Knutson, Craig Allen¹ 
(1) Pioneer Natural Resources, Irving, TX

ABSTRACT: Comparison of Gore and Microbial Surveys, Southern Tunisia Stratigraphic Play

A 325 km² area was surveyed to prioritize a well location for a Triassic (Tagi) stratigraphic play on the Hercynian unconformity at a depth of about 2000m. The source rock is Silurian and top seal is provided locally by 25m of tight Azizia shales and carbonates as well as 750m of Upper Triassic and Lower Jurassic regional anhydrites and salts. Both Gore and microbial geochemical techniques were used in conjunction with 6 control wells ranging from dry to productive. The Gore survey was able to discriminate between dry and hydrocarbon bearing in 6 of 6 control wells. The microbial was slightly less effective in “correctly” characterizing 5 of 7 wells, although re-interpretation of the control wells to be consistent with the microbial data is possible. The Jorf-1 well was drilled at a location interpreted to be geochemically anomalous by both techniques. The well was dry with no shows at the Triassic and Permian levels. The deeper Ordovician, a low rank objective, was not penetrated. Even though high microbial counts and high Gore similarities were observed in the area, reinterpretation of the data suggests that no anomaly actually exists. A statistical probability method is proposed for distinguishing between background and anomalies. In addition, the microbial data was strongly influenced by the presence of sand dunes which resulted in challenging interpretation. Finally, there was a disturbing lack of correlation between the microbial counts and the C2-C6 flux measured by Gore, suggesting that one or both of the methods was not reliable.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.