Karst-Controlled Diagenesis and Reservoir Development: Example from the Ordovician Main-Reservoir Carbonate Rocks on the Eastern Margin of the Ordos Basin, China
Bibliographic record
Abstract
Abstract The Ordovician Majiagou Formation contains the main reservoir of the Ordos Central gas field in the Ordos basin. The producing zone at the top of member 5 consists of carbonate rocks modified during a long period of subaerial exposure and karstification from the Late Ordovician to the middle Carboniferous. On the eastern margin of the Ordos basin, the porosity of the exposed carbonate rocks of this unit ranges from 0.5 to 15.1%, and permeability ranges from <0.01 to 1224 md. In the subsurface of the basin, the porosity of the reservoir rocks ranges from 0.5 to >8%, and permeability ranges from <0.1 to 5 md. The main reservoir porosity is a dissolution-enhanced vuggy porosity, associated with dolomite. The carbonate rocks show great heterogeneity, reflecting the varying effects of karstification in creating and modifying porosity. Petrographic and geochemical analyses of various components in these carbonates provided evidence for depositional and diagenetic processes. The reservoir carbonates were deposited in shallow and restricted hypersaline environments and were later modified by karstification and burial diagenesis. Dolomitization appears to have resulted from mixing of marine and meteoric waters and probably occurred in both shallow and deep burial settings. Cementation by calcite also occurred in both shallow and deep environments, under different hydrodynamic conditions. Both depositional settings and diagenetic processes, such as leaching by meteoric water, paleokarstification, dolomitization, and cementation, controlled reservoir development. The outcrop and subsurface samples show similar petrographic features, porosity types, and geochemical characteristics, but the exposed section of the formation shows evidence of more alteration by meteoric water.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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".