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摘要
The Middle Jurassic Walloon Subgroup is a prolific, low-rank coal seam gas (CSG) resource in the Surat Basin, Australia. This study applied a regionally consistent stratigraphic framework model to open-file gas desorption and coal property data from 69 cored CSG wells, to assess spatial trends in gas content and test the influence of large-scale geological heterogeneity on present-day gas distribution. Continued successful exploration and production, and possible future microbial regeneration of the Walloon CSG resource require an improved understanding of the controls on gas content distribution across the core region of production in the eastern Surat Basin.
The Walloon Subgroup can be subdivided into upper (Juandah) and lower (Taroom) coal measures separated by a relatively coal-barren and sandstone-dominated unit (Tangalooma Sandstone). Within these units, lateral variation in coal character is high, precluding a regionally agreed coal group or seam correlation. While the total dataset shows a general increase in gas content (dry-ash-free: d.a.f.) with increasing depth, as a function of hydrostatic pressure, there is considerable scatter in the distribution. Within individual wells, gas content follows one of three trends. Gas content either (1) increases; (2) increases, then decreases; or (3) decreases with depth. Trend 2 (parabolic) wells predominate, with the gas content maxima coinciding with the Tangalooma Sandstone, regardless of depth. As such, gas content layering follows, rather than cross-cuts the stratigraphy in the majority of wells. The change in gas content with depth between the different units and the wells was used to identify domains of different gas gradients, which could relate to differing gas saturations and geological setting.
The well profiles and gas gradients were analysed against available open-file coal quality data, without strong correlation to rank or type parameters. The majority of very high gas gradient values occur in coals within or above the Tangalooma Sandstone, suggesting that it forms a regional-scale barrier, both to possible gas migration from underlying and higher rank coals, and to meteoric and biogenic recharge from above. Increased ethane values in the upper Taroom and Tangalooma CSG support this interpretation. Alternatively, this distribution may be the result of alternating adsorption-desorption cycles linked to burial and uplift. The wells containing the highest mean gas gradients span a range of subtle rank and structural domains, but generally occur in structural highs under topographic lows with hydrostatic head. Wells from smaller-scale anticlines not associated with topographic surface lows mainly exhibit lower mean gas gradients. The study significantly expands the number of published gas content trends and proposes several factors influencing their formation and geographical distribution. The results form the basis for future targeted sampling to test for gas origins, migration, retention and biogenic recharge processes within the Surat Basin.
- 出版日期2012-11-1
