Abstract: Geochemical Effects of Cap-Rock Leakage Above Gas Reservoirs: Distribution of Light Hydrocarbons to Monitor Diffusive Losses
J. Konstanty, L. Schwark, D. Leythaeuser
An aspect of greater interest to hydrocarbon exploration in certain areas concerns the sealing efficiency of cap-rocks above gas reservoirs. In order to search for evidence of diffusive gas losses through cap-rocks, the distribution of light hydrocarbon (C1-C6) was studied in a gas-bearing sequence of NW-Germany.
From an exploration well in Northwest-Germany 340 fresh cutting samples covering a depth range from 2750 m down to 3986 m were sealed in metal cans and immediately analyzed by Headspace-Gas-Chromatography (HS-GC). Some method development had to be carried out to ensure satisfying gas recovery rates from cutting samples and reproducible compound separation and identification.
Two main intervals have been studied by closely spaced samples within the Permian Zechstein and the Triassic Bunter. The Zechstein sequence included the carbonate gas reservoir horizon (Ca2) and cap rock lithologies on anhydrites and halite of 300 m thickness. The Bunter interval mainly consisted of gas-bearing sandstones overlayed by thick homogeneous claystones of several meters thickness that could be examined for evidence of diffusive gas losses.
The analytical requirement was to fully separate the LHC from C1 to C6 in order to allow calculation of selected compound ratios which are likely to indicate redistribution of light hydrocarbon by diffusion, e.g. compound of high diffusivity in relation to compounds of low diffusivity. This approach does not require the determination of absolute amounts a gaseous hydrocarbons which causes problems with respect to accuracy and reproducibility. Furthermore, TC-, TOC-, TIC- and sulfur-contents were measured to verify whether indigenous organic matter contents lead to in situ generation of gaseous hydrocarbons in those cap-rock sequences.
The Zechstein gases are methane-dominated with C1/C3-ratios lower than 3 in the reservoir and no significant change in the bottom part of the cap-rock but show an significant increase of C1/C3-ratios in excess of 5 in the upper part of the cap-rock. Similar trends can be observed on the basis of C1/C2- or C1/(C2-C5)-ratios. These observations suggest that methane migrates by diffusion into the cap-rock to a much larger degree than the wet gases. Lithological control on light hydrocarbon distribution can however, not be ruled out as an alternative explanation.
A different scenario was observed in the Bunter interval, which does not bear any gas accumulations. Within the Bunter sequence drastic variation in permeabilities of sandstone layers versus homogeneous clay/siltstone intervals occur. When considering individual claystone intervals, preferential losses of methane from the shales into the overlying and the basal sandstones were recognized. Depletion towards the underlying and overlying strata points towards a non-buoyancy driven transport mechanism.
AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France