Petrology and hydrocarbon significance of the coaly source rocks from the Pinghu Formation in the Xihu Sag, East China Sea Shelf Basin

Abstract

The Xihu Sag in the East China Sea Shelf Basin contains abundant oil and gas reserves and is a focus for hydrocarbon exploration and development. Source rocks are mainly coals and coal-measures mudstones in the Paleogene Pinghu and Huagang formations. Samples from the Pinghu Formation in the Xihu Sag were collected for petrology, total organic carbon, and Rock-Eval analysis for the purpose of investigating macerals component and their contributions to hydrocarbon generation potential. The coaly source rocks from the Pinghu Formation are dominated by vitrinite (average 86.18%) but have an obviously elevated content of liptinite (average 12.59%) and a much lower amount of inertinite (average 1.23%). Liptinite of the samples is mainly composed of resinite, with a small amount of cutinite, sporinite and alginate in descending order. TOC values are 37.55%–65.58% (average 49.16%). Effective HI values are 167–281 mg HC/g TOC (average 223.5 mg HC/g TOC), suggesting the organic matter is type II kerogen. Relatively high HI values and macerals components suggest that the coaly source rocks can generate both oil and gas. Although the liptinite in the coaly source rocks has a content lower than vitrinite values, it makes a significant contribution to both total hydrocarbon and liquid hydrocarbon generation. The contributions of vitrinite, liptinite and inertinite to the total hydrocarbon generation approximately are 63.21%, 36.46% and 0.33%, respectively. The contributions of vitrinite and liptinite to the liquid hydrocarbon generation are approximately 40.95% and 59.05%, respectively. These results demonstrate that the coaly source rocks are dominated by vitrinite macerals with a relatively higher content of liptinite macerals, especially resinite, and these source rocks are more prone to both total hydrocarbon and liquid hydrocarbon generation. Paleogene coaly source rocks from other parts of the world should be considered for their oil-prone nature.

Hydrocarbon Generation Potential and Depositional Setting of Eocene Oil-Prone Coaly Source Rocks in the Xihu Sag, East China Sea Shelf Basin

“… 13 Liu suggested that effective HI values of the Pinghu coal were elevated about 0–99 mg·HC/g·TOC with an average of 33 mg·HC/g·TOC compared to the measured HI values. 27 As a result, the organic abundance in our analyzed samples could be an underestimate, and oil-prone source rocks should have occupied a larger proportion. Hydrocarbon expulsion will occur when EOM yields are >30 mg·HC/g·TOC.…”

“…A predominance of type II kerogen of the coaly source rocks in the Pinghu Formation has been reported by Liu et al based on petrology methods. 27 …”

“…A series of petroliferous structures have been discovered in recent years, including two oil and gas fields with reserves up to a hundred million tons of oil equivalent. 27 Four sets of source rocks have been found in the Xihu Sag, including the Baoshi, Pinghu, Huagang, and Longjing formations in ascending order ( Figure 2 ). 26 , 27 Coal and coal-bearing mudstones in the Pinghu Formation are widespread in the Xihu Sag and account for a significant portion of the four sets of source rocks.…”

“…43 Liu et al revealed a correlation between hydrocarbon generation potential and maceral composition. 27 Zhu et al illustrated the hydrocarbon generation potential characteristics of Paleogene source rocks in the Xihu Sag. 34 This study focuses on the origin and depositional environments of the organic matter to provide a better constraint on precursor plant associations of the Pinghu coaly source rocks in the Xihu sag.…”

Organic geochemistry and petrology of the coals in the Laisong Formation, Naga Schuppen Belt, NE India: Inferences on hydrocarbon potential and depositional environment

“…However, the Pinghu coal deposits contain significantly higher amount of liptinite (1.47-23.02%; Kang et al, 2020) than the Laisong coals. The vitrinite reflectance of the Pinghu coals (0.55-1.28%) is also much higher than the Laisong coal, owing to deeper depth of burial and steeper geothermal gradient in the Xihu Sag (Liu et al, 2020) (Kang et al, 2020). The coal measure of the Oligocene Yacheng Formation is the most important source rock sequence in the Qiongdongnan Basin in the South China Sea region that had entered a phase of mass gas generation, owing to its high degree of thermal evolution and subsidence (Gongcheng et al, 2019).…”