Biozonation (Benthic Foraminifera) Of Mishrif Formation At Majnoon And Zubair Oil Fields, Southern Iraq

Abstract

The Mishrif Formation represents a carbonate succession that deposited in major parts of the Arabian Plate during the Cenomanian stage. The formation is rich in fossils including foraminifera, rudists, and algae. This study includes determination of the biozones and biostratigraphic limits of three boreholes sections of Mishrif Formation (Cenomanian) from Majnoon and Zubair oil fields in southern Iraq within the Mesopotamian Basin. A new biostratigraphic scheme is introduced based on the study of benthic foraminifera that occur in the complete succession of Mishrif Formation. Depending on the vertical distribution of benthic foraminifera, four biozones in the studied sections have been established, they include: 1 - Miliolids Abundance Biozone that is characterized by the first appearance of miliolids to first appearance of Nezzazata conica, and include rudist, gastropoda, pelecypoda and algae, 2 - Nezzazata simplex – Nezzazata conica Concurrent Biozone (Middle Cenomanian) is distinguished by the first appearance of Nezzazata simplex, Nezzazata conica, Praealveolina cretacea, and Pseudorhapydionia laurinensis. Other associated benthic foraminifera include Nezzazata concave, Praealveolina tenuis, Chrysalidina sp., Cuneolina pavonia, Multispirina iranica, Biconcava bentori, Qataria dukhani, Dicyclina schlumbergari, Tabrina beingstani, Cisalveolina sp., and Carinoconus iraqiensis. 3 - Pseudorhapydionian laurinensis – Praealveolina cretacea Concurrent Biozone (Early Late Cenomanian) is identified by the last occurrence of Nezzazata simplex with the first occurrence of Psudorhapydionian laurinensis, and Praealveoilina cretacea, and the last occurrence of both taxa. This biozone also shows the occurrence of Cisalveolina fallax, Tabarian bingstani, Carinoconus iraqiensis, Spiroloculina sp., Chrysalidina gradata, Biconcava bentori, Qataria dukhani, Pseudotexularella sp., and Dicyclina schlumbergari. 4 - Pseudolituonella reicheli-Chrysalidina gradata Concurrent Biozone (Latest Cenomanian) is marked by the first and last occurrence of Pseudolituonella reicheli and Chrysalidina gradate. Other diagnosed foraminifera include: Spiroloculina sp., Rotalia sp., Pseudorhapydionia sp., Quinquelaculina sp., Nummulculina heimi, and Discorbis sp. Based on the stratigraphic ranges of the available fossils of the studied sections, the age of Mishrif Formation is estimated to be of Middle-latest Cenomanian.