Evaluation of Shaly Sands With Low Deliverability
Authors: Cannon Darrel  E. 
Published: March 2010

Historically, wells encountering only shaly sands with low deliverabilities have been abandoned as dry holes, and exploration for such reservoirs has been considered uneconomical. With an increasing demand for hydrocarbons, however, shaly reservoirs such as the Second White Specks, Milk River, Medicine Hat and Belly River of southern Alberta and Saskatchewan, formerly considered uneconomical plays, have now become sought-after prospects.

Evaluation of these very shaly reservoirs has proved to be neither simple not straightforward. Systems normally used for exploration and evaluation (logs, cores, drill-stem tests) were designed for clean sands with high deliverabilities. All these system have been hampered by the shale content of the sands.

Recently developed methods of log analysis can define and adjust for shaliness effects. After adjustment for shaliness, the logs are capable of supplying such information on the reservoir thickness, hydrocarbon content, hydrocarbon type and identification of producible zones. Available logging programs and their interpretation, including an adjustment for shaliness, are discussed in this paper. The manner of interpreting logs, in the above shaly formations is developed in the discussion of the field examples, and the results of log interpretation are compared to the results of Core analysis and production tests.

Introduction

THE Second White Specks, Medicine Hat, Milk River and Belly River sand formations have several common characteristics. They extend from southwestern Saskatchewan through the Alberta plains as shalysand reservoirs with proven productivity. Generally their deliverability is low - less than 1 MMcf/D or 50 BOPD. Evaluation of these shaly sands is complicated by large variations in shale content, variable lithologies and low permeabilities. Current logging equipment and recently developed log interpretation techniques are capable of providing rapid, reliable evaluation of these zones.

INTERPRETATION OF SHALY FORMATIONS

The readings of many logs are affected by shaliness. The shalier a sand, the greater the effect. The formations named in the preceding paragraph can be productive with up to 50 per cent (by volume) of shale. Before any meaningful evaluation of these reservoirs is possible, a shaliness adjustment must be made.

Shale, as seen by logs, consists of two components, silt and clay.(1) Silt is a very fine-grained lime, quartz, dolomite or other material that has infinite resistivity and may be radioactive. Clay is a hydrated mineral that is usually radioactive and has a low resistivity due to its ion-exchange properties. Both of these shale constituents affect log readings. Assuming the source rock to be sandstone, shaliness will cause the effects listed in the following table.

(Table in full paper)

In addition, the sonic and neutron logs are affected in sand-shale sequences by gas, which causes the sonic to read high and the neutron to read low. Lack of formation compaction also causes the sonic to read high. Log readings must be adjusted for shaliness effect, presence of gas, lithology and compaction before they can be used for accurate analysis.

In the following examples, it is shown that an interpretation based on a single porosity log or a gamma-ray neutron log is not adequate for shaly formations.