The Upper Triassic Braided River Thin-Bedded Tight Sandstone in the Yanchang Formation, Ordos Basin: Sedimentary Characteristics, Seismic Forecasting Method, and Implication

Abstract

In the Ordos Basin, Chang 81, a Member of the Yanchang Formation, features the development of braided river thin-bedded tight sandstones. These sandstones constitute one of the main production layers of tight oil and gas in the Yanchang Formation within the basin. This study integrates data from core samples, drilling, and seismic information to identify braided river thin-bedded sandstones in the Chang 81 Member at Daijiaping, Ordos Basin, using a method of constrained correlation between seismic waveform and seismic facies. This approach aids in determining the sedimentary microfacies types and reservoir characteristics of thin-bedded tight sandstones. We establish a quantitative fitting formula for the width-to-thickness ratio of braided channel sand bodies to finely characterize sand body stacking patterns and spatial distribution of thin-bedded tight sandstones in braided channels. Braided delta plain deposits in the Chang 81 Member at Daijiaping mainly comprise four types of sedimentary microfacies: braided channels, crevasse channels, floodplains, and swamps. The thickness of the reservoir sand body of Chang 81 member is mainly concentrated between 5–25 m, with low porosity and permeability, making it a typical thin-bedded tight sandstone reservoir. A method of constrained correlation between seismic waveforms and seismic facies was employed to identify sand bodies of braided river thin-bedded sandstones in the Chang 81 Member, summarizing four sand body stacking patterns: longitudinal incision type, longitudinal separation type, lateral shifting type, and single channel type. Furthermore, a quantitative forecasting formula of width-to-thickness ratio was established for the river channel scale, providing accurate guidance for well deployment. Horizontal wells deployed from the sand body’s side towards its center in a river channel yield a production 1.8 times higher than that of horizontal wells deployed in the opposite direction. Thin-bedded tight sandstones in braided channels, characterized by flat-top and convex-bottom lenticular seismic facies, hold practical significance in guiding the deployment of horizontal well patterns for tight oil and enhancing oil and gas recovery.