Geological Conditions for Upper Paleozoic Shale Gas Enrichment in the Ordos Basin, China

Zhao, Jingzhou ¹; Cao, Qing ¹; Fan, Yuanfang ¹; Bai, Yubin ¹; Wang, Xiaomei ¹
(1)School of Petroleum Resources, Xi'an Shiyou University, Xi'an, China.

The Ordos Basin is one of the largest basins in terms of natural gas resources. It is demonstrated that dark shales rich in organic matters are highly developed in Carboniferous, Permian and Triassic horizons. Among them the Carboniferous-Permian shales are most excellent and are even better in shale gas potential than many marine Paleozoic shales in south China. Those conditions include:

(1) Extensive distribution of dark shales. The Carboniferous-Permian shales, occurred in the Benxi, Taiyuan and Shanxi formations, are nearly basin-wide. Their thickness ranges from 30m to 300m, with thickest deposition occurred in the western part of the basin and thinnest in the east.

(2)High content of organic matter and appropriate thermal maturity. Deposited in marine to continental coal-bearing settings, the shales are rich in organic matter and their TOC ranges from 2.25% to 3.33%, bitumen“A”from 0.037% to 0.12%, and HC from 163.76 to 361.6ppm. Their kerogens are dominated by type III and thus are favorable for gas generation. Thermal maturity is mostly in the range of 1.0 to 2.8% Ro, with most part of the basin reaching thermal maturity greater than 1. 3 % Ro.

(3)Unique preserving condition. Owing to three factors, preserving condition for shale gas enrichment is extraordinary and unparallel compared with other regions in China, particularly with that for south China shales.

(4) Medium to shallow burial depth. The upper Paleozoic shales are buried shallow in the east and modest in the west part of the basin. With respect to the buried depth on the bottom of Shanxi Formation, it is less than 2800m in the east part, with a minimum depth of about 2300 m in the eastern margin.

(5) Abundant resources of natural gas and co-existence of shale gas with tight gas and coalbed methane. The Ordos Basin is confirmed being extremely abundant in natural gas and most of the gas resources are unconventional, of which tight gas, coalbed methane as well as possibly shale gas are all of great potential. These unconventional gases are co-existed and interbedded so closely in the Carboniferous-Permian strata that they can be explored and exploited simultaneously.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.