Remagnetization of lower Silurian black shale and insights into shale gas in the Sichuan Basin, south China

Abstract

The organic‐rich lower Silurian shale of the Longmaxi Formation in the Sichuan Basin is the most important target for shale‐gas exploration in China. Most Paleozoic rocks of the Sichuan Basin have experienced extraordinarily pervasive remagnetizations. To test a hypothesized connection between hydrocarbon generation and remagnetization and contribute to shale‐gas exploration in the region, we undertook an integrated magnetic, geochemical, and petrographic study of 160 specimens from the shale. The results suggest that the shale contains a reliable remanent magnetization (Dec = 41.4°, Inc = 40.8°, and α95 = 6.8°). The magnetization predates tilting, and the paleopole plots close to the Late Triassic segment of the south China apparent polar wander path. The rock magnetic data and scanning electron microscope (SEM) observations confirm that framboidal magnetites carry the bulk of the magnetization, which suggest a Late Triassic chemical remanent magnetization in the shale. 87Sr/86Sr and magnetic analyses indicate that the amount of magnetite was unaffected by fluid alterations around the veins but is strongly covariant with the amount of total organic matter. Moreover, SEM observations reveal possible evidence of the replacement of pyrite framboids by magnetite, probably in the presence of organic acids. These analyses, therefore, suggest that the remagnetization was caused by organic maturation rather than orogenic fluids and that the maturation occurred in the Late Triassic. This timing of organic maturation has been validated by independent modeling studies and provides important constraints on the complex thermal history of the Longmaxi Shale as well as contributing to shale‐gas exploration efforts.

A Widespread Early Mesozoic Remagnetization in South China

“…(a) The map showing the temporal‐spatial variation of the Triassic remagnetization in South China. The dark green, red, and light green pentagram denotes the location where Early‐Middle Triassic (T 1‐2 , Jiao et al, ), Middle Triassic (T 2 , this study), and Late Triassic (T 3 ; Zhang et al, ) remagnetization was observed, respectively. The curved arrow shows the regional younging trend of the Triassic remagnetization in South China.…”

“…Unlike the widely recorded Late Mesozoic remagnetization in South China (e.g., Dobson & Heller, ; Evans et al, ; Gao et al, ; Huang et al, ; Huang & Opdyke, ; Jing et al, ; Jing et al, ; Kent et al, ; Zhang et al, ), a Triassic overprint was previously reported by only a few studies (Jiao et al, ; Zhang et al, ). Zhang et al () reported a Late Triassic (T 3 ) remagnetization from Sichuan, western South China, and Jiao et al () recently reported an Early‐Middle Triassic (T 1–2 ) remagnetization component from Zhejiang, eastern South China. These records, together with the Middle Triassic (T 2 ) remagnetization recognized in middle South China (this study), present a temporal‐spatial pattern of the Triassic remagnetization that appears to exhibit a younging trend of remagnetization from the east to the west of South China (Figure ).…”

“…This age constraint on the timing of folding, however, is too crude to definitively conclude that the positive fold tests of the HTC indicate a primary remanence. Indeed, pre‐Jurassic remagnetization has been increasingly recognized in South China recently (e.g., Jiao et al, ; Jing et al, ; S. Zhang et al, ; Y. Zhang et al, ; Y. Zhang et al, ). As such, we compare the paleomagnetic pole of the HTC with the apparent polar wander path of South China.…”

“…While we are aiming to obtain a detailed paleomagnetic record from the relatively more expanded Late Cambrian succession from the Wa'ergang section to test and examine the Late Cambrian TPW in this study, we are fully aware of the extensive Mesozoic remagnetization that has been previously reported in South China (e.g., Dobson & Heller, ; Gao et al, ; Huang & Opdyke, ; Jing et al, ; Kent et al, ; Liu et al, ; Wang & Van der Voo, ; Zhang et al, ; Zhang et al, ). Although various remagnetization mechanisms have also been proposed (e.g., Gao et al, ; Huang & Opdyke, ; Jing et al, ; C. Liu, Ge, et al, ; Liu et al, ; Zhang et al, ), most previous studies focused on local processes leading to remagnetization. Few studies investigated the temporal‐spatial features of the remagnetization and the underlying mechanisms.…”

The Chemical Remagnetization of Ediacaran Dolomite in the Taishan Paleo‐Reservoir, South China

“…To constrain the chemical processes associated with hydrocarbon migration into the carbonate reservoir rocks, an integrated paleomagnetic, rock magnetic, and petrographic study was conducted along a transect across the outcrop. This investigation also aimed to provide important insights into the relationship between pervasive remagnetization and hydrocarbon occurrences in south China (Wang & Van Der Voo, ; Zhai, ; Zhang et al, ). The results have the potential to improve yield from Ediacaran dolomite formation, which is the largest single carbonate gas reservoir in China.…”