Reservoirs characteristics and environments evolution of lower permian transitional shale in the Southern North China Basin: Implications for shale gas exploration

Marine-continental transitional shale gas is an important resourceinaddition to shale gas in marine facies. Transitional facies strata are rich in type III organic matter and shales with obvious lamellation, but there has been no "breakthrough" for shale gas. The main reason for this may be that the sedimentary environment and characteristics of favorable shale reservoirs differ from those of marine shale. It is imperativeto understand the environment and reservoir characteristics to advance the exploration of transitional shale gas.Typical lower Permian transitional shale has been collected continuously in an exploratory well in the Southern North China Basin. A complete set of experimental procedures, including elemental geochemical methods (XRF and ICP-MS) and petrophysical techniques (N2/CO2 adsorption, MIP, NMR and FE-SEM), were used. The results indicate that, in addition to the contribution of mesopores (2–50 nm), transitional shale in the SNCB has a higher macropore (>50 nm) volume (average = 0.00616 cm3/g) and proportion (average45.85%) than in marine shale. The precipitation and dissolution of dispersed pyrite in the Shanxi Formation formed more macropores (average = 0.00649 cm3/g) than pyrite and calcite aggregates (filled with bitumen and clays) in the Taiyuan Formation. Total organic carbon (TOC) has no obvious relationship with micropores and mesopores, but has a weak positive correlation with macropores, unlike marine shale. Sulfur enrichment in the presence of organic matter mixed with terrigenous iron causes dispersed pyrite (FeS) precipitation under anoxic conditions, which improves the reservoir (intergranular and dissolution pores). With decreasing sea levels, the salinity of water decreases gradually, while weathering increases gradually; however, the shale reservoir conditions do not simply improve or deteriorate. Favorable shale reservoirs are mainly lagoonal facies characterized by a low SiO2content (<58 wt%), high TOC (>2.0%), intermediateV/Cr ratios (1.5–2), and a high chemical index of alternation (CIA; >88) and formed in an environment with little clastic input, hypoxia and strong weathering.However, due to the strong connectivity, high proportions of macropores and thin single-layers, it is necessary to study the preservation capacity in future to determine whether the natural gas formed in shale is lost.