Callovian-Oxfordian sedimentary microfacies in the middle of Block B on the right bank of the Amu Darya Basin, Turkmenistan

The right bank of the Amu Darya Basin enjoys abundant natural gas resources, on which the Callovian-Oxfordian strata in the middle of Block B serve as the major horizons for natural gas production. However, the characteristics and distribution patterns of the sedimentary microfacies in these strata are yet to be further explored. Based on the analysis of data on drilling, logging, cores, and thin sections from 29 typical wells, as well as the regional sedimentary background, this study inferred that the middle of Block B evolved from the Callovian ramp platform into the Oxfordian rimmed platform. Moreover, this study determined that the inner-ramp intertidal-subtidal shallow-water subfacies mainly developed during the Callovian and transitioned into the shallow shelf subfacies during the Oxfordian. This study identified eight sedimentary microfacies, namely reef knoll, reef-shoal complex, bioclastic shoal, psammitic shoal, bioherm, lime mud mound, intershoal (intermound), and static-water mud. Based on research into the high-precision sequence-sedimentary microfacies framework, this study built a geological model for the development of sedimentary microfacies in the study area. According to this geological model, the sedimentary microfacies in the study area are characterized by vertical alternation of reef-shoal complex, bioclastic (psammitic) shoal, bioherm, and intershoal microfacies. Moreover, they show the development of reef knoll, reef-shoal complex, bioclastic (psammitic) shoal, and bioherm (or lime mud mound) laterally from west to east, with the physical properties of the reservoirs deteriorating from west to east accordingly. The microfacies of reef-shoal complex and the bioclastic (psammitic) shoal predominate in the study area, and their deposition and development are controlled by sequence boundaries and are also affected by paleo-landforms. The Oxfordian reef-shoal complexes were largely inherited from the Callovian uplifts and show lateral seaward progradation.