Oil & Gas Geology ›› 2025, Vol. 46 ›› Issue (5): 1430-1445.doi: 10.11743/ogg20250504

• Petroleum Geology • Previous Articles    

Impacts of terrigenous inputs on organic matter type: A case study of shales in the Chang 73 oil sub-group of the Triassic Yanchang Formation, Ordos Basin

Chuang ER1,2(), Hongbo GUAN1, Wei LIU3, Ni CHENG4, Jie BAI5, Chong HU6   

  1. 1.School of Earth Sciences and Engineering,Xi’an Shiyou University,Xi’an,Shaanxi 710065,China
    2.Shaanxi Key Laboratory of Petroleum Accumulation Geology,Xi’an,Shaanxi 710065,China
    3.Research Institute of Petroleum Exploration & Development,PetroChina,Beijing 100083,China
    4.Yanchang Oil Field Co. ,Ltd. ,Yan’an,Shaanxi 716000,China
    5.School of Geoscience and Technology,Southwest Petroleum University,Chengdu,Sichuan 610500,China
    6.Changqing Branch,China National Logging Corporation,Xi’an,Shaanxi 710200,China
  • Received:2025-08-02 Revised:2025-08-29 Online:2025-10-30 Published:2025-10-29

Abstract:

This study aims to explore the impacts of terrigenous inputs on the organic matter type of semi-deep- to deep-lacustrine shales. Focusing on shales in the 3rd sub-group of the 7th oil group of the Triassic Yanchang Formation (also referred to as the Chang 73 oil sub-group) in the Ordos Basin, we conduct comprehensive petrological, sedimentological, and geochemical analyses on core samples from four wells located in the northeastern, northwestern, and southwestern parts of the lacustrine basin during the deposition of the Yanchang Formation. Accordingly, the impacts of terrigenous inputs on the organic matter type are delved into. The results indicate that the samples involved are all rich in organic matter, with maturities ranging from 0.91% to 1.03%, suggesting medium to low maturity. The organic matter comprises types Ⅱ1 and Ⅱ2 kerogens, with kerogen macerals dominated by sapropelinite, followed by vitrinite. Notably, maceral types differ greatly across different wells. Shales in the Chang 73 oil sub-group comprise seven lithofacies types: layered shale, shale with discontinuous to continuous laminae, silt-bearing shale with massive-discontinuous laminae, silty shale with massive-discontinuous laminae, layered silty shale, laminated silty shale, and massive silty shale. The layered shale, among others, is the least affected by terrigenous inputs, whereas the last four lithofacies are strongly affected. Shales from well W336 are weakly influenced by terrigenous clastic inputs. Correspondingly, type Ⅱ1 kerogen occurs in this well, with macerals dominated by sapropelinite (with a content of 64% ~ 83%). In contrast, shales from the other three wells are more strongly affected by terrigenous clastic inputs. In these wells, type Ⅱ2 kerogen predominates, with vitrinite content ranging from 15% to 42%. Despite the strong influence of terrigenous inputs, the parent materials of organic matter in the semi-deep to deep lacustrine environment remain dominated by aquatic organisms, producing favorable organic matter types. However, in the anoxic bottom water, higher plant debris accompanied by terrigenous inputs such as quartz and feldspar coexist with algae, leading to the formation of mixed type Ⅱ1-Ⅱ2 kerogen and comparatively less favorable organic matter types.

Key words: kerogen type, terrigenous material, maceral, shale lithofacies, organic-rich shale, Yanchang Formation, Triassic, Ordos Basin

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