Vertical Migration in Theory and in Practice

We use the term “vertical migration” to describe the mechanism of mass transport that moves trace petroleum hydrocarbons from subsurface accumulations to the surface. Vertical migration is the heart of geochemical exploration for petroleum. While our use of geochemical exploration for petroleum has been accelerating, our understanding of the mechanism that makes the process successful lagged behind. Early vertical migration models were based on a molecular process called diffusion. Diffusion remained the accepted mechanism during the early years of geochemical exploration even though the diffusion mechanism could not explain many observations, including migration that appeared predominately vertical. About 30 years ago buoyancy driven migration mechanisms were modeled. The models predicted how gases below the water table could migrate vertically as a gas phase with buoyancy providing a mechanism for predominately vertical gas migration. Buoyancy models explained how surface expressions observed from vertical migration measurements related to petroleum reservoirs including small lateral offsets. Buoyancy models also explained gradients and data contrasts observed in many surface geochemical features including what they mean and predicted fast vertical migration rates that have been verified by field observations. In addition, buoyancy models predicted how petroleum reservoirs could be mapped using gas data and how surface expressions of faults and fractures could be mapped using liquid data. Those data can be obtained from direct measurement of trace hydrocarbons in near-surface fluid samples. Since reservoir rock samples are not required, vertical migration provides a way to obtain information about a reservoir prior to exploratory drilling. Characteristics of these migrating fluids and their surface patterns are the foundation of modern geochemical exploration for petroleum.

Application of the Mineralogy and Mineral Chemistry of Carbonates as a Genetic Tool in the Hydrothermal Environment

The mineralogy and mineral chemistry of carbonates from various hydrothermal deposits, including volcanic-hosted Au-Cu epithermal, “Chilean Manto-type” Cu(-Ag), stratabound Mn, and Ag-Ba vein deposits from Spain and Chile, were investigated. Dolomite-ankerite (±siderite) was found in variable amounts within the epithermal deposits and associated hydrothermal alteration, whereas calcite was found either within barren veins or disseminated within the regional alteration. Calcite is the major gangue phase within the stratabound deposits, which tend to lack dolomite/ankerite and siderite. Carbonates precipitated from hydrothermal ore fluids are typically Mn-rich, up to 3.55 at. % in siderite, 2.27 at. % in dolomite/ankerite, and 1.92 at. % in calcite. In contrast, calcite related to very low-grade metamorphism or regional low-temperature alteration is Mn-poor but sometimes Mg-rich, possibly related to a higher temperature of formation. Chemical zonation was observed in the hydrothermal carbonates, although no unique pattern and chemical evolution was observed. This study suggests that the chemical composition of carbonates, especially the Mn content, could be a useful vector within ore-forming hydrothermal systems, and therefore constitutes a possible tool in geochemical exploration. Furthermore, Mn-poor calcites detected in some deposits are suggested to be linked with a later episode, maybe suggesting a predominance of meteoric waters, being not related to the main ore stage formation, thus avoiding misunderstanding of further isotopic studies.

Quantitative Remote Sensing of Metallic Elements for the Qishitan Gold Polymetallic Mining Area, NW China

The recent development in remote sensing imagery and the use of remote sensing detection feature spectrum information together with the geochemical data is very useful for the surface element quantitative remote sensing inversion study. This aim of this article is to select appropriate methods that would make it possible to have rapid economic prospecting. The Qishitan gold polymetallic deposit in the Xinjiang Uygur Autonomous Region, Northwest China has been selected for this study. This paper establishes inversion maps based on the contents of metallic elements by integrating geochemical exploration data with ASTER and WorldView-2 remote sensing data. Inversion modelling maps for As, Cu, Hg, Mo, Pb, and Zn are consistent with the corresponding geochemical anomaly maps, which provide a reference for metallic ore prospecting in the study area. ASTER spectrum covers short-wave infrared and has better accuracy than WorldView-2 data for the inversion of some elements (e.g., Au, Hg, Pb, and As). However, the high spatial resolution of WorldView-2 drives the final content inversion map to be more precise and to better localize the anomaly centers of the inversion results. After scale conversion by re-sampling and kriging interpolation, the modeled and predicted accuracy of the models with square interpolation is much closer compare with the ground resolution of the used remote sensing data. This means our results are much satisfactory as compared to other interpolation methods. This study proves that quantitative remote sensing has great potential in ore prospecting and can be applied to replace traditional geochemical exploration to some extent.

The (U-Th)/He Chronology and Geochemistry of Ferruginous Nodules and Pisoliths Formed in the Paleochannel Environments at the Garden Well Gold Deposit, Yilgarn Craton of Western Australia: Implications for Landscape Evolution and Geochemical Exploration

Ferruginous nodules and pisoliths that cap deeply weathered profiles and transported cover are characteristic of the Yilgarn Craton, Western Australia. Here we show how ferruginous nodules and pisoliths formed in the paleochannel sediments during Miocene can be used to locate buried Au mineralization. Three types of ferruginous nodules and pisoliths were identified in paleochannel sediments and saprolite, representing different parent materials and environments covering the Garden Well Au deposit: (i) ferruginous nodules formed in saprolite on the flanks of the paleochannel (NSP), (ii) ferruginous pisoliths formed in the Perkolilli Shale in the middle of the paleochannel (PPS) and (iii) ferruginous nodules formed in the Wollubar Sandstone at the bottom of the paleochannel (NWS). The appearance, mineralogy and geochemistry of ferruginous nodules and pisoliths vary according to their origin. The PPS and NWS are goethite-rich whereas NSP is a mixture of goethite and hematite which make them all suitable for (U–Th)/He dating. The average age of goethite in the NSP is 14.8 Ma, in the NWS is 11.2 Ma and in the PPS is 18.6 and 14 Ma. The goethite ages in ferruginous nodules and pisoliths are thought to be younger than the underlying saprolite (Paleocene-Eocene) and were formed in different environmental conditions than the underlying saprolite. Anomalous concentrations of Au, As, Cu, Sb, In, Se, Bi, and S in the cores and cortices of the NWS and the PPS reflect the underlying Au mineralization, and thus these nodules and pisoliths are useful sample media for geochemical exploration in this area. These elements originating in mineralized saprolite have migrated both upwards and laterally into the NWS and the PPS, to form spatially large targets for mineral exploration.