Types of Carbonate Laminations and Characteristics of Vertical Heterogeneity Distribution in the Fengshen Area of the Dongying Depression, China

This study focuses on the carbonate-laminated mud-shale from the upper segment of the Sha-4 Formation in the Fengshen area of the Dongying Depression. Employing a comprehensive suite of analytical techniques, including X-ray whole-rock diffraction, pyrolysis experiments, core scanning, thin-section analysis, and well logging curve analysis, we conducted an in-depth investigation of mudstone samples from a depth range of 3573.00m to 3637.86m. The research revealed that the primary mineral compositions of the mud-shale in this region include carbonate minerals (such as calcite and dolomite), clay minerals, quartz, along with minor amounts of feldspar, anhydrite, siderite, and pyrite. Quartz, as the most common terrigenous detrital mineral, comprises between 4% and 35% of the samples, with an average content of 22.72%.

By integrating thin-section identification with core scanning results, three fundamental types of laminae were identified: felsic laminae, clay mineral laminae, and carbonate laminae. Further, three binary combinations of laminae were distinguished: "organic-rich + micritic calcitic," "organic-rich + aragonitic," and "organic-rich + felsic." These distinct laminae types reflect the diagenetic processes occurring under different depositional environments.

The study found that the distribution of carbonate laminae within the mud-shale exhibits pronounced heterogeneity, with individual layer thicknesses primarily ranging from 1mm to 10mm, a development frequency of 50 to 350 layers per meter, and occupying 7% to 100% of the total mud-shale thickness. Notably, the thickness distribution of carbonate laminae shows a power-law characteristic, conforming to fractal geometry principles, which can be described using the N-S (Number-Size) model. According to this model, the number of layers and cumulative thickness of carbonate laminae in different thickness intervals can be calculated, with estimated values closely matching the actual measurements, thereby validating the effectiveness of the N-S model.

Moreover, the study demonstrated a positive correlation between the fractal dimension value D of carbonate laminae and their heterogeneity; that is, carbonate laminae with stronger heterogeneity have higher fractal dimension values D, and vice versa. This characteristic makes the fractal dimension value D an effective indicator for determining shale lithofacies types and their depositional environments, aiding in the reconstruction of paleoenvironments and identifying potential hydrocarbon enrichment zones, providing crucial scientific support for the exploration and development of shale oil and gas resources.

In summary, this study not only deepens our understanding of the types of laminae and their vertical heterogeneity in the mud-shale of the upper segment of the Sha-4 Formation in the Fengshen area of the Dongying Depression but also establishes a distribution model for carbonate laminae based on fractal characteristics. This provides valuable reference materials for subsequent geological studies and assessments of hydrocarbon resources.