Occurrence Characteristics of Mobile Hydrocarbons in Lacustrine Shales: Insights from Solvent Extraction and Petrophysical Characterization

Understanding the occurrence mechanism of mobile hydrocarbons is important for "sweet spot" identification in liquid-rich shale reservoirs. This paper investigated the occurrence and distribution characteristics of mobile hydrocarbons in a set of lacustrine shales, collected from the Upper Triassic Yanchang Formation, in the Ordos Basin. A comprehensive suite of petrophysical (pore structure and pore size distribution) and geochemical (total organic carbon quantification and Rock-Eval pyrolysis) experiments were performed on shales before and after solvent extraction. The chemical components of the extracted hydrocarbons were analyzed using liquid chromatography fractionation. Scanning electron microscopy was utilized to verify the petrophysical analyses. Then, the selective occurrence characteristics of different mobile hydrocarbon compounds in multi-scaled pores of different minerals were discussed. Results show that the specific surface area, pore volume, and pore size distribution are significantly enhanced after solvent extraction. The mobile hydrocarbons in the studies shales are controlled by organic matter richness and feldspar. The free oil content, determined in programmed pyrolysis at 300 degree celsius, increases with the increasing total organic carbon and feldspar contents. These are related to the sorption storage and volumetric storage of shale oil, respectively. An extremely high clay content is unfavorable for hydrocarbon adsorption storage as a result of the decreasing pore volume and limited pore accessibility of clay minerals. Different mobile hydrocarbon compounds prefer to store in different pore sizes. The non/low-polar hydrocarbons (aliphatic and aromatic hydrocarbons) reside in micro- and mesopores (pore diameter of <50 nm), whereas the strong polar hydrocarbons (resins and asphaltenes) prefer to reside in macropores (pore diameter of >50 nm). Direct image analyses by electron microscopy scanning show that mobile hydrocarbons remarkably exist in the dissolution-related pores and interparticle pores. Scanning electron microscopy observations support that volumetric storage of hydrocarbon in dissolution-related pores of feldspar accounts for the "sweet spot" of the investigated shales.