Research on architecture of multi-type sand bodies inside distributary channel of delta plain: a case study in the Daniudi Gas Field, Ordos Basin

Abstract

The distributary channel reservoirs in the delta plain are important reservoir bodies at home and abroad, which have internal sand bodies of various shapes and sizes with a conplicated quantitative relationship. Due to the lack of systematic research on the sedimentary mode and architectural characteristics of favorable reservoirs, the development effect of natural gas is restricted to a certain extent. Based on the integrated application of modern deposit research, field outcrop anatomy, well log identification, and other related techniques, this study comes up with a method for architecture analysis and sand body description of distributary channels. This method uses modern deposit research to determine the sand body type of distributary channels, uses depositional model to study on architecture anatomy, and uses well infomation of dense well pattern to find out the scale of sand bodies, then plots typical charts showing stack modes, grain size changes, lithologic association, and logging response for different sand body types. This study also researches the development patterns and internal architechtural characteristics of different sand body types in distributary channels, and innovatively forms a technique of identifying streamlines by “beading method” and describing the association relationships between channels and different sand bodies by “mosaic method,” which clarifies the formation mechanisms and internal architecture of five sand body types, as well as their distribution patterns in channels. There are mainly three results in this study. Firstly, the shape and internal architecture of the channel bars are controlled by hydrodynamic intensity. More specifically, the shape of a channel bar is constrained by the symmetry of the channel, while the scale of that is controlled by the flow intensity. Secondly, there are five types of sand bodies deposited in the distributary channels which are channel bar, convex-point bar, straight-point bar, branching-point bar, and combining-point bar. Also, the statistical results present that both the length–width ratio of bars from different channels and the width ratio of channels and bars have strong regularity. Thirdly, the widths of channel bars in distributary channels of the delta plain from Daniudi Gas Field are from 300 to 600 m, while the lengths are from 1200 to 2300 m, and the average length–width ratio is 4:1. There are great difference in various kinds of channel bars, and the main beneficial reservoir body types are channel bar, convex-point bar, straight-point bar, and branching-point bar. The method has a good guiding significance for sweet spot evaluation and effective development of fluvial sedimentary system.