Unravelling the formation and implications of illitic grain coats for reservoir quality in the Permian tight sandstones, northern Ordos Basin, central China
Bibliographic record
Abstract
ABSTRACT The presence of clay grain coats retards syntaxial quartz overgrowths, leading to well‐preserved porosity in deeply buried sandstones. However, the genetic link between clay coat formation and epitaxial quartz outgrowths remains poorly evaluated. This study investigates the formation and reaction pathways of clay grain coats in Permian tight sandstones from the northern Ordos Basin, China, and evaluate their influence on epitaxial quartz cementation and reservoir quality. Petrographic and mineralogical analyses reveal that the clay coats are dominated by illite or mixed‐layer illite/smectite clay minerals, consisting of inner smectitic cutans and outer illitic rims. Smectitic cutans, enriched in Fe, Ti and K, form tangentially to detrital grains through mechanical clay infiltration and syn‐depositional volcanic matrix input. Illitic rims, higher in K and depleted in Fe concentrations, result from a two‐stage illitization process: (1) initial and progressive smectite illitization at elevated temperatures, releasing silica that promotes quartz precipitation and (2) advanced smectite illitization induced by K‐feldspar dissolution during organic acids influx, enhancing illitic coats growth and quartz cementation. The identification of clay rim‐associated and serrated authigenic quartz supports the close genetic link between illitic grain coats formation and epitaxial quartz cementations. Fluid inclusion microthermometry reveals two generations of quartz cements at 60 to 100°C and 100 to 140°C corresponding to the two‐stage illitization process. Lower δ 13 C PDB values (−16.05 to −9.35‰) of calcite cements suggest carbon sourcing from organic acids' thermal decarboxylation. Geochemical modelling and mass balance calculations demonstrate that the silica released during illitization is largely re‐precipitated as localized quartz cements in a closed system. These findings reveal the dual role of illitic grain coats on reservoir quality evolution. While they limit porosity modification by inhibiting syntaxial quartz overgrowths, their growth decreases permeability by occluding pore throats. This study enhances the understanding of clay coat dynamics and their implications for reservoir quality in tight sandstone.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| 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) | 0.000 | 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 teacher head, 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".