A study of reservoir rocks in the northwestern Sichuan Basin points to concentrated seawater from the rocks’ own geological period as the predominant fluid involved in forming the dolostones. It also proposes that their pore space passed through alternating phases of dissolution, cementation, compaction and renewed pore formation. The evidence is interpretive rather than a direct sample of the ancient fluid.
The work focuses on the T2l3 interval and classifies its dolostones into four petrographic groups: D1 mud-microcrystalline, D2 algal, D3 grain-dominated and D4 crystalline. The groups provide the framework for comparing the study’s temperature estimates, dolomite ordering and pore features.
The seawater signal
To build the picture, the authors combined petrographic and cathodoluminescence observations with carbon, oxygen and strontium isotope measurements, major and trace element data, and rare-earth-element analysis. The dataset contained 27 representative samples from multiple wells and outcrops: 23 dolostones and four limestones. More than 150 petrographic thin sections were examined.
Carbon and strontium isotope values for the four dolostone types generally fell within ranges reported for seawater from the same period. A salinity index was about 130, and the rare-earth patterns broadly resembled both micritic limestone and normal seawater. The profiles showed relative light rare-earth enrichment, weak negative cerium anomalies and weak positive europium anomalies, with stronger anomalies in D4. Taken together, the authors inferred that concentrated seawater from the same geological period was the predominant fluid involved in forming these dolostones.
A gradient from shallow to deep
The proposed temperature pattern ran in the same direction. D1 was assigned an estimated formation temperature of about 33 to 41 degrees Celsius and an evaporative, near-surface oxidizing setting. D2 and D3 were placed at about 43 to 56 degrees Celsius in mainly reflux, shallow-burial settings, while D4 was placed at about 61 to 76 degrees Celsius under intermediate to deep burial conditions. The authors treat these as estimates based on oxygen-isotope assumptions, not direct readings of the old fluids.
An ordering measure in the dolomite also increased across the groups. Mean values were 0.497 for D1, 0.573 for D2, 0.578 for D3 and 0.752 for D4. D2 and D3 were therefore similar at an intermediate level, while D4 had the highest ordering. The trend is descriptive, since no formal statistical test or confidence interval was reported.
Elemental chemistry pointed in the same direction. D2 and D3 had similarly low mean iron and manganese contents, while D4 had higher levels: 0.006% iron and 0.0033% manganese, compared with 0.0024% and 0.0025% in D2 and 0.0026% and 0.0026% in D3. The authors interpreted that contrast as evidence of more thorough diagenesis with increasing burial depth, although the comparison was based on group means and petrographic observations.
Pore space was rebuilt
The pore system was divided into three broad categories: primary or fabric-related pores, secondary dissolution pores and fractures. D1 was described as dense, with almost no dissolution pores and only occasional microfractures. That contrast fits the authors’ wider model, in which the amount and type of open space changed through different diagenetic stages.
In that model, the initial pore system is linked to early dissolution. Shallow-burial cementation and compaction are linked to a denser reservoir, while intermediate to deep burial dissolution is linked to secondary-pore regeneration. Late dissolution plus hydrocarbon charging are associated with the final high-quality reservoirs. The sequence is a proposed reconstruction from petrography, geochemistry and relationships between minerals, not a directly timed record of every event.
A local interpretation
The study’s reach is limited. Its 27 representative samples came from selected wells and outcrops, and the fluid sources, temperatures and burial stages were inferred from isotope, elemental, rare-earth, cathodoluminescence and ordering proxies. The paper therefore does not establish that the same four-part sequence predicts reservoir quality outside the sampled T2l3 area, nor does it prove that seawater from the same period was the only fluid source.
The supplied metadata classifies the work as a peer-reviewed research article and reports funding from the National Science and Technology Major Project under identifier 2025ZD1400403.
Paper data and sources
Original title: Genesis of Dolomite and Diagenetic-Pore Evolution Model of High-Quality Reservoirs in the Third Member of the Middle Triassic Leikoupo Formation, Northwestern Sichuan Basin
Authors: Wu X, Huo F, Huang H et al.
Journal/Repository: ACS omega
Status: Peer-reviewed
First online: 2026-10-09
DOI: Not available
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