Preliminary Research on the Potential of Terrestrial Shale Oil in China: A Case Study of Upper Triassic Shale in the Ordos Basin

Zou, Cai-neng; Wu, Songtao; Yang, Zhi; Zhu, Rukai; Tao, Shi-zhen; Bai, Bin; Zhai, Xiufen

In this paper, Shale oil is defined to be the mature oil that is stored in organic-rich shale stratum with nano-sized pore-throats, and it is a very promising exploration field in future. Based on the data of thin sections, Field Emission-SEM, Nano-CT, EDS, Mercury injection, N2 adsorption and production data, we carried out the preliminary research on the potential of shale oil in the Ordos Basin. This study is trying to determine whether terrestrial shales can be reservoirs and whether the commercial oil production is available in such shales.

The samples are black shale and dark grey mudstones developed in the semi-deep to deep lacustrine facies, with I-type and II1-type kerogen. The thickness of shale with TOC higher than 2% and Ro higher than 0.7% is 21-36m, and the exploration area can reach 20,000 km2. EDS data indicate that total percentage of brittle minerals is 45~59%, with quartz>20%, feldspar=10%~15%, calcite minerals=5~18%, pyrite+hematite=5~18%, which is similar to the mineral composition of gas shale in North America. With such high content of brittle minerals, Upper Triassic shale are prone to generate artificial fractures after hydro-fracturing. FE-SEM data indicate that intra-feldspar pores, inter-chlorite pores, inter-pyrites pores and parallel bedding fractures dominate the storage space. Organic matter (OM) pores are limited, and most of them are located between organic matters and surrounding matrix. Nano-CT data indicate that connected pores account for around 60% of whole space. The porosity of the samples is 0.6%~3.5%, and permeability is 0.00072~0.0023mD. The pore-throats in Upper Triassic shale are continuously distributed, ranging from less than 10nm to 15μm. The percentage of pore-throats with radius less than 1000 nm is 96.5%, among which pore-throats with radius less than 100 nm is more than 60%. Thus, nanometer-scale pore-throats dominate the reservoir spaces of Upper Triassic shale, which may be the fundamental reason for the special characteristics of unconventional tight reservoirs. The oil testing indicates that oil production can reach 20.4 tons per day for a well with 3 perforated intervals. Moreover, the API of crude oil is 35-58°, and the viscosity is 0.7-5 lcP.

This study may be helpful in knowing more about the potential of terrestrial shale/mudstone oil, and providing references for the unconventional petroleum exploration in the Ordos Basin.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013