Sedimentary Characteristics and Sandbody Architecture of Shallow Delta Front in Ordos Basin: A Case Study of Chang 9 Member in Shiwanghe Section in Yichuan

摘要/Abstract

摘要： As an important reservoir for storing oil and gas, the sandbodies in delta front are found with enormous petroleum exploration potential. However, there are few studies on architecture of sandbodies in shallow delta front through field outcrops. Guided by sedimentology and reservoir architecture theories, the outcrop observation and sampling was combined with the results of laboratory experiments and statistical analysis to clarify the sedimentary characteristics and sandbody architecture of Chang 9 member in the Shiwanghe section in Yichuan, Ordos basin. The results show that during the deposition, the Chang 9 member in Shiwanghe section lied in a warm and humid environment, especially an oxidation to weak-reduction transitional freshwater environment that was not evidently stratified, and shallow delta front subfacies was mainly developed, including microfacies such as underwater distributary channel, estuarine bar, sheet sand and interdistributary bay. The single sandbodies of shallow delta front in Chang 9 member in the study area can be divided into two vertical stacking styles such as non-connected and connected, and two lateral contact styles such as butted and cut-stacked. The accommodation growth rate and sediment supply rate jointly controlled by terrain slope and lake level rise/fall are important factors affecting the spatial development style of the composite sandbodies. The gentle slope allows the channels to incise weakly and present the characteristics of plane intersection. The rise of lake level and the decrease of sediment supply increase the ratio of accommodation growth rate to sediment supply rate, which may lead to the weakening of sandbody connectivity. The architecture models of sandbodies like river-river cut stacking and river-bar cut stacking are favorable for hydrocarbon accumulation.

关键词: Ordos basin, Yichuan area, Shiwanghe section, Chang 9 member, delta front, sedimentary characteristic, sandbody architecture

Abstract: As an important reservoir for storing oil and gas, the sandbodies in delta front are found with enormous petroleum exploration potential. However, there are few studies on architecture of sandbodies in shallow delta front through field outcrops. Guided by sedimentology and reservoir architecture theories, the outcrop observation and sampling was combined with the results of laboratory experiments and statistical analysis to clarify the sedimentary characteristics and sandbody architecture of Chang 9 member in the Shiwanghe section in Yichuan, Ordos basin. The results show that during the deposition, the Chang 9 member in Shiwanghe section lied in a warm and humid environment, especially an oxidation to weak-reduction transitional freshwater environment that was not evidently stratified, and shallow delta front subfacies was mainly developed, including microfacies such as underwater distributary channel, estuarine bar, sheet sand and interdistributary bay. The single sandbodies of shallow delta front in Chang 9 member in the study area can be divided into two vertical stacking styles such as non-connected and connected, and two lateral contact styles such as butted and cut-stacked. The accommodation growth rate and sediment supply rate jointly controlled by terrain slope and lake level rise/fall are important factors affecting the spatial development style of the composite sandbodies. The gentle slope allows the channels to incise weakly and present the characteristics of plane intersection. The rise of lake level and the decrease of sediment supply increase the ratio of accommodation growth rate to sediment supply rate, which may lead to the weakening of sandbody connectivity. The architecture models of sandbodies like river-river cut stacking and river-bar cut stacking are favorable for hydrocarbon accumulation.

Key words: Ordos basin, Yichuan area, Shiwanghe section, Chang 9 member, delta front, sedimentary characteristic, sandbody architecture

REN Yilin, ZHAO Junfeng, CHEN Jiayu, GUAN Xin, SONG Jinggan. Sedimentary Characteristics and Sandbody Architecture of Shallow Delta Front in Ordos Basin: A Case Study of Chang 9 Member in Shiwanghe Section in Yichuan[J]. 新疆石油地质, 2022, 43(zk(English)): 63-72.

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