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.
Highlights
Coals can be one of the most important source rocks for oil and gas (Ayinla et al, 2017; Petersen et al, 2010; Snowdon, 1991)
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
Despite numerous viewpoints proposed by different researchers, there are still many unanswered questions about the hydrocarbon generation potential of coal macerals as coals with high hydrogen-rich maceral contents are rarely seen in geological history
Summary
Coals can be one of the most important source rocks for oil and gas (Ayinla et al, 2017; Petersen et al, 2010; Snowdon, 1991). Previous studies for the sources rocks in the Xihu Sag are mainly concentrated on their depositional environments (Yi et al, 2018; Zhou et al, 2016), accumulation and distribution (Tian et al, 2019; Yu, 2019), and geochemistry (Zhu et al, 2012), and only a few studies are dealing with the coal petrology and the relationships between the maceral types and the hydrocarbon generation. A coal petrology study is especially crucial for recognition of the oil-prone properties of the different macerals in the coals, and the assessment of the hydrocarbon resources in the Xihu Sag. This study will provide a detailed assessment of the petrology and hydrocarbon generation of the coaly source rocks in the Pinghu Formation from the Xihu Sag, and quantitatively determine the contributions to oil and gas of major maceral groups. Samples were crushed into about 1 mm fragments, and the sample was divided into two parts, one for coal petrology analysis and another for organic geochemistry analysis
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