The Application of Standard Microfacies Types to the Study of Carbonate Microfacies and Depositional Environments——A case study from the Ordovician in the Yubei area, southwestern Tarim Basin

Key words: 
• carbonate rocks /
• standard microfacies types /
• logging characteristics /
• sedimentary evolution /
• Ordovician /
• Yubei area

Abstract: The Standard Microfacies (SMF) Types are virtual categories that summarize microfacies with identical criteria. SMF 1 to SMF 26 are divided following the order of the facies zones in the rimmed carbonate platform going from the basin to the subaerial exposure areas, and the plates of the SMF Types, as well as the model of the SMF Types distributed in facies zones are developed by Flügel. Based on that, the SMF Types of the Ordovician carbonate in the Yubei area were studied. Nine of the SMF Types have been recognized in total, which always occur in certain assemblages. The assemblages are SMF 15-C, SMF 16-N_ON-LAMINATED and SMF 17 in the Penglaiba-lower Yingshan Formations (O₁p-O_1-2y^lower), SMF 16-N_ON-LAMINATED, SMF 17 and SMF 23 in the upper Yingshan-Yijianfang Formations (O_1-2y^upper-O₂yj), SMF 6 and SMF 12-C_RIN, as well as SMF 1-B_URROWED in the Qiaerbake Formation (O₃q), SMF 11 and SMF 9, as well as SMF 15-C, SMF 16-N_ON-LAMINATED and SMF 17 in the Lianglitage Formation (O₃l). Additionally, the characteristics of the log responses were analyzed for the assemblages of SMF Types, whose spatial distributions were outlined by comparison of the gamma log curves along the well-tie sections. The originally depositional environments were recovered for the Ordovician formations according to assemblages of the SMF Types and their spatial distribution. The study results indicated that the Ordovician had experienced the evolution of four depositional environments in the Yubei area, that is, open marine (O₁p-O_1-2y^lower), restricted marine (O_1-2y^upper-O₂yj), slope-deep shelf (O₃q) and platform margin-open marine (O₃l) in sequence.