Geochemistry of Tri- and Tetracyclic Terpanes in the Palaeozoic Oils from the Tarim Basin, Northwest China
Bibliographic record
Abstract
Tri- and tetracyclic terpanes in oils and gas condensates from the cratonic region of the Tarim Basin have been characterized by gas chromatography–mass spectrometry to investigate their geochemical applications in organic source input and maturity assessment. Most oils show typical marine source input characterized by the dominance of C 23 tricyclic terpane (TT), low C 19 TT, and low C 24 tetracyclic terpane (TeT), while some oils/condensates show typical terrigenous input signatures as indicated by the dominance of C 19 TT and C 24 TeT, which is obviously in conflict with early Palaeozoic geological settings. Commonly used organic source input parameters, such as C 19 TT/C 20 TT, C 24 TeT/C 26 TT, and C 20 TT/C 23 TT, show complicated correlation to maturity parameters. C 19 TT/C 20 TT ratios are positively correlated to n -C 18 /phytane (Ph), while C 24 TeT/C 26 TT and C 20 TT/C 23 TT ratios increase with n -C 18 /Ph in a narrow maturity range and then decrease at an extremely high maturity level. Unusually high C 19 TT/C 20 TT, C 20 TT/C 23 TT, and C 24 TeT/C 26 TT ratios occur only in samples with very low summed tricyclic terpane concentrations, suggesting extensive thermal cracking. An abnormal heating influence may cause an elevated C 20 TT/C 23 TT ratio when summed tricyclic terpane concentrations remain high. Experimental results from gold tube pyrolysis of two heavily biodegraded oil sands illustrate that ratios of C 19 TT/C 20 TT, C 24 TeT/C 26 TT, and C 20 TT/C 23 TT increase with the heating temperature at a low temperature and then decrease at a high temperature. The longstanding controversial issue regarding the relative thermal stability of tricyclic terpane components might be solved because previous studies have only covered part of thermal evolution history. Nevertheless, the distribution patterns of tri- and tetracyclic terpanes are no longer necessarily valid as an oil–source correlation indicator. The so-called Cambrian-sourced end member or unknown source rocks in the Tarim Basin are most likely derived from the thermal alteration of normally distributed tri- and tetracyclic terpanes.
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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 | 0.002 | 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) | 0.000 | 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 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".