澳大利亚是全球第四大煤炭生产国和第二大煤炭出口国，Bowen-Surat 盆地是澳大利亚最大的产煤盆地，Surat 盆地发育中侏罗统Walloon组含煤层系，沉积环境为河流相沉积，需厘清煤层在辫状河和曲流河及转换中的差异沉积特点和规律。

运用综合地质分析方法，结合地震、测井、岩心资料，建立沉积模式。利用地震地层追踪和基准面旋回划分，把研究区划分为6个四级地层旋回；依据37口井测井曲线的形态特征、接触关系等分为5种组合类型；通过研究区14口取心井岩心资料分析，识别出6种岩相；结合测井曲线和岩相特征，划分出曲流河和辫状河2种沉积相，4种亚相，8种微相。选取横向和纵向2条连井剖面，刻画煤剖面展布样式，分析煤在辫状河−曲流河及转换过程中的差异沉积特征。

结果表明：当基准面位于高处时，为曲流河环境，煤发育好，多层叠置。当基准面下降，曲流河慢慢转换为辫状河沉积，岩性由细到粗，层理构造幅度变大，高能水流冲刷先期煤层，辫状河道内零星残留煤沉积。当基准面位于最低附近时，为辫状河沉积，煤发育最差，漫溢砂内少有发育；随着基准面回升，由辫状河转为曲流河的过程中，水体能量变小，岩相依次发生变化Gm-St-Sp-Sh-Fr/C，煤层沉积趋好。曲流河−转换带−辫状河相变过程中，煤在三维空间上的展布表现为，平面上煤的连片性、范围变差；剖面上煤的横向连续性，垂向叠置状及厚度都变差。基准面变化过程中，最有利的聚煤环境为泛滥泥中的煤沼泽，其次河漫砂和漫溢砂有少量煤发育。研究成果可用来指导分析煤层分布范围，在Surat区块井位部署时，有助于煤层气开发目的层“甜点”的识别确认。

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.

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.

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.