CSEM technology as a value driver for hydrocarbon exploration

The use of the controlled source electromagnetic (CSEM) method as a risk reduction tool in marine hydrocarbon exploration is gaining increased acceptance in the oil industry. This is related to the ability to map resistivity contrasts in the sub-surface and thus aid the detection of hydrocarbons which are typically more resistive than surrounding rocks. Whereas acoustic (seismic) energy allows for mapping sub-surface structures that may contain hydrocarbons, electromagnetic (EM) energy can often say something about the fluids contained within the structures. Numerous successful CSEM case stories have been published over the past several years. However, there are also quite a few stories about “failure”-cases, although not well documented in the literature. Such “failure”-cases may reflect the lack of understanding of the CSEM technology and how CSEM data can act as a value driver. In order to understand this, it is necessary to handle uncertainties and decisions associated with the technology, These include geological uncertainties, noise models, survey designs, forward modelling parameters, inversion/migration parameters and pre-processing of real data. A proper handling of these uncertainties and decisions will aid in defining and constraining the chance of geologic success and geologic-success-case net present value for prospects prior to drilling wells. As such, the CSEM technology has a significant potential to increase exploration efficiency, if applied correctly.     

Erosion of the eastern United States observed with 10Be

The problem of identifying areas of accelerated erosion in a dynamic landscape is complicated. The limited history of sediment yield measurements makes this task difficult even if geomorphic evidence is available. Beryllium-10, a cosmogenic isotope produced by cosmic rays interacting with the earth's atmosphere and surface, has chemical and physical properties that make it useful as a tracer for erosion and sediment transport processes. The rarity of the stable isotope, ⁹Be, allows ¹⁰Be to be detected with accelerator mass spectrometry in natural materials at extremely low levels. Backgrounds for rocks and sediments below 10⁵ atom per g are now attainable, a value to be compared with an average deposition rate of 1.3 × 10⁶ atom cm^?2 yr^?1. The affinity of Be for the components of soil and sediment is sufficiently high that it is effectively immobilized on contact, thereby allowing ¹⁰Be to function as a tracer of sediment transport. To a good approximation all the ¹⁰Be transport out of a drainage basin is on the sediment leaving it. The number of ¹⁰Be atoms passing the gauging station can be determined by measuring the concentration of the isotope in the sediment, if the annual sediment load is known. The ratio of the ¹⁰Be carried from the basin by the sediment to that incident upon it, called the erosion index, has been determined for 48 drainage basins within the same physiographic province, which allows them to be reasonably compared, all of which have sediment yield data. Basins located in the Atlantic coastal plain have an average index of 0.3 with the maximum observed being 0.9. Basins located between the fall line and the mountains, a region called the Piedmont, have an average value of 2.2 with individual values ranging from 0.6 to 6.7; this marked difference is thought to result from two centuries of farming on land of moderate gradient. Basins in the highland regions reflect local conditions with low indices for those in grass and timber and high indices associated with destructive land use. The data allow an estimate of the erosion index for the pre-colonial Piedmont, which then allows the pre-colonial sediment yield to be calculated. A number of basins have also been examined world wide with similar conclusions derived. An important deviation from the rule is noted for rivers that erode large regions of loess, such as the Mississippi, Hwang Ho, and Yangtze. Large aeolian deposits were laid down during the ice age in these basins, deposits that brought inherited ¹⁰Be with them and that are easily eroded.     

Sedimentation rates, basin analysis and regional correlations of three Neoarchaean and Palaeoproterozoic sub-basins of the Kaapvaal craton as inferred from precise U–Pb zircon ages from volcaniclastic sediments

Calculation of sedimentation rates of Neoarchaean and Palaeoproterozoic siliciclastic and chemical sediments covering the Kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents. Zircons from tuff beds in carbonate facies of the Campbellrand Subgroup in the Ghaap Plateau region of the Griqualand West basin, Transvaal Supergroup, South Africa were dated using the Perth Consortium Sensitive High Resolution Ion Microprobe II (SHRIMP II). Dates of Ma and Ma for the middle and the upper part of the Nauga Formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below-wave-base Stratifera and clastic carbonate facies, southwest of the Ghaap Plateau at Prieska, was of up to 10 m/Ma, when not corrected for times of erosion and non-deposition. Dates of Ma for the upper Gamohaan Formation and for the upper Monteville Formation, indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 Ma to 62 Ma on the Ghaap Plateau. For these predominantly peritidal stromatolitic carbonates, decompacted sedimentation rates were of 40 m/Ma to over 150 m/Ma (Bubnoff units). The mixed siliciclastic and carbonate shelf facies of the Schmidtsdrif Subgroup and Monteville Formation accumulated with decompacted sedimentation rates of around 20 B. For the Kuruman Banded Iron Formation a decompacted sedimentation rate of up to 60 B can be calculated. Thus, for the entire examined deep shelf to tidal facies range, Archaean and Phanerozoic chemical and clastic sedimentation rates are comparable. Four major transgressive phases over the Kaapvaal craton, followed by shallowing-upward sedimentation, can be recognized in the Prieska and Ghaap Plateau sub-basins, in Griqualand West, and partly also in the Transvaal basin, and are attributed to second-order cycles of crustal evolution. First-order cycles of duration longer than 50 Ma can also be identified. The calculated sedimentation rates reflect the rate of subsidence of a rift-related basin and can be ascribed to tectonic and thermal subsidence. Comparison of the calculated sedimentation rates to published data from other Archaean and Proterozoic basins allows discussion of general Precambrian basin development. Siliciclastic and carbonate sedimentation rates of Archaean and Palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical, chemical and biological processes were active in the Precambrian as found for the Phanerozoic. Particularly for stromatolitic carbonates, matching modern and Neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite-building microbiota. The new data also allow for improved regional correlations across the Griqualand West basin and with the Malmani Subgroup carbonates in the Transvaal basin. The Nauga Formation carbonates in the southwest of the Griqualand West basin are significantly older than the Gamohaan Formation in the Ghaap Plateau region of this basin, but are in part, correlatives of the Oaktree Formation in the Transvaal and of parts of the Monteville Formation on the Ghaap Plateau.     

SLIP RATES OF THE RIYUE MT. FAULT AT DEZHOU SEGMENT SINCE LATE PLEISTOCENE

The Riyue Mt. Fault is a secondary fault controlled by the major regional boundary faults (East Kunlun Fault and Qilian-Haiyuan Fault). It lies in the interior of Qaidam-Qilianshan block and between the major regional boundary faults. The Riyue Mt. fault zone locates in the special tectonic setting which can provide some evidences for recent activity of outward extension of NE Tibetan plateau, so it is of significance to determine the activity of Riyue Mt. Fault since late Pleistocene to Holocene. In this paper, we have obtained some findings along the Dezhou segment of Riyue Mt. Fault by interpreting the piedmont alluvial fans, measuring fault scarps, and excavating trenches across the fault scarp. The findings are as follows:(1) Since the late Pleistocene, there are an alluvial fan f_p and three river terraces T₁-T₃ formed on the Dezhou segment. The abandonment age of f_p is approximately (21.2±0.6) ka, and that of the river terrace T₂ is (12.4±0.11) ka. (2) Since the late Pleistocene, the dextral strike-slip rate of the Riyue Mt. Fault is (2.41±0.25) mm/a. In the Holocene, the dextral strike-slip rate of the fault is (2.18±0.40) mm/a, and its vertical displacement rate is (0.24±0.16) mm/a. This result indicates that the dextral strike-slip rate of the Riyue Mt. Fault has not changed since the late Pleistocene. It is believed that, as one of the dextral strikeslip faults, sandwiched between the the regional big left-lateral strike-slip faults, the Riyue Mt. Fault didn't cut the boundary zone of the large block. What's more, the dextral strike-slip faults play an important role in the coordination of deformation between the sub-blocks during the long term growth and expansion of the northeast Tibetan plateau.     

Geochemistry of meteoric diagenesis in carbonate islands of the northern Bahamas: 2. Geochemical modelling and budgeting of diagenesis

Geochemical characterization and numerical modelling of surface water and ground water, combined with hydrological observations, provide quantitative estimates of meteoric diagenesis in Pleistocene carbonates of the northern Bahamas. Meteoric waters equilibrate with aragonite, but water‐ rather than mineral‐controlled reactions dominate. Dissolutional lowering of the undifferentiated bedrock surface is an order of magnitude slower than that within soil‐filled topographic hollows, generating small‐scale relief at a rate of 65–140 mm ka^?1 and a distinctive pocketed topography. Oxidation of organic matter within the subsoil and vadose zones generates an average P of 4·0 × 10^?3 atm, which drives dissolution during vadose percolation and/or at the water table. However, these dissolution processes together account for <60% of the average rock‐derived calcium in groundwaters pumped from the freshwater lens. The additional calcium may derive from oxidation of organic carbon within the lens, accounting for the high P of the lens waters. Mixing between meteoric waters of differing chemistry is diagenetically insignificant, but evapotranspiration from the shallow water table is an important drive for subsurface cementation. Porosity generation in the shallow vadose zone averages 1·6–3·2% ka^?1. Phreatic meteoric diagenesis is focused near the water table, where dissolution generates porosity at 1·4–2·8% ka^?1. Maximum dissolution rates, however, are similar to those of evaporation‐driven precipitation, which occludes porosity of 4·0 ± 0·6% ka^?1. This drives porosity inversion, from primary interparticle to secondary mouldic, vug and channel porosity. In the deeper freshwater lens, oxidation of residual organic carbon and reoxidation of reduced sulphur species from deeper anaerobic oxidation of organic carbon may generate porosity up to 0·06% ka^?1. Meteoric diagenesis relies critically on hydrological routing and vadose thickness (controlled by sea level), as well as the geochemical processes active. A thin vadose zone permits direct evaporation from the water table and drives precipitation of meteoric phreatic cements even where mineral stabilization is complete. Copyright © 2007 John Wiley & Sons, Ltd.     

Photogrammetric Analysis of Front Range Rock Glacier Flow Rates

Flow rates for rock glaciers in the European Alps have been monitored using photogrammetric techniques; however, a program has not been initiated for similar Front Range, USA, rock glaciers. Horizontal rock glacier displacements were measured by tracking large surficial rocks on temporal orthophotos from 1978, 1990, and 1999. Vertical change was measured by creating digital elevation models (DEMs) from digital stereopairs, then subtracting elevations to detect change. Long‐term horizontal velocities ranged from 14 to 20 cm/yr on average, although uncertainty ranged from 4 to 5 cm/yr. On average, vertical elevation changes were negligible with most rock glaciers exhibiting a slight growth or thinning (1–2 cm/yr). Over shorter time scales (c. 10‐year periods), horizontal velocities have only increased by about 2 cm/yr. Because horizontal and vertical change is minimal, Front Range rock glaciers appear to be adjusted with current climate, unlike some rock glaciers in the European Alps that have shown increasing subsidence rates or significant increasing or decreasing horizontal velocities.     

Time series video analysis of bubble release processes at natural hydrocarbon seeps in the Northern Gulf of Mexico

This research quantifies the rate and volume of oil and gas released from two natural seep sites in the Gulf of Mexico: lease blocks GC600 (1200 m depth) and MC118 (850 m depth). Our objectives were to determine variability in release rates and bubble size at five individual vents and to investigate the effects of tidal fluctuations on bubble release. Observations with autonomous video cameras captured the formation of individual bubbles as they were released through partially exposed deposits of gas hydrate. Image processing techniques determined bubble type (oily, gaseous, and mixed: oily and gaseous), size distribution, release rate, and temporal variations (observation intervals ranged from 3 h to 26 d). A semi-automatic bubble counting algorithm was developed to analyze bubble count and release rates from video data. This method is suitable for discrete vents with small bubble streams commonly seen at seeps and is adaptable to multiple in situ set-ups. Two vents at GC600 (Birthday Candles 1 and Birthday Candles 2) were analyzed. They released oily bubbles with an average diameter of 5.0 mm at a rate of 4.7 bubbles s⁻¹, and 1.3 bubbles s⁻¹, respectively. Approximately 1 km away, within the GC600 seep site, two more vents (Mega Plume 1 and Mega Plume 2) were analyzed. These vents released a mixture of oily and gaseous bubbles with an average diameter of 3.9 mm at a rate of 49 bubbles s⁻¹, and 81 bubbles s⁻¹, respectively. The fifth vent at MC118 (Rudyville) released gaseous bubbles with an average diameter of 3.0 mm at a rate of 127 bubbles s⁻¹. Pressure records at Mega Plume and Rudyville showed a diurnal tidal cycle (24.5 h). Rudyville was the only vent that demonstrated any positive correlation (ρ = 0.60) to the 24.5 h diurnal tidal cycle. However, these observations were not conclusive regarding tidal effects on bubble release.     

城市富营养化湖泊的外源污染负荷与贡献解析——以武汉市后官湖为例

为探究城市湖泊富营养化的污染负荷来源及其贡献率,选择长江流域中下游典型富营养化湖泊(武汉后官湖)为研究对象,结合实测法、平均浓度法与排污系数法,解析了后官湖的外部污染来源(包括点源和面源,面源又进一步划分为农村生活、农业种植、水产养殖、畜禽养殖和城市径流).结果表明:化学需氧量(COD)主要来源于点源,入湖贡献为54%;总磷主要来源于面源,入湖贡献为71%.进一步筛选面源中污染物的主要来源发现,总磷主要来自农业种植和水产养殖,入湖贡献分别为41%和30%;面源中COD、氨氮和总氮贡献最大的是城市径流,入湖贡献率分别为61%、53%和37%.行政区污染排放贡献率中,COD、氨氮、总氮和总磷排放贡献率最大的分别为沌口街办(25. 24%)、蔡甸街办(23.28%)、永丰街办(24.04%)和大集街办(26.15%).开展沌口街办和永丰街办的点源治理可有效消减入湖COD与氨氮负荷,而大集街办的农业面源污染防治则是后官湖流域总磷治理的重点.     

Variations in sediment yield over the advance and retreat of a calving glacier, Laguna San Rafael, North Patagonian Icefield

Bathymetric and sub-bottom acoustic data were collected in Laguna San Rafael, Chile, to determine sediment yields during the Little Ice Age advance and subsequent retreat of San Rafael Glacier. The sediment volumes and subaqueous landforms imaged are used to interpret the proglacial dynamics and estimate erosion rates from a temperate tidewater glacier over a complete advance-retreat cycle. Sediment yields from San Rafael Glacier averaged 2.7 × 10⁷ m³/a since the end of the Little Ice Age, circa AD 1898, corresponding to average basin-wide erosion rates of 23 ± 9 mm/a; the highest erosion rates, 68 ± 23 mm/a, occurred at the start of the retreat phase, and have since been steadily decreasing. Erosion rates were much lower during glacial advance, averaging at most 7 mm/a, than during retreat. Such large glacial sediment yields over two centuries of advance and retreat suggest that the contribution of sediments stored subglacially cannot account for much of the sediment being delivered to the terminus today. The detailed sub-bottom information of a proglacial lagoon yields important clues as to the timing of erosion, deposition and transfer of glacigenic sediments from orogens to the continental shelves, and the influence of glacier dynamics on this process.     

苏北岸外辐射潮流沙脊群东沙浅滩沉积速率空间分布特征

江苏岸外发育典型的辐射状沙脊群,沙脊群的地貌与沉积变化是海岸海洋动力地貌研究的热点领域。东沙是辐射沙脊群的重要组成部分,2012年在东沙浅滩采集了18个柱状岩心,利用210Pb方法分析了柱状岩心沉积物210Pb比活度,计算获得了东沙地区10个站位的平均沉积速率。东沙地区平均沉积速率有"西部边缘低、东部边缘高"和"南北高、中间低"的空间分布特点。辐射沙洲东沙附近地形格局和潮流对沉积速率有至关重要的影响。