Sequence Stratigraphy and Microfacies of the Middle Ordovician Yijianfang Formation in the Tarim Basin, NW China

Abstract It is of great significance for further exploration in deep-burial carbonates to clarify the distribution of sedimentary microfacies within the high-precision sequence framework and to analyze how the sequence stratigraphy and lithologic types control the karst carbonate reservoirs in the Tarim Basin. For the study area, research on the identification and distribution of fourth-order sequence boundaries is somewhat inadequate. Moreover, methods for identifying sedimentary microfacies using logging data still need to be explored and advanced. Based on field outcrop, drilling cores, thin sections, full-bore microscan imaging (FMI), and conventional logging data, the high-precision sequence stratigraphy and sedimentary facies of the Yijianfang Formation in the Tahe Oilfield have been studied to clarify how they control the formation and distribution of carbonate intra-platform shoals. One third-order sequence and three fourth-order sequences were identified, and the high-precision sequence stratigraphic framework was established. The thicknesses of intra-platform shoal are large in the south and middle parts of this study area, and they are thin in the west and north parts. In addition, eight lithofacies types were identified (oolitic grainstone, intraclastic grainstone, bioclastic grainstone, bioclastic packstone, intraclastic packstone, intraclastic wackestone, bioclastic wackestone, and mudstone) and four subsequent facies associations (inter-shoal sea, low-energy shoal, medium-energy shoal, and high-energy shoal) were then defined within the sequence framework according to different hydrodynamic conditions. Based on the response difference of microfacies types between FMI and conventional logging, four response models were recognized (blocky, linear, porphyritic, and compound). Favorable reservoirs are jointly controlled by sequences and sedimentary facies. Shoals developed in high-energy shoal facies, as the dominant lithofacies, provide the material basis for favorable reservoirs. High-precision sequence boundaries control the development of dissolution, and favorable reservoirs are developed near sequence boundaries and HST; therefore, this work provides a basis for reservoir prediction in the later stage.