The influences of sedimentary environments on organic matter enrichment in fine-grained rocks of the Paleogene Shahejie formation in Nanpu Sag, Huanghua Depression, Bohai Bay Basin

Min Li,Zhi Xu,Guoyong Liu,Di Chen,Bo Pang,Hong Pang,Xiongqi Pang,Fangxin Guo,Lingjian Meng

doi:10.1177/01445987211031034

Abstract

The fine-grained rocks in the Paleogene Shahejie Formation in Nanpu Sag, Huanghua Depression, Bohai Bay Basin, are extremely important source rocks. These Paleogene rocks are mainly subdivided into organic-rich black shale and gray mudstone. The average total organic carbon contents of the shale and mudstone are 11.5 wt.% and 8.4 wt.%, respectively. The average hydrocarbon (HC)-generating potentials (which is equal to the sum of free hydrocarbons (S1) and potential hydrocarbons (S2)) of the shale and mudstone are 39.3 mg HC/g rock and 28.5 mg HC/g rock, respectively, with mean vitrinite reflectance values of 0.82% and 0.81%, respectively. The higher abundance of organic matter in the shale than in the mudstone is due mainly to paleoenvironmental differences. The chemical index of alteration values and Na/Al ratios reveal a warm and humid climate during shale deposition and a cold and dry climate during mudstone deposition. The biologically derived Ba and Ba/Al ratios indicate high productivity in both the shale and mudstone, with relatively low productivity in the shale. The shale formed in fresh to brackish water, whereas the mudstone was deposited in fresh water, with the former having a higher salinity. Compared with the shale, the mudstone underwent higher detrital input, exhibiting higher Si/Al and Ti/Al ratios. Shale deposition was more dysoxic than mudstone deposition. The organic matter enrichment of the shale sediments was controlled mainly by reducing conditions followed by moderate-to-high productivity, which was promoted by a warm and humid climate and salinity stratification. The organic matter enrichment of the mudstone was less than that of the shale and was controlled by relatively oxic conditions.

Highlights

With the replacement of conventional oil and gas resources, shale oil and gas have become the focus of world oil and gas exploration (Hu et al, 2021)
The analysis of the sporopollenin and algae fossils of the shale and mudstone samples shows that these samples are characterized by high contents of angiosperm pollen, followed by gymnosperm pollen and fern spores (Figure 3)
The proportion of angiosperm pollen is higher in the shale samples than in the mudstone samples, while gymnosperm pollen shows the opposite trend

Summary

Introduction

With the replacement of conventional oil and gas resources, shale oil and gas have become the focus of world oil and gas exploration (Hu et al, 2021). OM enrichment is largely controlled by the depositional environments and benefits from high primary production and strong preservation conditions (Demaison and Moore, 1980; Doner et al, 2019; Pedersen and Calvert, 1991; Quan et al, 2019). Previous studies have focused on the controlling effects of environmental changes on OM enrichment at different scales. The differences in OM enrichment among different types of fine sedimentary rocks, such as shale and mudstone, have rarely been studied. Influenced by high-frequency environmental vibration, lacustrine fine-grained sediments display strong heterogeneity in rock type and OM enrichment degree (Hu et al, 2018; Liang et al, 2018; Quan et al, 2017). How the sedimentary environments control the enrichment of OM in different rock types needs further study

Methods

Results

Discussion

Conclusion