic stabilization of carbonate constituents with present-day waters of various geologic environments led to the conclusion that the nearshore population was stabilized in a near-surface, meteoric, well-oxygenated aquifer. In contrast, the basinal population was deposited and stabilized in waters of marine parentage.