Emerging Shale Gas and Shale Oil Plays in China
The investigation of regional source rocks , paleogeography
and tectonic evolutions in China reveal China has vast potential shale plays
spanning from Pre-Cambrian to Quaternary in three depositional settings of
marine, lacustrine and transition. During Pre-Cambrian Sinian period,
Doushantuo marine shale was mainly distributed in South China in shelf and
slope setting. The Cambrian Qiongzhusi, Ordovician Wufeng and Silurian Longmaxi
marine shales or their equivalent shales were deposited mainly in the ancient
passive margin setting in South China Yangtze platform. These shales are thick
(40-500m), widely distributed and have high TOC (1-8%), Ro (1.5-6%) and brittle
mineral composition. Meanwhile, thick Cambrian and Ordovician shales with TOC
of 0.5-5% and Ro of 0.4-3% were mainly deposited in restricted platform and
basin setting in Tarim basin. The kerogens in these marine shales are mainly
type I and II. During Carboniferous-Permian marine, the shales associated with
coal were mainly deposited in transitional setting in north China, Tarim basin
in NW China and Yangtze platform in South China, e.g., the typical transitional shales with type III kerogen in North China have high TOC of 2-20% and variable
Ro of 0.6-2.5%. Since late Triassic, the organic rich shales are dominated by lacustrine shales, e.g., the Triassic Yangchang shale in Ordos basin, Triassic
to Jurassic shales in Tarim basin, Jurrassic Badaowan and Xishanyao shales in
Junggar, Jurrassic Ziliujing shales in Sichuan basin, Cretaceous Qingshankou
shales in Songliao Basin, Cenozoic shales in East China (e.g. Shahejie shale in Bohai Bay basin), Quaternary shale in Qaidam basin, etc. They are mainly
featured by type I and II kerogen , low Ro (<1.2%) and high TOC (up to 30%).
The regionally extensive shales in the three depositional settings were
generally formed during stable basin subsidence period or high sea or lake
level period. The marine shales have decent brittle mineral content and almost no smectite. The lacustrine shales usually have sandstone interbeds and have more
clay mineral content than marine shales. The Paleozoic marine shales are
emerging thermogenic shale gas plays. The Meso-Cenozoic lacustrine shales with
high TOC and medium or low Ro are emerging shale gas or shale oil plays
respectively, while the shallow organic rich shales are usually biogenic shale
gas plays. These emerging shale plays have been gradually demonstrated by
historical and recent drilling.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California