Pore-scale oil distribution in shales of the Qingshankou formation in the Changling Sag, Songliao Basin, NE China

Abstract A total of 19 representative lacustrine shale (core) samples from the Upper Cretaceous Qingshankou Formation in the Changling Sag, Songliao Basin, northeast China have been characterized to determine the types of shale oil reservoir lithofacies and investigate pore structures and pore-scale oil distribution using an integrated petrographic, geochemical and petrophysical approach. Based on the sedimentary structures and mineral compositions, four lithofacies types have been recognized: foliaceous argillaceous mudstone (FAM), laminated diamictic mudstone (LDM), laminated felsic mudstone (LFM) and interbedded felsic mudstone (IFM). Sequential extraction experiments via a Soxhlet extractor were performed using different solvents to recover oils of varying polarities: weak-polar oil (WPO) was extracted with n-hexane under room temperature, while strong-polar oil (SPO) was extracted with a mix of dichloromethane and methanol (9:1 v/v) extraction. Low-pressure N2 sorption data acquired from the original and solvent extracted samples indicates that the pore volumes occupied by WPO (VWPO) decrease progressively in LFM (averaging 68.32 × 10−4 cm3/g), LDM (averaging 50.41 × 10−4 cm3/g), FAM (averaging 48.86 × 10−4 cm3/g) and IFM (averaging 39.39 × 10−4 cm3/g); while pore volumes occupied by SPO (VSPO) increase progressively in IFM (averaging 8.00 × 10−4 cm3/g), LFM (averaging 12.62 × 10−4 cm3/g), LDM (averaging 38.85 × 10−4 cm3/g) and FAM (averaging 65.08 × 10−4 cm3/g). Major factors controlling pore-scale oil distribution in the shale reservoirs include pore structures, TOC and clay mineral contents. Pore structures can affect the distribution of WPO and SPO to a great extent. WPO is mainly distributed in the mesopores, while SPO is present in all pores in the four lithofacies. SPO is mainly distributed in mesopores in FAM and LDM, and in macropores in LFM and IFM. TOC and clay mineral contents have strong impacts on the distribution of SPO. Carbonate minerals in LDM and IFM are dominated by ostracoda shells and authigenic calcite cement, respectively. The carbonate mineral content shows a positive correlation with WPO in LDM, in which the carbonate minerals are dominated by ostracoda shells with well-developed pores due to organic acid dissolution. TOC shows a positive correlation with WPO for TOC less than 1.2 wt%, while shows a negative correlation for TOC greater than 1.2 wt%. In the study area, LDM and LFM are identified as favorable lithofacies or “sweet spots” for shale oil exploration because they have relatively high TOC (0.7–2 wt%), high percentages of in situ WPO and a high mineral brittleness index.