Vertical Structure and Dominating Factors of Chang 9 Sandbody from Yanchang Formation in Ordos Basin

Key words: 
• vertical structure /
• stacking patterns /
• sandbody /
• delta /
• Chang 9 Reservoir Group /
• Ordos Basin /
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Abstract: With the reunderstanding of hydrocarbon geological conditions in Ordos Basin, a new discovery and breakthrough have been found in Chang 9 Reservoir Group form the lower part of Upper Triassic Yanchang Formation in recent years, and it has become the important succeed layer of strategic exploration. Through comprehensive study of outcrop section, drilling, core, well logging and tests, combining with the background analysis of sedimentary paleogeography, based on sedimentary characteristics and facies marker, it is considered that the genetic types of Chang 9 sandbody within the basin mainly include onshore and underwater distributary channel, natural levee, crevasse splay, sheet sand and other sandbodies associated with traction current effect in delta environment, the rivermouth bar, distal bar, sheet sand and other sand bodies in delta front, and the sandy shoal and bar, sheet sand and other sand bodies related to lakewave and lakeflow reworking in shore to shallow lake environment.  According to the detailed description of core sections and the analysis of sedimentary microfacies, combined with logging interpretation, it is pointed out that Chang 9 single sand body mainly develops B, C, I, F and D basic vertical structure types and multistory sandbodies have complicated and variable vertical stacking patterns, with BB, CC, CB, II, CI, BC, BI and FF as the common types. It is indicated that sedimentary environment, depositional facies and baselevel fluctuating cycle are main geologic factors controlled on vertical structure of individual sandbody, vertical stacking pattern and distribution of multistory sandbodies from the analysis of dominating factors of vertical structure of Chang 9 sandbodies.