Modeling of NMR Response in the Distributary Bar Sequence, Matagorda Island Field Gulf of Mexico

Abstract

Nuclear-magnetic resonance (NMR) measurements provide petrophysicists with porosity distributions that are lithology-independent. Forward modeling allows analyzing NMR, taking into account all data available. Based on the study of distributary bar system, we found that: (1) mouth bar facies studied are characterized by unimodal distribution of T₂, “free-fluid” maximum at 20.0–40.0 ms and a “tail” associated with the chlorite coating and partially dissolved feldspar microporosity at 0.5–3.0 ms; (2) Delta front facies have bimodal distribution with maximums at ≈2.0 and ≈30.0 ms, a 4.8 ms and ≈65 ms cut-off values were established to define clay-bound and free-fluid volumes; and (3) Distal bar sands with calcite cementation have cut-offs of 12.0 ms and 90+ ms. To characterize delta front facies, an additional case has been modeled to illustrate the dependence of clay-bound water cut-off point upon mineralogy: (a) chlorite—mixed layer grain coating have ≈5.0 ms cut-off, and (b) kaolinte pore filling has a cut-off value of ≈20 ms.

Keywords:

• Gulf of Mexico (GOM)
• modeling
• mouth bar
• nuclear-magnetic resonance (NMR)
• porosity

Acknowledgments

The authors would like to acknowledge Amoco Offshore Business Unit and TNK (Tyumen Oil Company) which provided software, materials, and funding, The OBU Grant to the senior author was invaluable. The authors would also like to thank Paul Bienvenue for his advice and suggestions. The help extended by Mike Sparacino of Schlumberger and James Buchanan of Numar (Halliburton) is also greatly appreciated.

Notes

^a PHIT = Total porosity, or in NMR terminology—MSIG.

^b R₃₅—pore throat aperture radius corresponding to a mercury saturation of 35% during capillary pressure test (Kolodzie, 1980).

^c KPhi = k/PHIT, or k/MSIG permeability-porosity ratio (Kolodzie, 1980).