Geochemistry characterization of diagenesis from Mishrif reservoir rocks in the Central Mesopotamian Basin, Iraq: Implication for the differential diagenesis and petroleum significance

Abstract

The Cenomanian Mishrif Formation is one of the most important carbonate reservoirs in the Mesopotamian Basin. Detailed petrographic analysis, aided by geochemistry and burial history establishment on Mishrif carbonates from the AD oilfield, located at the margins of the Central Mesopotamian Basin of Iraq, presents a distinctive differential diagenetic mechanism that significantly impacts reservoir quality and exhibits a pronounced relationship with tectonic evolution and hydrocarbon accumulation. Analyses of petrographic features, carbon‑oxygen isotopes, and minor and rare-earth elements reveal that echinoderm shells and rim calcites are results of marine waters; sparry calcites, matrix, and vein calcites from host rock primarily precipitate from meteoric fluid; microspars and vein calcites in the concretionary limestone are derived from brine water. The development of concretionary limestone mainly involves a complex process that comprises extensive bioturbation in marine waters, substantial fabric-selective dissolution in the meteoric zone, and significant sparry calcites occluded dissolved spaces in shallow burial environments. Winding and skew seams of argillaceous sediment around concretionary limestones, as well as the excellent preservation of skeletal remains inside the concretionary zone, manifest that the development of the concretionary limestone accomplished during the early diagenesis stage and pre-compaction. These centimeter-to-decimeter scaled concretionary limestones intermixed with host rock holding abundant open porosities shape the Mishrif reservoir and result in an inconspicuous, dual permeability system. The burial history, integrated with the diagenetic sequence and tectonic evolution, elucidates four stages that clarify interactions of diagenesis, porosity preservation, and hydrocarbon accumulation. The Cenomanian to Turonian period was a critical stage for the development of effective reservoir capacity. The end of the Cretaceous, marked by the contraction of the Neo-Tethys and the complete closure of the South Neo-Tethys, witnessed an early hydrocarbon accumulation event that enhanced reservoir porosity preservation. During the Paleocene to the Early Miocene epoch, the Neo-Tethys tended to be closing. As a result, the Mishrif Formation experienced shallow burial and generated typical concretionary limestone, which provided a rigid framework against compaction. From the Early Miocene to the present, the total closure of the Neo-Tethys and the Zagros Orogeny initiated a significant late oil-filling event that acted as the main accumulation moment, coinciding with efficient trap development and other optimal petroleum conditions. This study contributes to (1) comprehending the genesis of the differential diagenesis and its effect on pore structures and reservoir heterogeneity, and (2) linking diagenetic sequence to tectonic evolution and hydrocarbon accumulation of foreland basins evolved from Neo-Tethys tectonics.