Formation mechanism of reservoir oolitic dolomite in Lower Triassic Feixianguan formation, northeastern Sichuan Basin, southwest China
来源期刊：中南大学学报(英文版)2014年第8期
论文作者：CHEN Pei-yuan(陈培元) 谭秀成 LIU Hong(刘宏) MA Teng(马腾) 罗冰 JIANG Xing-fu(姜兴福) YU Yang(于洋) JIN Xiu-ju(靳秀菊)
文章页码：3263 - 3274
Key words：Lower Triassic Feixianguan formation; early exposure; seepage reflux; dolomitization; Sichuan Basin; northeastern Sichuan
Abstract: A series of marine natural gas fields were recently discovered in oolitic dolomites of the Lower Triassic Feixianguan formation, northeastern Sichuan Basin, southwest China. The mechanism forming these reservoir dolomites is debatable, limiting the ability to characterize these reservoir successfully. Based on the investigation of the representative Dukouhe, Luojiazhai, and Puguang areas, this issue was addressed by examining the distribution, petrology, and geochemistry of the dolomites, the most comprehensive study to date was provided. Dolomitization occurred at a very early stage of diagenesis, as shown by the petrological features of the rock fabric. Vadose silt, which is composed primarily of dolomitic clasts, is found in the primary and secondary pores of the oolitic dolomite. This indicates that the overlying strata were subjected to dolomitization when the Feixianguan formation was located in the vadose zone. Therefore, it may be inferred that the dolomitization which occurred before the formation was exposed to meteoric conditions. The spatial distribution and geochemical characteristics of the dolomite indicate that dolomitization occurred as a result of seepage reflux. The degree of dolomitization decreases with increasing distance from the evaporative lagoon. Furthermore, the type and porosity of the dolomite vary in different zones of the upward-shoaling sequence, with the porosity gradually decreasing from the highest layer to the lowest layer. This reflects a close relationship between dolomitization and seawater evaporation during the formation of the dolomite. Geochemical analysis provided further evidence for the relationship between the dolomitization fluid and the coeval seawater. The 87Sr/86Sr and δ13C isotopes, as well as the abundances of trace elements, Fe and Mn, indicate that seawater concentrated by evaporation acted as the dolomitization fluid. These results also show that dolomitization most likely occurred in a semi-closed diagenetic environment. Therefore, the main mechanism of oolitic dolomite formation is seepage reflux, which occurred at an early stage of diagenesis.
CHEN Pei-yuan(陈培元)1, 2, TAN Xiu-cheng(谭秀成)1, 2, LIU Hong(刘宏)2, MA Teng(马腾)2, LUO Bing(罗冰)3, JIANG Xing-fu(姜兴福)4, YU Yang(于洋)2, JIN Xiu-ju(靳秀菊)5
(1. State Key Laboratory of Oil and Gas Geology and Exploration (Southwest Petroleum University),
Chengdu 610500, China;
2. School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China;
3. Research Institute of Petroleum Exploration and Development,
Post-doctoral Workstation of PetroChina Southwest Oil and Gas Field Company, Chengdu 610051, China;
4. Department of Exploration Industry, PetroChina Southwest Oil and Gas Field Company, Chengdu 610051, China;
5. Research Institute of Petroleum Exploration and Development, SINOPEC,
Zhongyuan Oilfield Company, Puyang 457001, China)
Abstract:A series of marine natural gas fields were recently discovered in oolitic dolomites of the Lower Triassic Feixianguan formation, northeastern Sichuan Basin, southwest China. The mechanism forming these reservoir dolomites is debatable, limiting the ability to characterize these reservoir successfully. Based on the investigation of the representative Dukouhe, Luojiazhai, and Puguang areas, this issue was addressed by examining the distribution, petrology, and geochemistry of the dolomites, the most comprehensive study to date was provided. Dolomitization occurred at a very early stage of diagenesis, as shown by the petrological features of the rock fabric. Vadose silt, which is composed primarily of dolomitic clasts, is found in the primary and secondary pores of the oolitic dolomite. This indicates that the overlying strata were subjected to dolomitization when the Feixianguan formation was located in the vadose zone. Therefore, it may be inferred that the dolomitization which occurred before the formation was exposed to meteoric conditions. The spatial distribution and geochemical characteristics of the dolomite indicate that dolomitization occurred as a result of seepage reflux. The degree of dolomitization decreases with increasing distance from the evaporative lagoon. Furthermore, the type and porosity of the dolomite vary in different zones of the upward-shoaling sequence, with the porosity gradually decreasing from the highest layer to the lowest layer. This reflects a close relationship between dolomitization and seawater evaporation during the formation of the dolomite. Geochemical analysis provided further evidence for the relationship between the dolomitization fluid and the coeval seawater. The 87Sr/86Sr and δ13C isotopes, as well as the abundances of trace elements, Fe and Mn, indicate that seawater concentrated by evaporation acted as the dolomitization fluid. These results also show that dolomitization most likely occurred in a semi-closed diagenetic environment. Therefore, the main mechanism of oolitic dolomite formation is seepage reflux, which occurred at an early stage of diagenesis.
Key words:Lower Triassic Feixianguan formation; early exposure; seepage reflux; dolomitization; Sichuan Basin; northeastern Sichuan
