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Bioturbation of Mishrif Formation in M Oilfield, Iraq
LI Fengfeng1, WANG Zhenbiao1, GUO Rui1, LIU Lifeng1, YU Yichang1, SONG Shiqi2
(1.Research Institute of Petroleum Exploration & Development, Beijing 100083,China;2.College of Geoscience in China University of Petroleum (Beijing), Beijing 102249, China)
Abstract:
Taking the bioclastic limestone of Mishrif Formation in M Oilfield, Iraq as an example, the environment preference, the infilling genesis and controlling factors, and the mechanism of bioturbation were studied. The results show that the research area is a ramp with weak rimmed platform. Bioturbation is more inclined to develope in low-energy environment,especially in the lagoon. According to the ratio of the bioturbated area to the core section area, four grades are defined , which are separated by 20%, 50% and 80% connectivity. The four bioturbation grades are:Grade Ⅰ porphyritic, Grade Ⅱ laminated, Grade Ⅲ labyrinth and Grade Ⅳ thick layers. There are three kinds of filling materials, syngenetic clastic, dolomite and calcite in the bioturbation channel. The clastic is the physical genesis and is controlled by sedimentation, while dolomite and calcite are the products of late diagenesis.Dolomite can bedivided into the microcrystalline discrete dolomite, the euhedral fine-grained dolomite and the euhedral-semieuhedral mosaic dolomite. Among them, the microcrystalline discrete dolomite has little effect on reservoir physical properties, and the euhedral fine-grained dolomite is beneficial to improve physical properties, while the euhedral-semi euhedral mosaic dolomite reduces physical properties. The bioturbation mode of Mishrif Formation in Moilfield was established by combining petrology, degree of bioturbation, filling material and influences on physical properties. The sedimentary environment determines the substrate and preservation condition, the degree of bioturbation reflects the strength of reconstruction, and the filling material and diagenesis control the transformation properties. The greater the bioturbation intensity is, the more the euhedral fine-grained dolomite is, and the stronger the dissolution is, the better the reservoir properties are.
Key words:  bioturbation  Mishrif Formation  dolomite  bioturbation mode