Palaeogeographic reconstructions: Revealing the lithological variability of the Gage Sandstone and South Perth Shale, Offshore Vlaming Sub-basin

The Early Cretaceous Gage Sandstone and South Perth Shale are a prospective reservoir-seal pair in the Vlaming Sub-basin. Plays include post-breakup pinch-outs of the Gage Sandstone against the Valanginian Unconformity with the South Perth Shale forming a top seal. The Gage reservoir has core porosities of 9.5-23.9% and permeabilities from 52-1340 mD. It was deposited in palaeotopographic lows of the Valanginian breakup unconformity and is the lowstand component of the thick deltaic South Perth (SP) Supersequence. To characterise the reservoir-seal pair, a sequence stratigraphic analysis was conducted by integrating 2D seismic interpretation, well log analysis and new biostratigraphic data. Palaeogeographic reconstructions for the SP Supersequence were derived by mapping higher-order prograding packages and documenting changes in relative sea level and sediment supply. High resolution reconstructions of the Gage reservoir were based on seismic facies mapping. The Gage reservoir forms part of a sand-rich submarine fan system and ranges from canyon-confined inner fan deposits to middle fan deposits on a basin plain. Directions of sediment supply are complex, with major sediment contributions from a northern and southern canyon adjacent to the Mandurah Terrace. The characteristics of the SP Supersequence differ between the north and south due to variations in palaeotopography and sediment supply. Palaeogeographic reconstructions for the SP Supersequence portray a complex early post-rift depositional history in the central Vlaming Sub-basin. They reveal a series of regressions and transgressions leading to infilling of the palaeodepression.

APPEA

早白垩世（Early Cretaceous）盖奇砂岩（Gage Sandstone）与南珀斯页岩（South Perth Shale）是弗拉明次盆地（Vlaming Sub-basin）内一套具有勘探潜力的储盖组合。该储盖组合的圈闭类型包括盖奇砂岩在凡兰吟阶不整合面（Valanginian Unconformity）之上的裂后尖灭体，南珀斯页岩则充当顶部盖层。盖奇储层的岩心孔隙度为9.5%~23.9%，渗透率介于52~1340毫达西（mD）之间。该储层沉积于凡兰吟阶破裂不整合面的古地形低洼区，属于厚层三角洲成因的南珀斯（SP）超层序（South Perth (SP) Supersequence）的低位体系域组分。为表征这套储盖组合，研究人员通过整合二维地震解释、测井分析与最新生物地层数据开展了层序地层分析。针对南珀斯超层序的古地理重建，通过刻画高级次进积单元并记录相对海平面与沉积物供给的变化完成。盖奇储层的高精度重建则基于地震相制图工作。盖奇储层隶属于富砂海底扇系统，其沉积序列涵盖盆地平原上受峡谷限定的内扇沉积至中扇沉积。沉积物供给方向较为复杂，主要补给来源为曼杜拉台地（Mandurah Terrace）周边的北部与南部峡谷。受古地形与沉积物供给差异的影响，南珀斯超层序的特征在盆地南北区域存在显著分异。南珀斯超层序的古地理重建结果揭示了弗拉明次盆地中部复杂的裂后早期沉积演化历史，展现出一系列海退与海进事件，最终完成了古凹陷的充填过程。APPEA