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The diversity and multiscale characteristics of pore types in carbonate rocks usually result in extremely complex permeability–porosity relationships. Clarifying the main controlling factors of permeability and their response mechanisms is essential for improving permeability prediction. In this study, a digital image analysis (DIA) framework was developed to reveal the variation trend of permeability with pore structure parameters, and a rock-typing method consisting of flow zone indicator (FZI) and discrete rock type clustering technology (DRT) was applied to the carbonate samples to establish the binary permeability–porosity model. Results showed that permeability was approximately positively correlated with pore equivalent diameter, dominant pore size, and γ and negatively correlated with perimeter over area and fractal dimension. The commonly used multivariate linear regression (MLR) method failed to describe the response relationship between permeability and pore structure parameters. However, different discrete rock types exhibited special permeability–porosity relationships. Analysis showed that different discrete rock types were mainly controlled by the product of shape factor and the square of tortuosity. Further, three permeability prediction schemes were designed using the BP neural network and random forest algorithm based on the pore structure parameters derived from the thin sections. Compared with the direct prediction models trained by the optimized BP neural network and random forest algorithm, the indirect permeability prediction method based on FZI prediction showed better generalization ability with the highest R 2 of 0.936, demonstrating that the combination of digital image analysis, rock typing, and random forest algorithm is a robust and reliable method to realize the permeability prediction of carbonate rocks based on thin-section images.

Paleosalinity is an important environmental feature but it is difficult to evaluate. In the present study, paleosalinity was assessed during the deposition of three sets of source rocks in the western Pearl River Mouth Basin, South China Sea, utilizing four different kinds of methods, i.e., saturated hydrocarbon biomarkers, strontium abundance, non-pollen microalgae assemblies, and carbon-sulfur relationships. Results show that the second member of the Eocene Wenchang Formation (E2w2) was deposited in a freshwater environment and the Oligocene Zhuhai Formation (E3z) was deposited in a shallow marine environment. The Oligocene Enping Formation (E3e), which was believed to be deposited in a freshwater environment, was actually deposited in a brackish water environment. Mechanisms of salinity increase during the non-marine E3e deposition were mainly deep hydrothermal fluid input through the south boundary fault and episodic marine transgressions, not evaporation. The effect of salinity on organic matter content and type was investigated. Results show that salinity has no significant influence on total organic carbon (TOC) and hydrogen index (HI) of the E2w2, which was caused by the balance between freshwater algae and euryhaline algae. TOC and HI decrease with increasing salinity for samples from the E3z, which is contrary to the conventional hypothesis that marine transgressions promote source rock deposition. The decrease of TOC with carbon/sulfur ratios for samples from the E3e actually reflect the influence of thermal maturity but thermal maturity only plays the second role in HI. The effect of salinity on HI during the E3e deposition can be divided into two stages. During the first stage, the increase of salinity was mainly caused by deep hydrothermal fluid input without an oxygen level increase. HI values, therefore, remained relatively stable. During the secondary stage, the increase of salinity was mainly caused by marine transgressions which increased the oxygen level and as a consequence, HI decreased sharply with increasing salinity. This study provides a long-term salinity evolution of the western Pearl River Mouth Basin and suggests that salinity is an important factor controlling source rock deposition. In addition, this study presents an example that goes against conventional wisdom that marine transgressions promote source rock deposition in a shallow marine environment. This study also suggested that marine transgressions had already begun at the end of the early Oligocene.

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