Evaluation of Terrestrial Gas Shales: Case Study from the Ordos Basin, North China

Zhao, Jingzhou; Bai, Yubin; Cao, Qing; Er, Chuang; Shen, Wuxian

In this paper, we attempt to demonstrate that the terrestrial-deposited black shales from the Carboniferous-Permian and Triassic Yanchang Formation in the Ordos Basin are favorable for shale gas exploration and exploitation. The favorable factors include (1) Extensive distribution of black shales. The Carboniferous-Permian black shales are nearly basin-wide and their thickness ranges from 30 m to 300 m, while those of the Triassic Yanchang Formation are mostly distributed in the Chang-7 play and also in the Chang 9 play, of which the Chang-7 black shales are up to 30m to 160m and the area of the thickness greater than 30m is about 25,000 square kilometers or more. (2) High content and various types of organic matter. Deposited in fluvial-deltaic coal-bearing sedimentary settings, the mean TOC of the Upper Paleozoic black shales ranges from 2% to 3% and the kerogens are dominated by humic type. In contrast, the Chang-7 and Chang-9 black shales were deposited in the lacustrine environment with organic matter being principally humic-sapropelic type and their mean TOC ranges from 2% to 5%. (3) Shallow to medium burial depth and medium to high thermal maturity. The Upper Paleozoic shales are buried at the depth of 2300 m to 3800 m and their maturity is usually from 1.0 to 2.8 % Ro. The Triassic black shales are mainly buried at the depth of 600 m to 2000 m and their thermal maturity is moderate, with Chang-6 to Chang-9 plays at 0.65% to 1.1 % Ro. (4) Highly heterogeneous reservoirs and multivariate "sweet spots". Both the Upper Paleozoic and Triassic black shales exhibit strong reservoir heterogeneity, but various sweet spots are developed. (5) Excellent preserving condition. Thanks to the stable tectonic settings and excellent cap rocks, the preserving condition for shale gas enrichment in the Ordos Basin is extraordinary in comparison with other regions in China, particularly with that for south China shales. (6) Abundant resources of natural gas and co-existence of shale gas with tight gas and coalbed methane. These unconventional gases are co-existed so closely in the Carboniferous-Permian sequence that they can be explored and exploited simultaneously. Nevertheless, despite that the black shales from the Upper Paleozoic and Triassic Yanchang Formation are comparable in many aspects, the Triassic ones are mostly at the "oil window" and therefore are less favorable for gas generation.

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