Numerically Reconstructed Methane-seep Signal in Soil Gases over Devonian Gas Pools and Prospects (Northeast British Columbia): Surface Microseeps and Postsurvey Discovery
Hans H. von der Dick, Dane A. Bosman, Kent R. Barrett, 2002. "Numerically Reconstructed Methane-seep Signal in Soil Gases over Devonian Gas Pools and Prospects (Northeast British Columbia): Surface Microseeps and Postsurvey Discovery", Surface Exploration Case Histories: Applications of Geoschemistry, Magnetics, and Remote Sensing, Dietmar Schumacher, Leonard A. LeSchack
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A soil-gas survey was conducted in the Kotcho-Kyklo area of northeast British Columbia, with the objective to test for, evaluate, and verify methane-seep patterns that could provide critical exploration information on deep (about 1990 m) Devonian reef complexes in the Slave Point and Keg River Formations.
Survey conditions were difficult from both a surface-topography and a subsurface point of view. First, the terrain is partial muskeg, with pervasive in-situ soil-gas generation and therefore the potential for masking or confusing surface-seep signals. Second, multiple gas-bearing horizons are present above the deep reefs, with the potential for confusion regarding seep-signal origin. Third, the region is dry-gas-bearing, with little or no compositional reservoir contrast.
High-quality soil-gas sampling and analysis, coupled with advanced vector analysis to reconstruct and explain soil-gas data in terms of source-gas families, allow us to recognize and numerically reconstruct a distinct group of surface methane-seep samples that are compositionally consistent with reservoir gases and that correspond to the deep reefs. As a result of this numerical evaluation effort, a tentative seismic anomaly was evaluated geochemically, resulting in a lower exploration drilling risk and a subsequent reef discovery.
Based on numerical analysis, three types of surface-methane samples can be discriminated. Soil-gas samples having very high methane content and essentially pure seep-methane portions are clearly related to Devonian gas pools. These very high methane contents, coupled with very high seep-methane portions in the samples, are apparently the cumulative effect of combined seepage from deeper and shallower horizons, facilitated and amplified along sediment drape features over Keg River reefs. A second group of methane samples is high in methane content, with medium to high seep-methane portions, a result of regional methane seepage from pervasive, shallower gas horizons and mixed with methane contributions from biological activity. This group is not related to the deep reef structures. A third group is relatively low in methane content, with low to marginal seep-methane portions.
A comparison of total methane content with seep-methane portions indicates that in fact, high methane concentrations are driven by seepage and are not biologically derived methane, which often is pervasive in wet and swampy environments. In addition, numerical evidence is provided indicating that ethane and higher alkanes present in the soil-gas samples are not related to seepage.
The migration pathway of gases to the surface is nearly vertical and appears to be amplified and focused along the sediment drape features over the reefs. Assuming that some of the fracture network is associated with the drape features, this would explain the exceptional seepage rates over the reefs and the tight clustering of reef-related seep samples at the surface over these reef structures.
The case study presented here stresses the argument for requiring the data processing of usually noisy soil hydrocarbon gas data to isolate a distinct seep signal and to classify information contained in these surface gas data.
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Surface Exploration Case Histories: Applications of Geochemistry, Magnetics, and Remote Sensing provides an overview of successful applications of surface exploration methods. Through a series of independent case histories, this volume presents clearly documented evidence that demonstrates how surface exploration methods can significantly reduce exploration risk and finding costs: geochemical, magnetic, and remote sensing. The 19 chapters in this volume reflect the broad scope of applications for these methods: frontier basin reconnaissance, prospect development, prospect evaluation, and field development and production. The case histories span the globe: 1. North America 2. Africa 3. South America 4. Europe 5. Middle East 6. Australia. This book will interest explorationists and managers who seek to get the most out of each exploration dollar.