Pitfalls in Assessing Lacustrine Shale Versus Marine Shale Prospects: Lessons from the Frederick Brook Shale of New Brunswick
Most organic shales that are evaluated for oil and gas potential were deposited in a marine environment. Explorationists commonly incorporate a number of techniques and analyses to quickly evaluate or screen the quality of these shales.
However, these techniques are not always appropriate for the evaluation of lacustrine shales. This is due to the significant differences in the water and rock chemistries, and the organic deposition, between marine and lacustrine environments.
Newly deposited organic material can extract uranium ions from seawater that results in increased radioactivity in marine shales. High gamma ray counts often signal high total organic carbon (TOC) shales. Uranium concentrations in lake waters are quite variable and generally much lower than ocean waters. Thus in some cases, the highest TOC shales are marked by lower gamma ray signatures than associated non-prospective shales.
Lakes can be internally, or poorly, drained, which can lead to the precipitation of a wide range of minerals depending upon the lake chemistry. In some cases these minerals have a high density that can mask the TOC content and porosity of the rock.
RockEval analyses from lacustrine shales may also be misleading. Lacustrine shales are generally rich in Type I organic material, whereas marine shales are rich in Type II organic material. Type I organic carbon contains approximately one third more generative organic carbon than Type II. Thus marine shales have more non-generative organic carbon that is of little value. In mature lacustrine shale, this results in low TOC values that may condemn the play whereas, in reality, there have been significant hydrocarbons generated and significant secondary porosity created.
Examples from the Frederick Brook shale of southern New Brunswick will be used to illustrate these points.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013