Characteristics and origin of desorption gas of the Permian Shanxi Formation shale in the Ordos Basin, China

Abstract

The Lower Permian Shanxi Formation marine-continental transitional organic-rich shale is one of the most important potential shale gas plays in the Ordos Basin, China. However, the content and origin of desorbed gas from the Shanxi Formation are poorly documented, limiting the understanding of gas generation and potential play elements. Geochemical characteristics of desorbed gas, including content and origin, are analyzed from 17 core samples of the Shanxi Formation from well SL-1. The results show that the Shanxi Formation shales in the study area are characterized by high total organic carbon content of 1.17-2.63%, type III organic matter, and high T max between 493 and 513 C. The desorbed gas content of the shale samples varies from 0.22 to 0.50 m 3 /t, with an average of 0.37 m 3 /t, and shows a positive correlation with total organic carbon. The gases are dominated by methane (69.57-89.02%), with small amounts of ethane (0.01-0.09%). The carbon isotopic signature d 13 C 1 ranges from À49.5 to À45.3%, and the d 13 C 2 ranges from À23.3 to À14.7%. In addition, gases released from the Shanxi Formation core samples are thermogenic in origin and possibly coal derived, as the Whiticar chart and the diagram of ethane versus d 13 C 2 suggest.

Intensifying aeolian activity following the end‐Permian mass extinction: Evidence from the Late Permian–Early Triassic terrestrial sedimentary record of the Ordos Basin, North China

“…The logged sections are located in the northeastern part of the Ordos Basin, the largest cratonic basin on the North China Craton which covers an area of 400 000 km 2 and is bordered to the north, east, south and west by the Yin, L€ uliang, Qinling, Liupan and Helan mountains, respectively (Yang et al, 2005;Sun et al, 2017). The Ordos Basin was developed on rigid Archean granulites and early Proterozoic greenschists (Yang et al, 2005), and is a broadly rectangular structure with a largely undeformed interior bounded by linear fold and fault belts around the margins, which switched from being divergent in the early Palaeozoic to convergent in the late Palaeozoic and Mesozoic (Yang et al, 2005).…”

“…Fission track analysis indicates that uplift and deformation of the L€ uliang Mountains mainly occurred after the late Early Cretaceous (Zhao et al, 2016) and would therefore not have exerted any control on Permo-Triassic sedimentation. Around the PTB, the Ordos Basin was part of a stable cratonic foreland that received sediment from the Yinshan-Yanshan tectonic belt to the north and passed southward into shallow marine environments along the eastern seaboard of the Pangea supercontinent (Sun et al, 2017). 200 m thick composed of laterally and vertically variable cross-bedded medium-coarse-grained sandstone.…”

Geochemistry and shale gas potential of the lower Permian marine-continental transitional shales in the Eastern Ordos Basin

“…As the shale samples in Table 1. Geochemical characteristics for the shale samples of Shilou and Xixian area in Ordos Basin (Sun et al, 2017;Xu et al, 2017 Ordos Basin are in a highly mature evolution stage, carbon isotope fractionation occurs during the diagenesis and thermal evolution. Therefore, the organic carbon isotopes of the shale samples are heavier than those in the sedimentary matter during the deposition.…”

Natural gas migration and accumulation model and favorable exploration targets in Ordovician dolomite in Jingxi, Ordos Basin

“…According to the analysis of the contact relationships between source rocks and reservoirs in Jingxi, the source rocks that are in contact with the Majiagou Formation reservoirs comprise Permo-Carboniferous coal-bearing source rocks and Ordovician carbonate rocks (Cheng et al., 2007; He et al., 2005; Sun et al., 2017; Yang et al., 2011). Because of undeveloped faults in the Majiagou Formation, the natural gas here cannot migrate over a long distance vertically.…”