Abstract:

The shale lithofacies in the second member of the Funing Formation (hereafter referred to as Funing Formation Member 2) in the Gaoyou Sag of Subei Basin exhibits significant heterogeneity, with complex lamination types that are challenging to quantify using well logging, thus limiting the identification of favorable “sweet spots” for shale oil. Therefore, this study investigates the methods for quantitative characterization of shale lamination types and their development in Funing Formation Member 2 of Gaoyou Sag, by integrating data from core thin sections, whole rock diffraction, elemental logging, and well logging, based on the climatic and environmental evolution during different sedimentary stages. The results show that shale lamination types mainly include quartz-enriched, clay-enriched, calcite, and dolomite bands. Influenced by ancient climatic evolution, the proportions of different lamination types vary across intervals, and the vertical superposition and coupling of these lamination types lead to differential shale oil enrichment, with more developed laminations corresponding to higher oil enrichment. During the deposition of intervals Ⅴ-6 to Ⅴ-10, the sediments exhibit a high aridity index, low Sr/Cu ratio, significant variation in the Sr/Ba ratio, and high V/(V+Ni) ratio. These characteristics suggest a strongly reducing, semi-arid to arid saline water environment with fluctuating water depths and periodic variation in lake nutrients. Saline stratification and diagenesis facilitate the development of abundant bright calcite layers, fibrous calcite layers, and dolomite layers, providing favorable reservoir properties for shale oil. During the deposition of intervals Ⅴ-1 to Ⅴ-5, the Sr/Cu ratio increases significantly while the aridity index decreases. The overall environmental characteristics indicate a strongly reducing, arid saline water environment. The shale is predominantly composed of clay-rich to sandy-mixed lithology, with clay-enriched layers and clay-rich laminations as the dominant lamination types. Due to the influence of recrystallization degree, the proportion of bright calcite layers decreases while the proportion of mudstone-like calcite layers increases. During the deposition of intervals Ⅳ-5 to Ⅳ-8, the Sr/Cu ratio exhibits a periodic variation of “decrease followed by increase”, indicating a decrease in lake water salinity. The lithology primarily consists of clay-rich to sandy-mixed shale, with the development of clay-enriched layers, clay-rich laminations, bright calcite layers, fibrous calcite layers, and dolomite layers. These intervals demonstrate excellent reservoir properties and are regarded as high-quality sweet spot layers for shale oil. During the deposition of intervals Ⅳ1-Ⅳ4, the Sr/Cu ratio increases, indicating intensified arid conditions. The climate characteristics suggest a strongly reducing, arid saline environment. The recrystallization degree of calcite is higher, leading to the development of bright calcite, fibrous calcite, and dolomite layers. Additionally, the proportion of mudstone-like calcite layers increases, indicating a higher overall carbonate mineral content influenced by the depositional environment. During the deposition of the subinterval Ⅲ, the climate alternates between humid and arid conditions, with a higher degree of calcite crystallization and the development of bright calcite layers. Subintervals Ⅱ and Ⅰ exhibit a significant decrease in Fe/Mn and Sr/Ba ratios, indicating intensified humid conditions. Water depth increases, and the shale gradually transitions to blocky structure. The content of gray and muddy minerals decreases, limiting the development of gray and muddy laminations. The study further confirms a positive correlation between the degree of shale lamination development and shale oil enrichment. Based on the geological characteristics of the shale lamination distribution, further analysis was conducted using methods such as edge detection from electrical imaging well logging and shale deposition rate calculation. The study identified intervals Ⅳ-3 to Ⅳ-7 and Ⅴ-6 to Ⅴ-8 in Funing Formation Member 2 as having well-developed laminations and higher total organic carbon (TOC) compared to other intervals, marking them as vertical shale oil sweet spot layers. The image edge detection method using electrical imaging well logging offers high accuracy for shale bedding identification and is suitable for detailed geological evaluation of vertical shale oil sweet spot layers in different blocks. Furthermore, as the climate change during shale deposition becomes more frequent and the sedimentation rate varies more drastically, the vertical heterogeneity and lamination development of shale increase. Thus, sedimentation rate variations can serve as an indicator of shale lamination development. An analysis of stratigraphic cycles in the Huazhuang area's Funing Formation Member 2 revealed that natural gamma MTM spectrum analysis of well Huaye 7 identified eight dominant frequencies, corresponding to cycle thicknesses of 39.84, 11.76, 9.43, 4.20, 3.19, 2.32, 2.13, 1.82 m. The ratio of cycle thicknesses is 21.91:6.47:5.19:2.13:1.76:1.28:1.17:1.00, which is close to the theoretical cycle ratio of 21.32:6.58:5.26:2.74:2.00:1.21:1.16:1.00 for this period. Therefore, the shale deposition process of the Funing Formation Member 2 is controlled by the Milankovitch astronomical cycle. The optimal sedimentation rate for this interval was determined to be 10.8 cm/kyr. Using this optimal rate, the eCOCO statistical method was applied to track and analyze sedimentation rate variations in the Funing Formation Member 2. The results indicate significant differences in sedimentation rates among different sub-layers of the Funing Formation Member 2 due to the influence of periodic climatic fluctuations. Moreover, the degree of lamination development indicated by the sedimentation rate variation correlates well with the overall proportion of lamination development obtained from thin section analysis, and is consistent with the lamination development detected by imaging logging in different intervals. Consequently, this method can predict the spatial distribution of lamination development, providing guidance for three-dimensional shale oil exploration. In summary, this study provides insight on the lithological heterogeneity and quantitative logging characterization of the Funing Formation Member 2 in the Gaoyou Sag, Subei Basin. These findings contribute to the identification and evaluation of shale oil sweet spot layers, promoting shale oil exploration and development.

Key words: Gaoyou Sag, Funing Formation Member 2, shale lamination types, climate evolution, electrical imaging logging, image edge detection, Milankovitch cycles