Influence of HEM Drilling Fluid on Organic Geochemical Characteristics of Deep-Water Source Rocks in the Qiongdongnan Basin

Ziming Zhang; Dujie Hou; Xiong Cheng; Xiaze Yan; Weihe Chen

doi:10.1021/acsomega.4c02311

Influence of HEM Drilling Fluid on Organic Geochemical Characteristics of Deep-Water Source Rocks in the Qiongdongnan Basin

ACS Omega. 2024 Apr 24;9(18):20582-20592. doi: 10.1021/acsomega.4c02311. eCollection 2024 May 7.

Authors

Ziming Zhang^{1

2

3}, Dujie Hou^{1

2

3}, Xiong Cheng^{1

2

3}, Xiaze Yan^{1

2

3}, Weihe Chen^{1

2

3}

Affiliations

¹ School of Energy Resources, China University of Geosciences, Beijing 100083, China.
² Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Accumulation Mechanism, Ministry of Education, Beijing 100083, China.
³ Beijing Key Laboratory of Unconventional Natural Gas Geological Evaluation and Development Engineering, Beijing 100083, China.

Abstract

The influence of oil-based drilling fluid on the geochemical characteristics of source rocks has been widely reported in the northern South China Sea. However, contamination from water-based HEM drilling fluid has long been neglected in previous deep-water petroleum exploration. To further understand the impacts of HEM drilling fluid on deep-water source rocks, the organic matter abundance and type, kerogen maceral composition, and saturated biomarkers of the deep-water source rocks in the Qiongdongnan Basin were investigated. The influence of HEM drilling fluid on the organic geochemistry data of cuttings is significant but minor for sidecores. It is evident that the organic drilling additives in HEM drilling fluid can increase the organic matter abundance and optimize the organic matter types of shale cuttings. Specifically, the total organic carbon, S₁, S₂, and hydrogen index are increased by 28.5 ± 6.1, 90.8 ± 2.0, 34.2 ± 2.0, and 51.9 ± 4.0%, respectively. Furthermore, the organic drilling additives will greatly enhance the levels of C₂₉ regular steranes, especially for C₂₉-ααα-20R sterane, with the influence still persisting even when conducting GC-MS-MS analysis. The highly abundant 17α (H) -22, 29, 30-trisnorhopane, 17β (H)-22, 29, 30-trisnorhopane, C_29-3117α (H), 21β (H) hopanes, and C_29-31 17β (H), 21α (H) hopanes may directly originate from the organic drilling additives. It is dangerous to directly use organic geochemical data from deep-water source rock cuttings contaminated by HEM drilling fluid as it may lead to conclusions that are entirely inconsistent with the basin's geological background.