Oil Mobility of Low-Mature Lacustrine Shale and Controlling Factors: A Case Study from the Upper Cretaceous Nenjiang Formation, Gulong Sag, in the Songliao Basin

Abstract

Producing low-mature shale oil resource is becoming economically available as in situ organic matter or heavy oil conversion technologies are introduced to shale oil production. However, we still lack suitable evaluation parameters and criteria for characterizing oil content and mobility in low-mature shales, which are very important for low-mature shale oil economic production. With the aim to better understand the influence factors for low-mature shale oil content and mobility, 16 shale core samples were collected from low-mature Upper Cretaceous Nenjiang Formation (K2n), Gulong Sag, in the Songliao Basin, to investigate their mineral composition and organic geochemical and pore characteristics. The results show the following: (1) Four shale lithofacies were identified, such as mudstone, carbonate lamina shale, bioclastic shale, and mixed lamina shale. (2) Shale oil mobility [represented by the oil saturation index (OSI)] was mainly controlled by large-sized pore distribution. (3) Total organic carbon (TOC) content and lithofacies had great influence on the distribution of oil mobility in the shale. As for mudstone and bioclastic shale, TOC played the most important role in controlling shale oil mobility, whereas for carbonate lamina shale and mixed lamina shale, TOC and mineral variation both have impacts on oil mobility. (4) The average contents of TOC, S1, and OSI in K2n were higher than those in the deeper Qingshankou shale. The oil mobility controlling factors in K2n and K2qn were both TOC and preservation conditions. (5) A new index was proposed to weigh the content of shale oil, in situ OSI, and multiple isothermal pyrolysis parameter S2-2. The in situ OSI shows that K2n has high oil retention capacity, and the in situ oil mobility is 2–3 times the movable oil content.