NMR Application in Reservoirs with Complex Lithology: a Case Study

摘要

Nuclear Magnetic Resonance (NMR) logging has developed into a powerful reservoir characterization tool and was recently applied in the West Siberia basin, Russia. NMR typically is represented as a lithology-independent measurement of porosity, because hydrogen in the rock matrix does not contribute to the signal. Both the rock matrix and the cement composition, however, seriously influence the tool's response. We present four cases of NMR log response in reservoirs with complex lithology: Weathered-crust volcaniclastic conglomerates and coarse-grained sandstones, with an apparent shift in T_2 distribution of the free-fluid index (FFI) towards faster components, placing it below 33 ms because of a large amount of finely dispersed iron material; Fore-reef elastics composed of quartz sandstones with well-developed kaolinite cementation, low surface relaxivity, and bound water in the 33 to 128 ms range; Slump and debris-flow deposits with a complex mixture of elastic, carbonate material, and organic matter that results in variations of surface relaxivity, possible oil-wetting, and a general shift of T_2 distribution towards the bound-fluid interval (3 to 33 ms); NMR- "hostile" environments related to the black shales and the continental Tyumen formation. Excessive amounts of the iron-rich minerals leucoxene, pyrite, and siderite, which are associated with coals, and the presence of plant fragments and bitumen inclusions effectively "kill" the NMR signal.