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Abstract

AAPG Bulletin, V. 106, No. 8 (August 2022), P. 1557-1579.

Copyright ©2022. The American Association of Petroleum Geologists. All rights reserved.

DOI: 10.1306/02072220103

Astronomical forcing of meter-scale organic-rich mudstone–limestone cyclicity in the Eocene Dongying sag, China: Implications for shale reservoir exploration

Jianguo Zhang,1 Zaixing Jiang,2 Chao Liang,3 Timothy F. Baars,4 Youwei Wang,5 and Hemmo A. Abels6

1School of Energy Resources, China University of Geosciences, Beijing, China; [email protected], [email protected]
2School of Energy Resources, China University of Geosciences, Beijing, China; [email protected]
3School of Geosciences, China University of Petroleum (East China), Qingdao, China; State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Sinopec Group, Beijing, China; [email protected]; [email protected]
4Department of Geosciences and Engineering, Delft University of Technology, Delft, the Netherlands; [email protected]
5Department of Geosciences and Engineering, Delft University of Technology, Delft, the Netherlands; [email protected]
6Department of Geosciences and Engineering, Delft University of Technology, Delft, the Netherlands; [email protected]

Abstract

The middle Eocene Dongying sag in the Bohai Bay Basin of China has an estimated shale oil resource of approximately 1.1 billion t (8.06 billion bbl); flows of shale oil have been produced in the succession from tens of wells, where the daily production of a single well generally varies between 10 and 100 t (73.3–733 bbl). Therein, the mudrock successions composed of meter-scale mudstone–limestone couplets are the most important shale oil-producing layers. The controls on the deposition of the meter-scale mudstone–limestone couplets, however, remain enigmatic, constraining the analysis of lithofacies and, therefore, sweet spot distributions. Here, we analyze three continuously cored organic-rich successions of mudstone–limestone couplets (371 m [1217 ft] in total) in the middle Eocene Dongying sag, accompanied by decimeter- to meter-scale sampling and testing of mineralogy, organic geochemistry, and paleontology of the rocks. Our integrated cyclostratigraphic analysis shows that the observed mudstone–limestone couplets occur at periods that coincide with Milankovitch periodicities; 21-k.y. precession cycles are the main driver of the meter-scale mudstone–limestone couplets, with additional imprints of 41-k.y. obliquity cycles. Specifically, precession minima are associated with high summer insolation and consequently high summer monsoonal precipitation, which increased river discharge and terrigenous input to the basin, resulting in the deposition of siliciclastic-rich mudstones. In the study, low summer insolation during precession maxima led to decreased summer monsoonal precipitation, lower river discharge and terrigenous input, and increased lake water salinity, resulting in the deposition of authigenic lime mudstones. The shale reservoir quality kept pace with the orbital climate changes; compared with lime mudstones deposited during precession maxima, mudstones deposited during precession minima had higher total organic carbon, porosity, and oil content, but lower brittleness.

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