Logging identification and characteristic analysis of the lacustrine organic-rich shale lithofacies: A case study from the Es3L shale in the Jiyang Depression, Bohai Bay Basin, Eastern China

Abstract

The Es3L (lower sub-member of the third member of the Eocene Shahejie Formation) shale of Jiyang depression was a set of relatively thick and widely deposited lacustrine sediments with high organic richness and medium thermal maturation (0.7%≤Ro≤1.1%), which rapidly emerges an important and target shale-oil play in Eastern China. An important component for shale-oil reservoir evaluation is lithofacies that affect hydraulic fracture stimulation and constrain significant organic-matter and oil concentration. Based on core and thin section observation, X-ray diffraction and comparative analysis of well logging data, we can define the types of shale lithofacies in the Es3L and determine log response characteristics of each lithofacies to show effective methods for identifying or predicting the lithofacies from conventional well logs. Organic richness, porosity and permeability measurements and the analysis of scanning electron microscope (SEM) images were used to study the impact of lithofacies on total organic carbon (TOC), reservoirs properties and oil content. The Es3L shale is rich in carbonate minerals (most >50%). According to the mineral composition, sedimentary structure and genesis, the lithofacies can be divided into 5 types and 2 sub-categories: marl (including massive and bedded), calcareous shale (including massive and bedded), laminated bindstone, massive silty mudstone and argillaceous shale. By the analysis of log response characteristics of lithofacies from 17 wells, 8 sensitive logging curves (e.g., density (DEN), acoustic times (AC) and resistance (Rt)) were optimized to construct logging recognition models and conduct cross plots to qualitatively identify lithofacies. On this basis, using the Fisher Discrimination Analysis and Naïve Bayes Classification Function, 5 types of shale lithofacies were quantitatively identified and predicted. Combined with the identification of shale beddings from Formation MicroScanner Imaging (FMI), the 7 sub-categories of shale lithofacies can be further recognized. The statistics of lithofacies indicate that bedded marl and calcareous shale are the dominant lithofacies in the Es3L. TOC content has the wide variation between different lithofacies (0.06–12%). The argillaceous shale has the high TOC content, followed by calcareous shale and bindstone. TOC is positively correlated to clay and pyrite content, and negatively correlated to quartz. The measurements of reservoir properties indicate that bindstone and calcareous shale have high porosity (>7.0%) and good permeability. A general positive relationship between porosity and TOC, quartz or carbonate minerals indicates that organic matter (OM), recrystallized intercrystal and interparticle pores in BSE images can improve the porosity. Additionally, the oil accumulation index (OAI) of bindstone, calcareous shale and argillaceous shale is high. Considered reservoir properties and fracability, the laminated bindstone and bedded calcareous shale in the medium-low section of Es3L are the most advantageous lithofacies for shale-oil exploration and exploitation in this area.