The formation and evolutionary characteristics of organic matter and pyrites in the continental shales of the 3rd submember of Chang 7 Member, Yanchang formation, Ordos Basin, China
Through microscopic analyses (e.g., organic macerals, thin section observation, scanning electron microscope (SEM) imaging of fresh bedding planes via argon ion milling, and energy spectrum tests) combined with Rock-Eval analyses, this study systematically investigated the organic matter and pyrites...
| Main Author: | |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
KeAi Communications Co., Ltd.
2024-04-01
|
| Series: | Energy Geoscience |
| Subjects: | |
| Online Access: | http://www.sciencedirect.com/science/article/pii/S2666759223000963 |
| _version_ | 1797200775957446656 |
|---|---|
| author | Ruikang Bian |
| author_facet | Ruikang Bian |
| author_sort | Ruikang Bian |
| collection | DOAJ |
| description | Through microscopic analyses (e.g., organic macerals, thin section observation, scanning electron microscope (SEM) imaging of fresh bedding planes via argon ion milling, and energy spectrum tests) combined with Rock-Eval analyses, this study systematically investigated the organic matter and pyrites in the continental shales in the 3rd submember of the Chang 7 Member (Chang 73 submember) in the Yanchang Formation, Ordos Basin and determined their types and the formation and evolutionary characteristics. The results are as follows. The organic matter of the continental shales in the Chang 73 submember is dominated by amorphous bituminites and migrabitumens, which have come into being since the early diagenetic stage and middle diagenetic stage A, respectively. The formation and transformation of organic matter is a prerequisite for the formation of pyrites. The Ordos Basin was a continental freshwater lacustrine basin and lacked sulphates in waters during the deposition of the Chang 7 Member. Therefore, the syndiagenetic stage did not witness the formation of large quantities of pyrites. Since the basin entered early diagenetic stage A, large quantities of sulfur ions were released as the primary organic matter got converted into bituminites and, accordingly, pyrites started to form. However, this stage featured poorer fluid and spatial conditions compared with the syndepositional stage due to withdraw of water, the partial formation of bituminites, and a certain degree of compaction. As a result, large quantities of pyrrhotite failed to transition into typical spherical framboidal pyrites but grew into euhedral monocrystal aggregates. In addition, pyrites are still visible in the migrabitumens in both microfractures and inorganic pores of mudstones and shales, indicating that the pyrite formation period can extend until the middle diagenetic stage A. |
| first_indexed | 2024-04-24T07:37:01Z |
| format | Article |
| id | doaj.art-11e526660c0f4e7ca502d16d106d6987 |
| institution | Directory Open Access Journal |
| issn | 2666-7592 |
| language | English |
| last_indexed | 2024-04-24T07:37:01Z |
| publishDate | 2024-04-01 |
| publisher | KeAi Communications Co., Ltd. |
| record_format | Article |
| series | Energy Geoscience |
| spelling | doaj.art-11e526660c0f4e7ca502d16d106d69872024-04-20T04:17:55ZengKeAi Communications Co., Ltd.Energy Geoscience2666-75922024-04-0152100250The formation and evolutionary characteristics of organic matter and pyrites in the continental shales of the 3rd submember of Chang 7 Member, Yanchang formation, Ordos Basin, ChinaRuikang Bian0State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 102206, China; Key Laboratory of Shale Oil and Gas Exploration & Production Technology, Sinopec, Beijing 102206, China; SINOPEC Exploration & Production Research Institute, Beijing, 102206, China; SINOPEC Exploration & Production Research Institute, Beijing, 102206, China.Through microscopic analyses (e.g., organic macerals, thin section observation, scanning electron microscope (SEM) imaging of fresh bedding planes via argon ion milling, and energy spectrum tests) combined with Rock-Eval analyses, this study systematically investigated the organic matter and pyrites in the continental shales in the 3rd submember of the Chang 7 Member (Chang 73 submember) in the Yanchang Formation, Ordos Basin and determined their types and the formation and evolutionary characteristics. The results are as follows. The organic matter of the continental shales in the Chang 73 submember is dominated by amorphous bituminites and migrabitumens, which have come into being since the early diagenetic stage and middle diagenetic stage A, respectively. The formation and transformation of organic matter is a prerequisite for the formation of pyrites. The Ordos Basin was a continental freshwater lacustrine basin and lacked sulphates in waters during the deposition of the Chang 7 Member. Therefore, the syndiagenetic stage did not witness the formation of large quantities of pyrites. Since the basin entered early diagenetic stage A, large quantities of sulfur ions were released as the primary organic matter got converted into bituminites and, accordingly, pyrites started to form. However, this stage featured poorer fluid and spatial conditions compared with the syndepositional stage due to withdraw of water, the partial formation of bituminites, and a certain degree of compaction. As a result, large quantities of pyrrhotite failed to transition into typical spherical framboidal pyrites but grew into euhedral monocrystal aggregates. In addition, pyrites are still visible in the migrabitumens in both microfractures and inorganic pores of mudstones and shales, indicating that the pyrite formation period can extend until the middle diagenetic stage A.http://www.sciencedirect.com/science/article/pii/S2666759223000963Organic matterPyriteFormation and evolutionContinental shaleChang 73 submemberOrdos Basin |
| spellingShingle | Ruikang Bian The formation and evolutionary characteristics of organic matter and pyrites in the continental shales of the 3rd submember of Chang 7 Member, Yanchang formation, Ordos Basin, China Energy Geoscience Organic matter Pyrite Formation and evolution Continental shale Chang 73 submember Ordos Basin |
| title | The formation and evolutionary characteristics of organic matter and pyrites in the continental shales of the 3rd submember of Chang 7 Member, Yanchang formation, Ordos Basin, China |
| title_full | The formation and evolutionary characteristics of organic matter and pyrites in the continental shales of the 3rd submember of Chang 7 Member, Yanchang formation, Ordos Basin, China |
| title_fullStr | The formation and evolutionary characteristics of organic matter and pyrites in the continental shales of the 3rd submember of Chang 7 Member, Yanchang formation, Ordos Basin, China |
| title_full_unstemmed | The formation and evolutionary characteristics of organic matter and pyrites in the continental shales of the 3rd submember of Chang 7 Member, Yanchang formation, Ordos Basin, China |
| title_short | The formation and evolutionary characteristics of organic matter and pyrites in the continental shales of the 3rd submember of Chang 7 Member, Yanchang formation, Ordos Basin, China |
| title_sort | formation and evolutionary characteristics of organic matter and pyrites in the continental shales of the 3rd submember of chang 7 member yanchang formation ordos basin china |
| topic | Organic matter Pyrite Formation and evolution Continental shale Chang 73 submember Ordos Basin |
| url | http://www.sciencedirect.com/science/article/pii/S2666759223000963 |
| work_keys_str_mv | AT ruikangbian theformationandevolutionarycharacteristicsoforganicmatterandpyritesinthecontinentalshalesofthe3rdsubmemberofchang7memberyanchangformationordosbasinchina AT ruikangbian formationandevolutionarycharacteristicsoforganicmatterandpyritesinthecontinentalshalesofthe3rdsubmemberofchang7memberyanchangformationordosbasinchina |