--> ABSTRACT: Diagenesis in The Mardie Greensand of The NW Shelf of Australia and Its Effects on NMR Measurements, by P. Joe Hamilton and Mike Dowen; #90906(2001)

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P Joe Hamilton1, Mike Dowen1

(1) CSIRO Petroleum, Perth, Australia

ABSTRACT: Diagenesis in The Mardie Greensand of The NW Shelf of Australia and Its Effects on NMR Measurements

An NMR logging tool, or laboratory instrument measures the strength & decay with time of the signal induced by magnetization of hydrogen nuclear spins. Pore size distribution and nature of mineral-fluid contact govern the decay rate of NMR response. Paramagnetic minerals cause a significant increase in decay rate that may reduce apparent producible porosity.

Diagenetic studies of the Mardie Greensand demonstrate the presence of paramagnetic Fe-bearing minerals (pyrite, siderite and glauconite). Pyrite mainly occurs as small aggregates of cubic crystals located primarily in microporosity within glauconite pellets. This has negligible effect on NMR response from moveable fluid. Siderite occurring as pore occluding cement will cause significant increase in NMR decay rate from moveable fluid with which it is in contact and will result in underestimation of moveable fluid volume. Glauconite occurs in highly variable abundance and in different forms. As intergranular cement it has high Fe content and the constituent clay platelets present a large surface area to adjacent fluids resulting in potentially significant but non-constant effect on NMR decay rate. Reworked glauconite pellets have variable and often low Fe content. Exterior surfaces are often smooth and present low surface area to volume grains to adjacent free fluid. This glauconite type will have least impact on NMR signal. Autochthonous glauconite pellets have variable and often high Fe content. Exterior surfaces are generally rough and comprise clay plates that present large areas of paramagnetic effect on adjacent fluid. It will have the most impact on reducing NMR decay rates.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado