Cisuralian coals of the Mullaley Sub-basin, northern New South Wales

The Mullaley Sub-basin of the Gunnedah Basin extends from Quirindi in the southeast, to north of Narrabri, to west of Dunedoo in northern New South Wales. There have been more than 100 boreholes sunk to basement investigating the (lower Permian) Cisuralian coal and coal seam gas resources of the Mullaley Sub-basin since the early 1990s. A desktop review of this open file information has allowed the formal correlation and naming of six Cisuralian coal members attaining a maximum 35 m of cumulative thickness within an upward coarsening sedimentary package totalling no more than 150 m. In ascending order, the coal members are: Bibblewindi (0–10 m), Bohena (3–18 m), Collygra (0.5–3 m), Coxs (1.5–4 m), Tullamullen (0.5–4 m) and Mooki (0.5–3 m). Cisuralian coal seams in the Maules Creek Formation of the southern Mullaley Sub-basin are here correlated with those of the Greta Coal Measures at Werris Creek and Muswellbrook. It is apparent that basement paleotopography played a significant role in the Cisuralian coal development as coals are best developed where the sedimentary sequence is greater than 60 m thick, as there the thick seams (Bohena and Bibblewindi coal members) occur towards the base of the sequence. The maximum western limit of the Cisuralian coals (Rocky Glen Ridge) is further east than previously inferred with new drilling information showing the Porcupine Formation directly overlying the barren pelletoidal claystones of the Leard Formation or the underlying volcanics (Boggabri Volcanics/Werrie Basalt). Early marine transgressions at the top of the Maules Creek Formation have stopped development of the Mooki, Tullamullen and Coxs coal members in the northern and eastern Mullaley Sub-basin and allowed the development of localised paraconglomerate (diamictite) intervals up to 10 m thick. Thick (>20 m cumulative) coal occurrences are localised to the Jacks Creek and Pilliga East State Forest areas southwest of Narrabri. The coal resource potential of the Mullaley Sub-basin is estimated at 13–28 billion tonnes.

冈尼达盆地（Gunnedah Basin）马拉利次盆地（Mullaley Sub-basin）的分布范围为：东南起自奎伦迪（Quirindi），北至纳拉布里（Narrabri）以北，西抵新南威尔士州北部的杜尼杜（Dunedoo）以西。自20世纪90年代初以来，科研人员已施工超过100个钻至基底的钻孔，用于勘探马拉利次盆地的下二叠统乌拉尔阶（Cisuralian）煤炭与煤层气资源。本次对该公开档案资料的桌面地质调研，完成了6个乌拉尔阶煤组的正式地层对比与命名：这些煤组累计最大厚度可达35米，赋存于总厚度不超过150米的向上变粗沉积序列中。按地层上升顺序，该套煤组依次为：比布伦迪煤组（Bibblewindi，厚度0–10米）、博赫纳煤组（Bohena，3–18米）、科利格拉煤组（Collygra，0.5–3米）、考克斯煤组（Coxs，1.5–4米）、塔拉穆伦煤组（Tullamullen，0.5–4米）与穆基煤组（Mooki，0.5–3米）。本次研究将马拉利次盆地南部莫尔斯克里克组（Maules Creek Formation）内的乌拉尔阶煤层，与沃里斯克里克（Werris Creek）和马斯韦尔布鲁克（Muswellbrook）地区的格雷塔煤系（Greta Coal Measures）进行了地层对比。研究表明，基底古地形对乌拉尔阶煤层的发育具有显著控制作用：当沉积序列厚度大于60米时，煤层发育最佳，此时厚层煤层（博赫纳煤组与比布伦迪煤组）赋存于序列底部。基于新钻探资料，乌拉尔阶煤层的最西界（罗克利格伦岭，Rocky Glen Ridge）较此前推断的位置更偏东：新数据显示，波库平组（Porcupine Formation）直接覆于利尔德组（Leard Formation）的无煤球粒状黏土岩，或下伏的博加布里火山岩（Boggabri Volcanics）/韦里玄武岩（Werrie Basalt）之上。莫尔斯克里克组顶部的早期海侵事件，抑制了马拉利次盆地北部与东部地区穆基、塔拉穆伦与考克斯煤组的发育，并形成了累计厚度可达10米的局部分布似砾岩（杂砾岩，paraconglomerate/diamictite）夹层。累计厚度大于20米的煤层仅局部分布于纳拉布里西南部的杰克溪（Jacks Creek）与皮利加东州立森林（Pilliga East State Forest）区域。马拉利次盆地的煤炭资源潜力估算为130亿至280亿吨。