Abstract

In this paper, we describe porosity variations in Miocene to Holocene carbonates off the west coast of Australia and assess their compaction trends. The porosity values were measured using discrete samples of Sites U1459–U1464 obtained by the International Ocean Discovery Program Expedition 356. The carbonate deposits have been influenced by a range of textures and diagenetic conditions throughout a nearly continuous sequence of geological ages from the Miocene to Holocene and at core depths from 0 to 1100 m below the seafloor. The collected samples were mostly grainstone, packstone, wackestone and mudstone textures. Dolostones and dolomitic carbonates were described at the Miocene intervals. Compaction trends were estimated exponentially and linearly based on cored sites, carbonate textures and dominant mineralogies (dolomite, calcite/aragonite). At all six sites, porosity distribution and reduction were generally depth-dependent. The porosity converged to about 30% between 750 and 1100 m, which suggests that the carbonates were close to the densest packing by mechanical compaction at a burial depth of ∼750 m. The porosity deviations are associated with textures and dominant mineralogies. Increasing mud content from grainstone to mudstone is a controlling factor for initial porosity and porosity reduction rate. Dolomitisation, dolomitic cementation, aragonite needle-rich mud and non-skeletal grains cause deviations from the depth-dependent compaction trends. Reflux-related cementation generally decreases porosity in Miocene dolomitic intervals. Higher porosity values of the Quaternary wackestone and mudstone at Site U1461 resulted from the presence of aragonite needle-rich mud hosting abundant micropores and from a high sedimentation rate. The occurrence of non-skeletal grains, such as ooids and peloids, as well as occasional meteoric exposure led to porosity inversion, occluding interparticle permeability and the creation of moldic pores.

1. KEY POINTS

2. Porosity variations of Miocene–Holocene carbonates off the west Australian coast are primarily depth-dependent.

3. Mechanical compaction affected porosity reduction down to a value of ∼30% at a burial depth of ∼750 m.

4. Differences in texture-based compaction trends were affected by increasing mud content.

5. Major porosity deviations are associated with dolomitisation, dolomitic cementation, aragonite needle-rich mud and non-skeletal grains.

摘要

在本文中，我们描述了澳大利亚西海岸外中新世至全新世碳酸盐岩的孔隙度变化，并评估了它们的压实趋势。孔隙度值是使用由国际海洋发现计划远征 356 获得的 U1459-U1464 站点的离散样本测量的。碳酸盐沉积物在从中新世到全新世的几乎连续的地质年代序列中受到一系列结构和成岩条件的影响以及海底以下 0 至 1100 m 的核心深度。采集到的样品主要为颗粒岩、灰岩、泥岩和泥岩结构。白云岩和白云质碳酸盐被描述在中新世间隔。根据取芯地点、碳酸盐结构和主要矿物学（白云石、方解石/文石）以指数和线性方式估计压实趋势。在所有六个地点，孔隙度分布和减少通常与深度有关。孔隙度在 750 至 1100 m 之间收敛到约 30%，这表明碳酸盐接近于约 750 m 埋藏深度的机械压实作用最致密的堆积。孔隙度偏差与结构和主要矿物学有关。增加从颗粒岩到泥岩的泥浆含量是初始孔隙度和孔隙度减少率的控制因素。白云石化、白云质胶结、文石富针泥和非骨架颗粒导致与深度相关的压实趋势的偏差。回流相关的胶结作用通常会降低中新世白云岩层段的孔隙度。U1461 位点的第四纪泥岩和泥岩的较高孔隙度值是由于含有丰富的微孔的文石针状泥浆的存在和高沉积速率所致。非骨架颗粒的出现，如鲕粒和球粒，以及偶尔的大气暴露导致孔隙度反演，堵塞粒间渗透性和铸模孔的产生。

1. 关键点

2. 西澳大利亚海岸中新世-全新世碳酸盐岩的孔隙度变化主要取决于深度。

3. 机械压实使孔隙度降低至约 750 m 埋深约 30% 的值。

4. 基于质地的压实趋势的差异受到泥浆含量增加的影响。

5. 主要的孔隙度偏差与白云石化、白云质胶结、文石富针泥和非骨架颗粒有关。