Dynamic phase permeability for calculating oil locations in digital models

The article presents the author's formulas for calculating the residual oil saturation and the end points of relative phase permeabilities that dynamically depend on the filtration rate of reservoir fluids and capillary numbers. The dependences of the residual oil saturation and the end points of the phase permeabilities on the capillary number are investigated and described. An element of a five-point system for the development of an oil deposit case study shows the possibility of calculating oil targets using dynamic phase permeabilities. The difference between the model with static relative phase permeabilities and the model with dynamic phase permeabilities should be stressed. The text gives valuable information on the dependence of the simulation results on the parameters of the nonlinearity of the filtration processes with the traditional filtration-capacitance properties of the oil deposit model unchanged.

A New Model for the Genesis of Carboniferous Mn Ores, Longtou Deposit, South China Block

The lower Carboniferous Luzhai and Baping Formations (ca. 359 Ma) of the South China block, Guangxi Province, comprise an ca. 170-m-thick clastic-carbonate succession capped by Mn ore horizons near the town of Longtou. Excellent exposure of the stratigraphic succession provides an unparalleled opportunity to investigate the origin of carbonate-hosted Mn deposits, which are generally understudied. Lithofacies associations suggest inner and middle shelf clastic rocks accumulated with deposition of carbonates on a mesotrophic middle to outer shelf. In the Longtou region, carbonate deposition during marine transgression culminated with the precipitation of high-grade Mn deposits during maximum flooding. Mn ore horizons are composed of amalgamated alabandite-bearing rhodochrosite, Mn calcite, and braunite laminae. Mn carbonates have been largely interpreted as forming in oxic water columns via reduction of Mn oxides by organic matter. However, paragenetic relationships and δ13C values (similar to those of seawater) indicate the Mn carbonates of Longtou were formed during authigenesis by the emplacement of anoxic, Mn-rich water masses on the distal to middle shelf. Such anoxia is interpreted to have shut down the carbonate factory and diminished sedimentation, a prerequisite for the concentration and precipitation of Mn carbonates in pore water. This research supports the notion that areas of the Paleozoic deep ocean were persistently anoxic and periodically tapped by coastal upwelling to produce Mn- and Fe-rich deposits. Application of this emerging ore deposit model to other economically important carbonate-hosted Mn deposits may improve resource exploration.

The Neoarchean Conglomerate-Hosted Gold of the West Pilbara Craton, Western Australia

Recently discovered Au in boulder conglomerate between the Mesoarchean West Pilbara superterrane basement and the overlying volcano-sedimentary stratigraphy of the Neoarchean Fortescue Group in Western Australia has renewed comparisons with the Witwatersrand conglomerate Au deposits in South Africa. As such, this has reignited the question of the Pilbara and Kaapvaal cratons being linked as part of the postulated Vaalbara continent during the Archean. However, little is known about the origin of the Pilbara conglomerate Au and its host conglomerates, as they are hitherto unstudied, and their formation and/or source is uncertain. Here we present a detailed study on the textures, composition, and sedimentology of one newly discovered Pilbara conglomerate Au deposit at the base of the Neoarchean Fortescue Group in the northwestern Pilbara craton. The Pilbara conglomerate Au occurrences are characteristically Ag-bearing but Hg-poor polycrystalline discoid masses that are overgrown by Au-poor chloritic halos, which are further enveloped by a hydrothermal alteration halo of disseminated Au within chlorite. Both the discoids and the auriferous chlorite halo are Ag bearing, with up to ~9 wt % Ag, consistent with a hydrothermal (orogenic) origin. The discoids do not display any physical or chemical evidence for sedimentary transport; thus, their formation (placer versus hydrothermal) remains unclear. However, the position of the Au in the conglomerate, limited to the basal section of the conglomerate, is difficult to account for in a purely hydrothermal deposit model. We argue the Pilbara conglomerate Au represents a modified placer deposit from a primary orogenic Au source, with surface evidence for sedimentation removed by partial dissolution during later hydrothermal alteration in the host conglomerate and the crystalline basement. While the basal Fortescue Group conglomerate Au shares commonalities with the time equivalent (>~2.7 Ga) Venterspost Conglomerate Formation, which overlies the Witwatersrand Supergroup, inconsistencies remain, with different Au chemistries and tectonic, magmatic, sedimentary, and metamorphic-metallogenic histories of the Pilbara and Kaapvaal cratons prior to deposition of the >2.7 Ga conglomerate sequences. This collectively indicates the drivers of Au metallogenesis and ultimate Au deposition in conglomerate facies were fundamentally different in the Pilbara and Kaapvaal cratons.

A NEW SUBSEAFLOOR REPLACEMENT MODEL FOR THE MACMILLAN PASS CLASTIC-DOMINANT Zn-Pb ± Ba DEPOSITS (YUKON, CANADA)

Sedimentary exhalative (SEDEX) deposits are a subset of sediment-hosted massive sulfide deposits and provide our dominant resource of Zn. In the SEDEX model, base metals (Zn, Pb, Fe) are hydrothermally vented into sulfidic (euxinic) seawater and deposited coevally with the organic-rich mudstone host rock, resulting in laterally extensive layered mineralization. In the Selwyn Basin (Canada) at Macmillan Pass, two deposits (Tom, Jason) are well preserved in a succession of Upper Devonian mudstones and are considered type-characteristic examples of the SEDEX deposit model. As with a number of SEDEX deposits, at Macmillan Pass barite is abundant in the succession hosting hydrothermal mineralization. Early work presented a hydrothermal model for barite formation, in which barite coprecipitated with base metal sulfides in a redox-stratified water column. Recently, however, studies have both proposed an alternative diagenetic model for barite formation and provided more precise constraints on the chemistry of the hydrothermal fluid that entered the vent complexes. Here, we present a new model for Macmillan Pass in which sulfide mineralization occurred entirely within the subsurface. The introduction of hot (300°C) hydrothermal fluids into the shallow subsurface (<1-km depth) resulted in the thermal degradation of organic matter and generated CO2; this promoted barite dissolution, which both provided a source of sulfate for thermochemical sulfate reduction and increased the porosity and permeability within the system. Importantly, there was clear potential for the development of positive feedbacks and self-organization between diagenetic and hydrothermal processes, resulting in highly efficient ore-forming systems. In contrast to the SEDEX model, alteration footprints will be controlled by the mass transfer involved in (barite) replacement reactions rather than hydrothermal venting, and exploration criteria at a district scale should strongly favor highly productive continental margins.

Ore Geology, Fluid Inclusions, and (H-O-S-Pb) Isotope Geochemistry of the Sediment-Hosted Antimony Mineralization, Lyhamyar Sb Deposit, Southern Shan Plateau, Eastern Myanmar: Implications for Ore Genesis

The Lyhamyar deposit is a large Sb deposit in the Southern Shan Plateau, Eastern Myanmar. The deposit is located in the Early Silurian Linwe Formation, occurring as syntectonic quartz-stibnite veins. The ore body forms an irregular staircase shape, probably related to steep faulting. Based on the mineral assemblages and cross-cutting relationships, the deposit shows two mineralization stages: (1) the pre-ore sedimentary and diagenetic stage, and (2) the main-ore hydrothermal ore-forming stage (including stages I, II, and III), i.e., (i) early-ore stage (stage I) Quartz-Stibnite, (ii) late-ore stage (stage II) Quartz-calcite-Stibnite ± Pyrite, and (iii) post-ore stage (stage III) carbonate. The ore-forming fluid homogenization temperatures from the study of primary fluid inclusions in quartz and calcite indicate that the ore-forming fluid was of a low temperature (143.8–260.4 °C) and moderate to high-salinity (2.9–20.9 wt. % NaCl equivalent). Hydrogen and oxygen isotopes suggest that the ore-forming fluids of the Lyhamyar deposit were derived from circulating meteoric water mixed with magmatic fluids that underwent isotopic exchange with the surrounding rocks. Sulfur in Lyhamyar was dominated by thermochemical sulfate reduction (TSR) with dominant magmatic source sulfur. The lead isotope compositions of the stibnite indicate that the lead from the ore-forming metals was from the upper crustal lead reservoir and orogenic lead reservoir. On the basis of the integrated geological setting, ore geology, fluid inclusions, (H-O-S-Pb) isotope data, and previous literature, we propose a new ore-deposit model for the Lyhamyar Sb deposit: It was involved in an early deposition of pyrite in sedimentary and diagenetic stages and later Sb mineralization by mixing of circulating meteoric water with ascending magmatic fluids during the hydrothermal mineralization stage.