Objective Australia is recognized as the fourth largest producer and the second largest exporter of coals in the world. The Bowen-Surat Basin is the largest coal producing basin in this country. The Surat Basin exhibits coal-bearing layers of the Middle Jurassic Walloon Subgroup. This subgroup features a sedimentary environment of fluvial facies, necessitating clarifying the differential sedimentary characteristics and patterns of coal seams in braided river and meandering river facies and during their transitions.
Methods Using comprehensive geological analysis methods, as well as seismic data, logs, and core data, this study established the sedimentary pattern of the study area. Using seismic horizon tracking and base-level cycles, this study divided the study area into six fourth-order stratigraphic cycles and categorized the log curves of 37 wells into five combinations based on the morphological characteristics of the curves and contact relationships. The observations of cores from 14 wells revealed six typical lithofacies in the Walloon Subgroup. In combination with the characteristics of log curves and lithofacies, this study determined two sedimentary facies: braided river to meandering river facies. The sedimentary facies can be further divided into eight microfacies. Using two horizontal and vertical cross-well sections, this study characterized coal distribution and analyzed the differential sedimentary characteristics of coal seams in the braided river and meandering river facies and during their transitions.
Results and Conclusions In the case of a high base level, the study area was in a sedimentary environment of the meandering river facies. In this case, coals were well-developed, exhibiting the superimposition of multiple coal seams. As the base level dropped, the meandering river facies gradually transitioned to the braided river facies, with the lithofacies changing from fine- to coarse-grained rocks and the amplitude of bedding structures increasing. During this period, high-energy water flow washed away coal seams formed previously, leaving scattered coal deposits in braided river channels. When the base level approached its minimum, the study area exhibited a sedimentary environment of the braided river facies. This period witnessed the lowest level of coal development, with a minor presence of coals in overflow sands. As the base level rose, the braided river facies transitioned to the meandering river facies. During this period, the water body energy decreased, and the lithofacies varied, exhibiting Gm-St-Sp-SH-Fr/C sequentially. Meanwhile, an increasing number of coal seams were deposited. In the transition from the meandering river facies to the transition zone and then to the braided river facies, the three-dimensional coal distribution was characterized by increasingly reduced planar continuity and range, lateral continuity in sections, vertical stacking and thickness. As the base level changed, the sedimentary microfacies that was the most favorable for coal accumulation was coal swamps, followed by floodplains and overflow sands. The achievements of this study can serve as a guide for the analysis of the distribution range of coal seams. During well emplacement in the Surat block, the results of this study can help confirm the "sweet spots" of the targets for coal coalbed methane (CBM) production.
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