Anomaly annular

An apical anomaly comprises either a continuous zone of elevated gas concentrations or an area with erratic elevated gas concentrations directly over the oil or gas field. An annular or halo anomaly has the form of a doughnut of continuous or discontinuous high gas concentrations surrounding a central zone of lower or background values, the latter overlying the surface projection of the oil or gas field. In a linear or belt anomaly, high gas concentrations are found continuously or intermittently along a line or confined in a belt, usually associated with faults, fracture zones, or matured source beds. 

It is vital, but not necessarily easy, to recognise whether an anomaly is apical, annular or linear, because the interpretation placed upon each of them leads to quite different courses of action. The model used to explain an apical anomaly favours drilling the anomaly peak, whilst the model for an annular anomaly favours the area within the ring of high values. Follow-up of linear anomalies needs to take into account structural or lithological information. 

Subsequently the scientific research well was drilled as planned. When it penetrated Lower Ordovician carbonates it produced 16.3 x lO" m per day of natural gas. The scientific research well thus became the discovery well in this region. Since the well was drilled directly on the surface hydrocarbon gas anomaly, the soil gas survey traverse was thought to cross an apical or linear anomaly. Further work, however, was to reveal that the gas field has an annular anomaly. 

The resulting hydrocarbon gas patterns and Hg patterns are presented in Figs. 6-8 and 6-9, respectively (which also show the location of the traverse discussed above and shown in Fig. 6-7). The regional patterns comprise an arc of anomalous values extending from the southwest to the northeast. This arc is interpreted as part of an annular anomaly that is still open to the north and west. The exploration implication is that a gas field underlies the low values in the northwest of the survey region. In fact the earlier gas survey traverse and the scientific research well that became the discovery well were already located in this part of the region. 

The gas production layer is a Lower Palaeozoic karstic plateau at a depth of 3000 m, surrounded by palaeo-valleys filled with a Permo-Carboniferous coal series (which could be the source of the natural gas). The annular surface geochemical anomaly may be related to the edge of the buried plateau. In the production layer, the gas is under a pressure of 120 atmospheres. Therefore the average pressure gradient between the gas... 

Seismic data have demonstrated three sub-surface southwest-northeast trending faults in the area. The annular gas anomaly partially overlies an uplifted block, a stmcturally-favourable setting for an oil or gas trap. The linear anomaly follows the trend of another of the subsurface faults. 

Gas geochemical surveys of light molecular-weight hydrocarbons (C1-C5) and Hg have proved to be useful methods of detecting oil and gas fields in the Ordos Basin of northern China and of indicating potential oil or gas fields in the Lixian Depression of southern China. The annular anomaly is the type most usually found in these regions, although apical and linear anomalies occur under particular conditions. 

Finally, increasing attention has recently been given to an important circulation anomaly observed in the Northern Hemisphere (see Box 3.3) and characterized by a nearly symmetric oscillation in temperature, wind, and pressure between the Arctic and mid-latitudes (Thompson and Wallace, 1998). This annular mode of oscillation in the climate system is called the Arctic oscillation (AO), and extends from the surface to the upper stratosphere. The AO affects the strength and location of the Arctic vortex, with associated effects on stratospheric ozone (Wallace and Thompson, 2002). 

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