A Study on the Relationship Between Graptolites and Shale Gas in the Wufeng and Longmaxi Formations in the Sichuan Basin

doi:10.19657/j.geoscience.1000-8527.2019.02.13

Abstract

Abstract:

Previous research on graptolites mainly focused on stratigraphic division and sedimentary environment analysis, but rarely on the relationship between graptolite and shale gas. Using data from core well analysis, drilling and core logging and experimental analysis, the Ordovician Wufeng Formation in the Sichuan Basin is divided into four graptolites zones (WF1-WF4), whilst the Silurian Longmaxi Formation is divided into nine graptolites zones (LM1-LM9).In the WF2-LM4 zones,TOC contents are high and organic pores are well developed, and thus these zones are favorable for shale gas exploitation. We found that the graptolite evolution and organic matter enrichment have the same depositional environment, and the abundance of graptolites is positively correlated with TOC, organic porosity, and total hydrocarbon content measured by gas. The evolution has a certain contribution to the organic matter content and organic pore formation in the shale, and somehow affects its gas-bearing properties, which is an important factor in shale gas reservoir evaluation. The shale gas formation in the Wufeng and Longmaxi Formations in the Sichuan Basin was likely related to the formation and evolution of graptolites, which has major implications for shale gas reservoir studies.

Key words: graptolites, organic matter, shale gas, wufeng and Longmaxi formations, Sichuan Basin

CLC Number:

TE122

WANG Ziyi, GAO Zhiqian, SHI Wenrui, WANG Xingzhi, ZHAO Hongyan. A Study on the Relationship Between Graptolites and Shale Gas in the Wufeng and Longmaxi Formations in the Sichuan Basin[J]. Geoscience, 2019, 33(02): 379-388.

Figures/Tables 11

Fig.1 Location map of the Sichuan Basin

Fig.2 Core photos of graptolite types from the research area

Fig.3 Division of the graptolites zones of the Wufeng (WF) and Longmaxi (LM) Formations in the Yangtze region

Fig.4 Comparison of graptolitic shale in Wells JYA-W2-B-N2-C of the Wufeng and Longmaxi formations

Fig.5 Core photos of graptolites and pyrite from Well JYDA-E

Fig.6 Division of graptolite zones in Well JYDA-E of the Wufeng-Longmaxi formations

Fig.7 Core lithology integrated analysis histogram of Well JYDA-E

Fig.8 Diagram showing the variation trends of TOC content and number of graptolites in Well JYDA-E

Fig.9 Diagram showing the variation trends of organic porosity and number of graptolites in Well JYDA-E

Fig.10 Micrograph of graptolites from Well JYDA-E

Fig.11 Relation between the total hydrocarbon content and number of graptolites in Well JYDA-E

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