Discussion on the Contribution of Graptolite to Organic Enrichment and Reservoir of Gas Shale: A Case Study of the Wufeng—Longmaxi Formations in South China

Abstract

The graptolitic shales of the Wufeng and Longmaxi Formations were widely deposited across the Ordovician and Silurian transition in South China, which are the target of shale gas exploration and development in China. Based on the statistics of graptolite abundance from nearly 1000 shale samples and total organic content (TOC) analyses, different scales of scanning electron microscopy analyses of related samples, which are from the Wufeng formation and the bottom part of the Longmaxi formation in South and North margins of Yangtze plate, the contribution of graptolite to organic enrichment and reservoir of shale gas was discussed. The TOC content of the Wufeng-Longmaxi Formation graptolitic shales (including graptolite and non-graptolite, i.e., the surrounding host) is mainly controlled by that of its surrounding host, but less affected by the graptolite abundance, indicating that the graptolite has a relatively less impact on the organic enrichment. Graptolite consists of a large number of organic matter, with reticular biological tissue structure, accounting for 20% ~ 50% graptolitic area; it also developed honeycomb-shaped pores with pore sizes of 110nm-1.7um (an average of about 500nm), which are higher than the organic pores in the surrounding host (108nm-770nm, average 330nm), showing that graptolite has an important contribution to shale gas storage space. Because of the large number of graptolites occurred in shales from the Wufeng formation and the bottom part of the Longmaxi formation, the laminated and stacked local pattern of them on the laminae’s surfaces is not only provides abundant storage space for shale gas, but also serves as the predominant channel for shale gas flow. Therefore, the greatly abundant graptolite should be considered as one of the important factors controlling enrichiment and high productivity of shale gas in the Wufeng-Longmaxi Formations.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018