Shale Gas Characteristics in the Southeastern Part of the Ordos Basin, China: Implications for the Accumulation Condition and Potential of Continental Shale Gas
Tang, Xuan¹; Zhang, Jinchuan¹; Yu, Bingsong¹; Ding, Wenlong¹; Yang, Yiting¹; Wang, Long¹; Xiong, Jinyu²
¹China University of Geosciences, Beijing, China.
²Petroleum Exploration and Research institute, CNOOC Shenzhen branch, GuangZhou, China.
Marine shale gas has recently gained significant success in USA, and has triggered a world-wide fever on shale gas resources. In contrast there is little work done and less attention on the continental shale gas. Continental shales are widespread in northern China. They are characterized by low thermal maturity and high clay content, completely different from that of marine shale documented in USA and elsewhere. There is an urgent need to characterize continental shales and evaluate their gas potential as a viable resource.
The Mesozoic strata of the Chang-7 Member of the Yanchang Formation in the southeastern part of the Ordos Basin comprise typical continental shale. Recently some vertical wells were drilled and produced up to 3000 m³ per day by fracturing. Shale samples collected from the bottom of the Chang-7 Member were characterized in details. The results show that the Chang-7 shale and mudstone were developed in shallow to deep lacustrine environments. Clays account for 40% to 60% among the whole mineral compositions. The percentage of the brittle minerals is lower than that of the marine shales reported. The shales are characterized by low thermal maturity with vitrinite reflectance 0.7% to 1.5%, predominantly Type II-III Kerogen, and an organic abundance (TOC range) of 1.5% to 4%. The shale gas is primarily thermogenic gas associated with oil generation, as it is presently still in the oil to wet gas windows. The gas shale in the Chang-7 Member is interbedded with oil shale layers, which differ from those present successfully developed shale gas accumulations around the world. Desorption gas measured from fresh core yields 1.25-5.75 m³/t, more than that reported in the Barnett Shale, suggesting such low maturity shale (Ro: 0.7% to 1.5%) as the Chang-7 Member succession in the Ordos Basin could possess significant gas potential.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012