Sedimentary microfacies identification and inferred evolution of the Chang 7 Member of Yanchang Formation in the Longdong area, Ordos basin
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    Abstract:

    The sedimentary characteristics of core, outcrops and logging data of the Chang 7 Member of Yanchang Formation in Longdong area of Ordos Basin were summarized and evaluated based on the theory of gravity flow sedimentation. The analyses show that the sandy debris flow and turbidity current deposits are mainly developed in Chang 7 Member of Longdong, but slip and slump rocks are locally developed. The sedimented sand bodies in the study area are affected by multiple sources, lake basin bottom shape and lake shoreline migration and do not show an obvious fan- shape. Sandbodies in Jingchuan- Huachi area controlled by steep and narrow slopes in southwest and form sandy strips, and channel- type gravity flow deposits are also developed. The sedimentary microfacies are divided into six types: restricted channel, unconfined channel, flank overflow, inter- channel, leading edge lobate sand and deep lake mud. At the bottom of the lake basin, the Qingcheng- Huachi waterway intersects and overlaps forming a large area of inter- connected sand bodies. In the Ningxian- Heshui area dominated by the southern provenance, due to relatively gentle slope gradient and abundant source supply, a slope sedimentation model dominated by slump structure is formed. According to the slip distance, it can be divided into two subfacies: near- source sedimentation and far- source sedimentation. The sedimentary microfacies can be divided into four types: irregular slumping, sandy debris flow tongue, lobate turbidite, and deep lake mud. In the southern part of the study area, sandy debris flow tongue is the main body which overlaps both vertically and horizontally with many stages on the slope, forming a large area of continuous sand body of thin and thick interbedded sand. The identification and division of sedimentary microfacies in the Chang 7 Member in Longdong and the establishment of its sedimentary model provides important guidelines for detailed characterization of sand body distribution in the study area and provides a basis for scaled production of tight oil horizontal wells in the area.

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ZHANG Xiaohui, FENG Shunyan, LIANG Xiaowei, FENG Shengbin, MAO Zhenhua, REN Jisheng, CHEN Shaohua.2020. Sedimentary microfacies identification and inferred evolution of the Chang 7 Member of Yanchang Formation in the Longdong area, Ordos basin[J]. Acta Geologica Sinica,94(3):957-967

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History
  • Received:May 23,2019
  • Revised:October 12,2019
  • Adopted:November 26,2019
  • Online: November 30,2019
  • Published: