A systematic review and summary of the developmental history and advances in experimental techniques for carbonate reservoirs reveals that technological progress in experimentation serves as a fundamental driving force behind theoretical and technological innovations in the field. The research results are as follows. (1) The evolution from macro-scale reservoir characterization via visual inspection and magnifying lenses in the 1950s and 1960s to modern digital outcrop acquisition, 3D micro-to-nano pore imaging, and multi-scale reservoir heterogeneity characterization and modeling in the 21st century, underscores the pivotal role of theoretical and experimental technological advancements, particularly in carbonate petrology, sedimentology, sequence stratigraphy, and the deployment of sophisticated experimental equipment. (2) “Geochronology, thermometry, barometry, and fluid property tracing” represent the most cutting-edge geochemical testing techniques for carbonate reservoirs in the 21st century. The iterative upgrading from bulk-rock to in-situ micro-area analysis, from solution-based methods to laser ablation, and from point detection to area mapping defines the future trajectory of the field. Furthermore, digital transformation and intelligent development provide the impetus for these technical upgrades, which will yield more comprehensive and reliable data for investigating the genesis of diagenetic fabrics and diagenetic-porosity evolution. (3) The high-temperature and high-pressure (HTHP) visualization apparatus for reservoir formation simulation across tectonic stages in ultra-deep layers is the core equipment for investigating the genetic mechanisms of ultra-deep reservoirs. It can accurately replicate ultra-HTHP geological environments, enabling dynamic forward modeling of water-rock interaction processes across tectonic stages, and integrates in-situ real-time monitoring of fluid properties and rock permeability. This apparatus provides direct experimental evidence for unraveling the formation mechanisms, preservation conditions, and spatial distribution patterns of secondary porosity in ultra-deep carbonate reservoirs. The elucidation of hydrocarbon accumulation mechanisms in ultra-deep carbonate reservoirs via simulation experiments represents a defining future research trend. (4) Reservoir prediction is the ultimate objective of reservoir characterization, evaluation, genesis, and modeling. Given that geophysical techniques are the primary tools for reservoir identification and prediction, it is highly recommended to strengthen the organic integration of experimental carbonate reservoir techniques with geophysical methods.

This study systematically reviews and summarizes the history and advances of research on the geneses of dolomites and dolomite reservoirs, and outlines the prospects for future research orientations. The study of dolomites has had a history of over 200 years, while research on dolomite reservoirs has been kicked off since the 1950s. Previous studies established dolomitization models under varying geological conditions represented by seepage reflux and burial-compaction, and explained the geneses of dolomites with varying characteristics and occurrences in nature. Accordingly, the dolomite pores may mainly result from dolomitization as concluded. Over the past decade, progress has been made by incorporating modern sedimentary observations and simulation experiments, and four genetic types of dolomites have been identified, that is, the low-temperature organic precipitation type, the low- and high-temperature metasomatism types, and the high-temperature inorganic precipitation type. Based on this, the plot for identifying the geological and geochemical characteristics of dolomites is developed. It has been revealed that the genesis of dolomites cannot be simply equated with that of dolomite reservoirs. The porosity in dolomite reservoirs is mainly inherited from original porosity followed by modification, featuring more significant facies control and inheritance rather than modification. Although some dolomite pores are the products of burial dissolution, early-stage dolomitization is favorable for pore preservation. Two dolomitization pathways, i.e., dolomitization with protolith texture preserved and burial-based limestone dolomitization, have been identified, with the former exhibiting the optimal reservoir-forming effects. The advancements in experimental techniques have deepened the understanding of the geneses of dolomites, while simulation experiments provide deep insights into the geneses of dolomite reservoirs. In response to hydrocarbon exploration and production, future research should intensify the fine characterization and modeling of dolomite reservoirs, as well as logging-based reservoir identification and seismics-based reservoir prediction using the geological models of reservoirs.

Trace elements and rare earth elements (REEs) are of great value in reconstructing diagenetic environments and tracing the diagenetic fluid evolution. Presently, the solution method is recognized as a well-established, commonly used analytical method, but due to its high demand for powdered samples, it is only applicable to mixed samples of multiple diagenetic microfabrics with relatively low accuracy. With the support of the State Energy Key Laboratory for Carbonate Oil and Gas, we conduct the research and development of new techniques for analyzing trace elements and REEs in carbonate minerals, achieving three significant outcomes. First, upgrading the technique for testing trace elements and REEs in carbonate minerals with solution method. The required quantity of powdered samples gets reduced from 50 mg to 10 mg, meeting the sampling and testing requirements of microfabrics. Moreover, less time is needed for sample analysis and tests, so does the acid consumption, falling from 10 mL to 2 mL, resulting in a lower relative error of analysis and tests of 2 % ~ 5 % from 5 % ~ 10 %. Second, developing a new continuous laser mapping technique for trace elements and REEs using a laser ablation system and a triple quadrupole inductively coupled plasma mass spectrometer (ICP-MS) platform. With this technique, the lower limit of detection content is reduced from 1-10 ppb to a sub-ppb level, while the spatial resolution of images rises from ≥ 5 μm to ≤ 1 μm, and the time efficiency for image scanning and processing is increased by 10 times. Third, besides its applications in reconstructing the diagenetic environments of microfabrics and tracing the diagenetic fluid evolution, this new technique developed can improve the success rate and accuracy of laser-ablation U-Pb isotopic absolute dating.

Dolomites, distributed extensively across the world, are known to contain abundant hydrocarbon resources. However, questions regarding their formation mechanisms and reservoir space preservation are yet to be clarified. We delve into recent significant advances in the study of dolomites’ genetic mechanisms and factors governing their reservoir space, along with emerging experimental techniques. The findings indicate that low-temperature ordered dolomites can be successfully synthesized through dissolution-reprecipitation reactions and that dolomite formation is constrained by both thermodynamic and kinetic barriers. Various dolomitization models have been established, expanded, and improved, including the newly constructed organic matter- or microbially induced dolomitization model, the enhanced inorganically catalyzed dolomitization model, and the modified mixed-water-zone dolomitization model. Furthermore, the model for dolomitization by evaporative pumping and seepage reflux of brines has been expanded, and the burial dolomitization model has been refined. The findings highlight the significant controlling effects of the original sedimentary environment and late-stage diagenetic transformations on the evolution of dolomite reservoirs. Moderate dolomitization and secondary dissolution increase the porosity of dolomite reservoirs. In contrast, excess cementation, precipitation of substantial hydrothermal minerals, and mild to moderate recrystallization lead to a decrease in reservoir porosity. Novel experimental techniques have been developed, including element microanalysis, Mg-Ca isotopic tracing, in-situ U-Pb isotopic dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP/MS), clumped isotope thermometry, confocal laser scanning microscopy (CLSM) combined with CT scanning imaging, and nuclear magnetic resonance (NMR) combined with spectral induced polarization (SIP). Additionally, techniques for the forward modeling of dolomite reservoirs and the multiscale quantitative characterization of the 3D structures and fluid mobility of dolomite pores have also been developed. These emerging techniques provide significant technical support for research on the genetic mechanisms and reservoir space of dolomites.

The deeply buried Lower Cambrian Longwangmiao Formation and Upper Ediacaran Dengying Formation from the Sichuan Basin, China, have a total natural gas reserve up to 3 × 10¹² m³. The complex diagenetic evolution and their impacts on the present-day reservoir quality have not been systematically elucidated, hampering the current exploration. Crucially, the integration and comparation diagenetic study on these two formations, which may be able to shed new lights on reservoir formation mechanism, are yet to be systemically evaluated. By compiling geochemistry data, including carbonate U-Pb ages and petrophysics data, coupled with new petrology, trace elements, and strontium isotope data, of various types of diagenetic carbonates, this study aims to decipher the potential links between diagenesis and reservoir development of both formations. Intriguingly, similar diagenetic sequence, which contains five distinctive dolomite phases, is established in both formations. The matrix dolomite (D1) and early dolomite cement (D2) were likely formed by reflux dolomitization, as inferred by their nearly syn-depositional U-Pb ages and elevated δ¹⁸O caused by seawater evaporation. The subsequent moderate burial dolomite cement (D3) was most plausibly the product of burial compaction as indicated by its lighter δ¹⁸O and slightly younger U-Pb ages compared with D1 and D2. Whereas deep burial dolomite cements (D4 and D5) yield markedly depleted δ¹⁸O, elevated ⁸⁷Sr/⁸⁶Sr, along with much younger U-Pb ages and higher precipitation temperatures, suggesting that they were likely linked to hydrothermal fluids. Despite the wide occurrence of meteoric and organic acids leaching and thermochemical sulfate reduction, they may have only played a subsidiary role on these reservoirs development. Instead, superior reservoir quality is tightly linked to tectonics as inferred by higher reservoir quality closely related to the well-developed fractures and faults filled with abundant hydrothermal minerals. Notably, good reservoirs in both formations are mainly attributed to high permeability caused by tectonics. Hence, this new contribution emphasizes the crucial role of tectonics on spatially explicit reservoir prediction of deep to ultra-deep (up to > 8000 m) carbonates in the Sichuan Basin, as well as other sedimentary basin analogues in China.