The Upper Ordovician – Lower Silurian Wufeng – Longmaxi Formation Black Shale in the Upper Yangtze: Lithofacies and Depositional Environments
Bo Ran, Shugen Liu, Wei Sun, Di Yang, Shiyu Wang, Chao Luo, Yuehao Ye, and Xuan Zhang
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, China
The Upper Ordovician – Lower Silurian Wufeng – Longmaxi Formation in the Upper Yangtze area is one of the most important source rocks in the Lower Paleozoic Strata (Wang et al., 2008; Liang et al., 2009) and one of the most potential shale gas play (Zou et al., 2010; Liang et al., 2011; Wan et al., 2012). Although the recent popularity of organic-rich shale and mud rocks has led to a significant interest in analyzing Wufeng – Longmaxi shale reservoir characterizations (e.g., Chen et al., 2011; Ma et al., 2012), lithofacies and depositional setting of shale obviously lags behind shale reservoir studies. Even though some important understandings have been achieved for Wufeng – Longmaxi depositional environments in former work (Table 1; Chen et al., 2004; Yan et al., 2008, 2009a, 2009b; Gorjan et al., 2012), learning from examples in the shale reservoir of the famous Barnett Shale Gas system, these important reservoir parameters including the content of total organic carbon (TOC) and thickness of organic-rich shale and the content of brittle minerals are mainly controlled by lithofacies and depositional setting of black shale in reducing conditions (Table 1; Loucks and Ruppel, 2007; Guo et al., 2011).
In order to explore the whole lithofacies and depositional variations of Wufeng – Longmaxi black shale, geochemical, petrographical, pyrite morphology analysis were adopt in this study. In the Qilongcun section, Xishui country in Guizhou Province, eight general lithofacies are recognized on the basis of mineralogy, fabric, biota, and texture: (1) Clay-rich laminae, (2) Silt-rich laminae, (3) Dolomite-rich or calcareous laminae, (4) Graded mudstone, (5) Massive mudstone, (6) Lenticular mudstone, (7) Bioturbation mudstone, (8) Low-angle laminated mudstone. Then trace-element evidences indicate: (1) organic-rich units of the Wufeng Formation and Lower Longmaxi Formation were deposited under pervasive anoxic– euxinic water columns, (2) the Guanyinqiao Formation and Upper Longmaxi Formation were deposited under oxic-dysoxic water columns. Adding the evidences of framboidal pyrite size distribution, this study indicates that during the deposition process of the Wufeng – Guanyinqiao – Lower Longmaxi – Upper Longmaxi Formation, redox conditions in the bottom water fluctuated from euxinic-anoxic, characterizing in clay-rich laminae and massive mudstone facies, to oxic-dysoxic, characterizing in lenticular mudstone and bioturbation mudstone faceis, to euxinic-anoxic to oxic-dysoxic.
Based on this analysis of geochemical, petrographical, pyrite morphology methods for the Wufeng – Longmaxi Formation of Upper Yangtze, the following may be concluded: (1) the average TOC and content of brittle minerals of Wufeng and Lower Longmaxi black shale being formed in the euxinic-anoxic condition lacking of bioturbation are higher than of Guanyinqiao and Upper Longmaxi shale being formed in the oxic-dysoxic condition; (2) clay-rich laminae and massive mudstone of the Wufeng – Longmaxi Formation are conducive to shale gas exploration and development in the Upper Yangtze.
AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013