ABSTRACT: Recognizing High-Porosity/Low-Permeability Sandstone Reservoirs with Routine Logs: Case History

Hughbert Collier

For Mid-Continent-type sandstones, good porosity (15-20%) does not always signify good permeability (tens to hundreds of millidarcys). Characterization of the potential producibility of many reservoirs is based primarily on wireline logs because cores are seldom available. Unfortunately, log responses indicating low permeability can be subtle and easily overlooked. Failure to recognize low-permeability intervals results in errors in log interpretation, which cause (1) overestimation of net effective pay, (2) overly optimistic reserve calculations, and (3) errors in exploitation strategy.

A case study of Pennsylvanian deltaic sandstones in Concho County, Texas, shows the reservoir sandstones to have good porosity, but also to have numerous thin shaly zones of low permeability. Integration of log and core data reveals and/or confirms subtle indications of low-permeability intervals on the open-hole logs (dual induction, gamma ray, sp, density, and neutron). Permeability/porosity crossplots show that the quality of the reservoirs is determined by the depositional environment of the sand.

Integration of the log and core data necessitated a significant revision of the reservoir model. In some wells, 80-95% of the productive capacity is contained in 40-60% of the net pay. Vertical permeability barriers were discovered in the reservoir-quality sand (K_vertical of 1.5-2 md vs. K_horizontal of 74-196 md).

Careful analysis of open-hole logs, especially when some core data is

available from offset wells, can identify low-permeability intervals and greatly improve reservoir characterization.

AAPG Search and Discovery Article #90096©1990 AAPG Southwest Section Meeting, Wichita Falls, Texas, March 11-13, 1990