Downward hydrocarbon migration predicted from numerical modeling of fluid overpressure in the Paleozoic Anticosti Basin,eastern Canada

The Anticosti Basin is a large Paleozoic basin in eastern Canada where potential source and reservoir rocks have been identified but no economic hydrocarbon reservoirs have been found. Potential source rocks of the Upper Ordovician Macasty Formation overlie carbonates of the Middle Ordovician Mingan Formation, which are underlain by dolostones of the Lower Ordovician Romaine Formation. These carbonates have been subjected to dissolution and dolomitization and are potential hydrocarbon reservoirs. Numerical simulations of fluid‐overpressure development related to sediment compaction and hydrocarbon generation were carried out to investigate whether hydrocarbons generated in the Macasty Formation could migrate downward into the underlying Mingan and Romaine formations. The modeling results indicate that, in the central part of the basin, maximum fluid overpressures developed above the Macasty Formation due to rapid sedimentation. This overpressured core dissipated gradually with time, but the overpressure pattern (i.e. maximum overpressure above source rock) was maintained during the generation of oil and gas. The downward impelling force associated with fluid‐overpressure gradients in the central part of the basin was stronger than the buoyancy force for oil, whereas the buoyancy force for gas and for oil generated in the later stage of the basin is stronger than the overpressure‐related force. Based on these results, it is proposed that oil generated from the Macasty Formation in the central part of the basin first moved downward into the Mingan and Romaine formations, and then migrated laterally up‐dip toward the basin margin, whereas gas throughout the basin and oil generated in the northern part of the basin generally moved upward. Consequently, gas reservoirs are predicted to occur in the upper part of the basin, whereas oil reservoirs are more likely to be found in the strata below the source rocks. Geofluids (2010) 10 , 334–350     

Potential of palaeofluid analysis for understanding oil charge history

Fluid inclusion data, particularly the distribution of hydrocarbon fluid inclusions and their chemistry, can provide insights into oil charge in a petroleum-prospective region. Examples from the UK Atlantic margin show how we can understand thermal regime, timing and chemistry of oil charge. Data from the UK Atlantic margin based on fluid inclusion temperature profiles shows anomalously high temperatures which are highest at the top of the Triassic–Eocene sequence. This is interpreted as a product of hot fluid flow, probably reflecting hydrothermal activity related to intrusion of sills at depth. The preservation of high temperatures also implies rapid migration from depth through fracture systems. Ar–Ar analysis of oil-bearing K-feldspar cements, and petrographic studies of oil inclusion distribution help delimit timing and migration pathways for the hot fluid charge and later fluid migration events. Coupled with compositional data for oils measured destructively (organic geochemistry) or non-destructively (fluorescence), these approaches allow the development of oil charge histories based on real data rather than theoretical modelling.     

Sampling and analysis of geothermal fluids

Sampling of geothermal fluids presents some problems not encountered when sampling surface and nonthermal ground waters. Specific collection techniques are required to obtain representative samples because of the elevated temperature and boiling of these fluids, the effect of exposing them to the atmosphere and cooling of the samples. Sample treatment during collection depends on the analytical method to be used. When sampling wet‐steam wells, both the liquid and the vapour fractions should be collected at the same fluid separation pressure. When sampling fumarole steam, maximum information is obtained if the total discharge is collected into a single container without separating the gas and the steam condensate fractions. Silica polymerization affects the solution pH. The only way to obtain reliable pH measurement of a water sample supersaturated with respect to amorphous silica is to measure it on site, before the onset of polymerization. This paper provides an outline of the geothermal sampling techniques and analytical methods currently in use in Iceland. Sampling of hot‐water and wet‐steam wells is described, as is sampling of hot springs, fumaroles and gas bubbling through hot‐spring waters. Detailed procedures are given for the analysis of total carbonate carbon and total sulphide sulphur in geothermal water and steam condensate samples.     

On sampling the upper crustal reservoir of the NE Pacific Ocean

The oceanic upper crustal reservoir is a 600‐m thick layer of porous and permeable basaltic rock that forms the uppermost igneous basement underlying the global ocean. Pore spaces within this fluid aquifer contain a significant fraction of the global seawater, and active circulation through this reservoir has profound influence on the chemical composition of the ocean, strongly impacting the biological environment near the sea surface. Because of the relative inaccessibility of the deep seafloor, where hydrothermal fluid discharges and seawater re‐charges the oceanic crustal aquifer, our understanding of the dynamic physical, chemical and biological processes is strongly dependent on our ability to obtain uncontaminated samples from this challenging environment. Recent technological advances have addressed some, but certainly not all of these sampling problems, providing new data and samples that test our current hypotheses about the crustal fluid reservoir. Current scientific interest in the sub‐seafloor biosphere has focused on the uppermost igneous oceanic crust as likely to be one of the most habitable environments, because of its porosity and locus of hydrothermal circulation of chemical nutrients. Recent observations indicate that sub‐seafloor crustal environments harbor novel CO₂‐utilizing bacteria (primary producers) that could be a significant source of carbon‐fixation in the ocean, thus broadening possible habitable zones both on Earth and elsewhere where microbial life could exist independent of nutrient input from photosynthesis.     

Real‐time mud gas logging and sampling during drilling

Continuous mud gas loggings during drilling as well as offline mud gas sampling are standard procedures in oil and gas operations, where they are used to test reservoir rocks for hydrocarbons while drilling. Our research group has developed real‐time mud gas monitoring techniques for scientific drilling in non‐hydrocarbon formations mainly to sample and study the composition of crustal gases. We describe in detail this technique and provide examples for the evaluation of the continuous gas logs, which are not always easy to interpret. Hydrocarbons, helium, radon and with limitations carbon dioxide and hydrogen are the most suitable gases for the detection of fluid‐bearing horizons, shear zones, open fractures, sections of enhanced permeability and permafrost methane hydrate occurrences. Off‐site isotope studies on mud gas samples helped reveal the origin and evolution of deep‐seated crustal fluids.     

Hydraulic observations from a 1 year fluid production test in the 4000 m deep KTB pilot borehole

A long‐term pump test was conducted in the KTB pilot borehole (KTB‐VB), located in the Oberpfalz area, Germany. It produced 22 300 m³ of formation fluid. Initially, fluid production rate was 29 l min^?1 for 4 months, but was then raised to an average of 57 l min^?1 for eight more months. The aim of this study was to examine the fluid parameters and hydraulic properties of fractured, crystalline crusts as part of the new KTB programme ‘Energy and Fluid Transport in Continental Fault Systems’. KTB‐VB has an open‐hole section from 3850 to 4000 m depth that is in hydraulic contact with a prominent continental fault system in the area, called SE2. Salinity and temperature of the fluid inside the borehole, and consequently hydrostatic pressure, changed significantly throughout the test. Influence of these quantities on variations in fluid density had to be taken into account for interpretation of the pump test. Modelling of the pressure response related to the pumping was achieved assuming the validity of linear Darcy flow and permeability to be independent of the flow rate. Following the principle ‘minimum in model dimension’, we first examined whether the pressure response can be explained by an equivalent model where rock properties around the borehole are axially symmetric. Calculations show that the observed pressure data in KTB‐VB can in fact be reproduced through such a configuration. For the period of high pumping rate (57 l min^?1) and the following recovery phase, the resulting parameters are 2.4 × 10^?13 m³ in hydraulic transmissivity and 3.7 × 10^?9 m Pa^?1 in storativity for radial distances up to 187 m, and 4.7 × 10^?14 m³ and 6.0 × 10^?9 m Pa^?1, respectively, for radial distances between 187 and 1200 m. The former pair of values mainly reflect the hydraulic properties of the fault zone SE2. For a more realistic hydraulic study on a greater scale, program FEFLOW was used. Parameter values were obtained by matching the calculated induced pressure signal to fluid‐level variations observed in the KTB main hole (KTB‐HB) located at 200 m radial distance from KTB‐VB. KTB‐HB is uncased from 9031 to 9100 m and shows indications of leakage in the casing at depths 5200–5600 m. Analysis of the pressure record and hydraulic modelling suggest the existence of a weak hydraulic communication between the two boreholes, probably at depths around the leakage. Hydraulic modelling of a major slug‐test in KTB‐HB that was run during the pumping in KTB‐VB reveals the effective transmissivity of the connected formation to be 1 to 2 orders of magnitude lower than the one determined for the SE2 fault zone.     

节庆事件对区域旅游流空间网络的影响效应和机制研究

以洛阳牡丹花节为例,通过多源数据的融合,运用社会网络、因子分析方法,探究节庆事件对区域旅游流空间网络的影响效应及机制。结果表明：节庆事件对区域旅游流空间网络影响表现为协同、弱化及蜂聚三种效应,不同效应影响机制不同。其中,节点的等级和知名度优势非常明显的态势下,产生显著的协同效应;节庆游憩区内的低等级节点出现轻微的节庆弱化效应,区域外节点主要受到空间交通距离限制产生了显著的节庆弱化效应;旅游等级很强且旅游承载量优势非常明显的非节庆旅游节点,以及等级、知名度或地理区位较强的节庆旅游节点呈现出强蜂聚效应,以上条件均不强的节庆旅游节点仅发生弱集聚效应。     

Geomorphic reconstruction of formation and recession processes of waterfalls of the Kaminokawa river basin on Osumi Peninsula,southern Kyushu,Japan

To assess the geomorphological importance of waterfall recession in volcanic bedrock, we examined recession rates of six waterfalls in the lower reaches of Kaminokawa river basin on the Osumi Peninsula in southern Kyushu. The examination was performed with an empirical equation that uses a dimensionless parameter obtained by dimensional analysis of relevant measured factors, including erosive force, size of waterfall, and bedrock resistance. Welded Ata ignimbrite, formed at approximately 110 ka, may have played an initiating role to maintain such waterfalls because it resists weathering more than other local rocks. Estimated recession rates for the six waterfalls range from 0.2 to 3.0 cm/y, which compare with estimated rates for waterfalls in another region characterised by welded ignimbrite. Comparison of equation‐derived recession rates of waterfalls with actual recession distances from confluences supports the idea that an original waterfall will subsequently split into two distinct waterfalls when it recesses past an upstream junction of two channels. Our findings revealed that all six waterfalls likely would have been at almost the same point lower in the watershed in the past, marking the general site of the original waterfall. Moreover, the ancestral original waterfall is highly likely to have started from a point between the caldera rim and present river mouth. There, a knickpoint was likely caused by the river dropping into an inner part of the caldera, possibly just after the eruption of the Ata ignimbrite. The waterfall erodes upstream away from the caldera basin, and this happens to be to the east.