Lacustrine Shale Characterization and Shale Resource Potential in Ordos Basin, China

Abstract

The Ordos Basin is the second largest petroliferous basin in China. Petroleum resources of the basin are mainly accumulated in fluvio-lacustrine sandstone reservoirs of the Upper Triassic Yanchang Formation (Fm.). Organic-matter (OM)-rich intervals in the Chang 7, 8, and 9 members of the Yanchang Formation deposited during the Carnian and Norian stages (237.0-208.5 Ma) are the sources of oil for the sandstone reservoirs. Given the richness of the OM-rich intervals, thick Chang 7 shale is dominated by dark or gray mudstones, and these fine-grained rocks have the potential to be a shale resource target. However the deliverability of shale gas and oil from a lacustrine shale system containing high clay-mineral content remains contentious.

Geological controls of Chang 7 shale oil and gas resource potential have been investigated. The depositional setting and the relationships between rock attributes and wireline logs, mineral matrix source input, and sedimentary features in the Yanchang Formation have been studied by chemostratigraphic record. Four flooding surfaces were defined, each of which represents basin deepening and serves as a possible correlation event. Each event is marked by the superposition of OM-rich mudstones over more proximal arkosic siltstones and sandstones. Chemostratigraphy and TOC results indicate a significant linkage between TOC and %Al (a proxy for clay mineral content), and a positive relationship between enhanced TOC and %S (a proxy for pyrite content).

Compaction and organic maturation are the main controls on porosity evolution in the lacustrine Yanchang Formation. Variation in texture, as expressed by silt content, and OM-content affect the pathways of porosity modification during compaction and maturation. Silt content is associated with the preservation of intergranular mineral-hosted, larger pores (> 20 nm). OM content, which is higher in relatively silt-poor samples, is associated with the loss of large pores and the generation of very small OM-hosted pores. Organic matter type varies in different depositional facies, and organic matter type is dominated by oil-prone Type I and II kerogens in deep lake facies, a mixture of Type II and III kerogens in prodelta facies, and Type III kerogen in delta front. The lower part of the Chang 7 member at oil window, a 25-m-thick organic-rich interval, contains a large amount of free oil and gas. It is the target for shale gas/oil exploration in terms of potential resources.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018