Print this pageLow Resistivity, Low Contrast Pays
SNEIDER, ROBERT M., and JOHN T. KULHA
Major hydrocarbon accumulations have been produced over the past 40 years in low resistivity, low contrast (LRLC) sands in the Gulf of Mexico Basin (GOM). LRLC reservoirs were commonly considered wet, tight, misidentified as a shale or overlooked, but are being re-evaluated now in other world basins. Seismic response, drill cuttings, cores, log response, petrophysical models, and production testing provide an integrated LRLC evaluation.
Causes of LRLC pay in the GOM include: laminated clean sands with shales; silts or shaly sands; clay-coated sands; glauconitic sands; sands with interstitial dispersed clay; sands with disseminated pyrite or other conductive minerals; clay-lined burrows; clay clasts; altered volcanic/feldspathic framework grains; and very fine-grained sand with very saline water. LRLC depositional systems include: deepwater fans, with levee-channel complexes; delta front and toe deposits; shingle turbidites; and alluvial and deltaic channel fills.
Geological and petrophysical models developed in the GOM for evaluation of
LRLC pay are applicable in other world basins. An 
Archie
clean sand or
Waxman-Smits shaly sand model are commonly used to evaluate LRLC anomalies.
Often, shaly sand models are not necessarily suited for LRLC evaluation. The
Archie
lithology exponent (m) and saturation exponent (n) for many LRLC
reservoirs range from 1.4 to 1.85, and 1.2 to 1.8, respectively. In thinly
laminated LRLC reservoirs, net sand distribution is identified with high
resolution logging tools, rock examination and interval testing.