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.