Comments regarding hydrothermal dolomitization and porosity development in the paper "Formation mechanism of deep Cambrian dolomite reservoirs in the Tarim basin, northwestern China" by Zhu et al. (2015)
Abstract
Zhu et al. (2015) present a case study that is interpreted as showing porosity creation in the deep subsurface by hydrothermal dolomitization. Zhu et al. (2015) describe the strata as consisting of finely to coarsely crystalline, dark colored "matrix dolomite" containing dissolution pores and fractures partly filled by coarsely crystalline, light colored "filling dolomite." The matrix dolomite and are suggested to have formed from seawater in near-surface hypersaline environments (presumably in Cambrian time), whereas the filling dolomites formed from hydrothermal water that moved upward along faults and fractures as a result of magmatic activity during Permian time. Pores in the dolostones "are most likely … related to dissolution by meteoric water near the sequence boundaries" (Zhu et al., 2015, p. 240), but most of the present porosity "can be attributed primarily to hydrothermal dissolution" (Zhu et al., 2015, p. 240). We conclude, however, that (1) the data in this study do not support the interpretation of hydrothermal dolomitization and (2) compelling evidence is lacking for increase in total porosity by during deep burial.
Hydrothermal dolomitization has not been demonstrated because the data do not show that the temperatures of the dolomitizing waters exceeded the ambient formation temperature, as is required by the definition of this term (Machel and Lonnee, 2002). Our point is that, although hydrothermal conditions may well have occurred, this has not been shown to be the case. The fluid inclusion temperatures (80-160 °C) are not greater than the present bottom-hole temperature and are thus within the range than can be expected during the burial history. High and variable iron contents in dolomite and the presence of saddle morphology cannot be considered to be evidence of hydrothehermal activity.- Publication:
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Marine and Petroleum Geology
- Pub Date:
- 2016
- DOI:
- 10.1016/j.marpetgeo.2015.09.008
- Bibcode:
- 2016MarPG..76..480E