关键词: 龙马溪组上层系, 层序格架, 沉积微相, 沉积微地貌, 海相强非均质页岩储层

Abstract: The marine organic-rich high-quality shales in the upper member of Silurian Longmaxi Formation of southern Sichuan Basin exhibit pronounced heterogeneity characterized by limited thickness, vertical discontinuity, and rapid lateral pinch-out. Based on integrated datasets including drilling, elemental logging, core descriptions, thin-section identification, whole-rock X-ray diffraction (XRD), and scanning electron microscopy (SEM), this study characterizes the sequence stratigraphic framework and sedimentary microfacies of the organic-rich shales from the upper member of Longmaxi Formation in Luzhou area. Furthermore, the control of the sequence framework and microfacies on the distribution of these strongly heterogeneous shale reservoirs are discussed. The research results indicate that the lithologies of the upper member of Longmaxi Formation in Luzhou area are dominated by argillaceous shale, clay-bearing siliceous shale, and mixed shale, with the muddy shallow-water shelf and muddy semi-deep-water shelf being the most prevalent sedimentary microfacies. Within the shelf interior, five distinct microgeomorphologic units are identified:shelf plain, shelf highland, shelf depression, shelf valley, and shelf slope. The upper member of Longmaxi Formation in Luzhou area comprises two third-order sequences:the lower portion corresponds to the highstand system tracts (HST) of the first sequence, while the upper portion consists of the transgressive system tracts (TST) and HST of the second sequence. Type Ⅰ organic-rich shale reservoirs are primarily distributed in the middle-to-upper TST and the lower HST, and their development is predominantly controlled by the muddy semi-deep-water shelf plain microfacies and its associated microgeomorphology. The long duration of the shallow-water highstand during the early sequence of the upper member of Longmaxi Formation, coupled with the slow rate, limited magnitude of sea-level rise, and high sedimentation rates during the early transgression of the late sequence, collectively resulted in thin high-quality reservoirs with rapid lateral pinch-out and pronounced heterogeneity. This was further compounded by the rapid lateral variations in sedimentary microfacies and micro-geomorphology. The unique architectural combination of sequence, sedimentary microfacies, and micro-geomorphologies constitutes the critical factor controlling the intense vertical and lateral heterogeneity of the upper member of Longmaxi Formation in Luzhou area. Research in this field provides essential guidance for predicting the distribution of strongly heterogeneous marine high-quality shale reservoirs and optimizing target intervals for horizontal well development.

Key words: upper member of Longmaxi Formation, sequence stratigraphic framework, sedimentary microfacies, sedimentary microgeomorphology, marine strongly heterogeneous shale reservoir