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Abstract: Low Resistivity, Low Contrast Pays: Part I--Concepts and Methodology for Identification and Evaluation

Robert M. Sneider, John T. Kulha

Major hydrocarbon accumulations have been found and produced in low resistivity, low contrast (LRLC) sands in the Gulf of Mexico Basin (GOM). In the past in the GOM, LRLC reservoirs were commonly considered wet, tight, misidentified as a shale, or overlooked. Examples of many offshore GOM producing wells are documented now in a joint publication of the Houston and New Orleans geological societies. These examples provide models for identification and evaluation of wells with similar geologic-petrophysical occurrence in the world, including southeast Asia.

LRLC pays in the GOM are caused by one or more of the following: (1) thin beds or laminae of clean sands alternating with shales, silts, or shaly sands, (2) clay-coated sands, (3) glauconite-rich sands, (4) sands with interstitial dispersed clay, (5) sands with disseminated pyrite or other conductive minerals, (6) clay-lined burrows, (7) clay clasts in clean sand, (8) altered volcanic/feldspathic framework grains, and (9) very fine-grained sand with very saline water. Similar types of LRLC pay and potential pay sands are being recognized in Indonesia, Malaysia, Australia, Philippines, and Korea. Common depositional systems containing LRLC production in the GOM are (1) deep-water fans, including levee-channel complexes, (2) delta front and toe deposits, (3) shingle turbidites, and (4) lluvial and deltaic channel fills. Similar depositional systems are found in southeast Asia.

GOM geological-petrophysical models prove very useful in determining the appropriate petrophysical models to identify and evaluate pay in similar lithologic sequences in southeast Asia. Evaluation models used to calculate water saturation (SW) depend on the reason for the low resistivity. We find that modified Archie or Waxman-Smits equations are the common ones used to identify anomalies to recommend for production testing. The Archie lithology exponents (m) and saturation exponent (n) used in these equations depend upon the rock type (s) causing low resistivity. In the GOM, Archie m and n values range from 1.4 to 1.85, and 1.2 to 1.8, respectively. In thin bed or laminated LRLC reservoirs, net sand distribution is identified with higher vertical resolution logging tools.< P>

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994