Abstract: Hydrothermal dolomites in Paleozoic rocks of eastern Canada: a story from identification to recent hydrocarbon production record
Bibliographic record
Abstract
In North America, world-class conventional hydrocarbon discoveries in the last decade were made in hydrothermal dolomites of Ordovician (Trenton-Black River play in the Appalachian basin in eastern USA), Devonian (Ladyfern and Tay River discoveries in the Western Canadian sedimentary basin) and, speculatively to Mesozoic (Deep Panuke in the Atlantic margin of Nova Scotia) age rock units. Controversies exist on the exact defi nition and criteria for recognition of hydrothermally altered carbonates in the rock record. By defi nition, a hydrothermal fl uid has temperatures over that of the ambient burial fl uid. A difference of at least 15°C is required to designate such fl uid as hydrothermal. Therefore, it is important to note that even if late burial allows the rock unit to be exposed to temperatures higher than those recorded by an early hydrothermal event, it is the difference of temperature at that specifi c event which is critical for the hydrothermal alteration process. Any later high temperature, sedimentary to tectonic burial events will only serve to mask and render diffi cult the recognition of hydrothermal alteration. Whatever the age of the host rock, effi cient (e.g., porosity/ permeability generating) hydrothermal alteration of a carbonate precursor will follow a rather unique suite of tectono-diagenetic events that will be recorded in an almost ubiquitous mineral paragenetic succession. High temperature and saline fl uids will move upwards along early extensional to transtensional faults and fl ow laterally into a porous precursor carbonate host. It can be demonstrated for most hydrothermal reservoirs that alteration occurred within the fi rst 500–1000 m of burial. The initial faulting will enhance permeability through brecciation. However, an economic reservoir can only be formed if fl uids are confi ned to specifi c intervals and do not breach any upper seal layer that preserve the integrity of the newly formed reservoir. The high temperature fl uids will dissolve a signifi cant volume of limestones and with increased carbonate ion saturation, eventually precipitate the peculiar high temperature saddle dolomite cement, the latter also occuring as a matrix-replacive phase. Following porosity generation and saddle dolomite precipitation, some late calcite and sulphate cements are commonly observed in most hydrocarbon fi elds. Diverse geoscience tools are used for the recognition of hydrothermal alteration: regional tectonic scenario, petrography and geochemistry of the diagenetic phases and the seismic expression of brecciation known as platform sag. In 1995, the discovery of a hydrothermal dolomite reservoir in Lower Ordovician rocks in the Port au Port #1 well was a milestone for hydrocarbon exploration in the Appalachians of eastern Canada. Since that discovery, hydrothermal dolomites have been documented in Lower and Middle Ordovician, in Lower and Upper Silurian and in Lower Devonian carbonate rock units in eastern Canada. Small hydrothermal dolomite hydrocarbon fi elds have been put into production (both oil and gas fi elds in Lower Devonian Upper Gaspe limestones in eastern Gaspe peninsula) and major Canadian exploration companies have started to extensively explore the Paleozoic belts in eastern Canada looking for hydrothermal dolomite reservoirs.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.003 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.004 | 0.006 |
| Science and technology studies | 0.002 | 0.002 |
| Scholarly communication | 0.003 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.003 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".