Prologue The standard model for relating bulk formation resistivity to porosity and water saturation was introduced to the petroleum industry in 1941; it remains the industry standard to this day. The model was discovered empirically by means of graphical analysis. Basically, G.E. Archie discovered that when the logarithm of formation resistivity factor was plotted against the logarithm of porosity the resulting trend could be fitted by a straight line. A similar relationship was discovered connecting the logarithms of resistivity index and water saturation. When these two power laws are combined into a single equation, it can be solved for water saturation (which is not observable from a borehole) in terms of bulk formation resistivity, interstitial brine resistivity, and porosity (all of which can be estimated from observations made in boreholes). This revolutionized log interpretation. There has always been a problem with the model in terms of its "explainability". That is, it cannot be derived in any straightforward way from accepted first principles of physics. It does not contradict any first principle, but neither does it seem to follow ineluctably from them. However, since the model works, most formation evaluators have memorized the relationships that follow from the model and simply "get used to them". That remains the situation to this day. However, there is a path around this obstacle to understanding formation resistivity at a fundamental level, and that way forward is to abandon the resistivity formulation in favor of its reciprocal, conductivity. It is surprising that such a seemingly trivial change could open a new vista into the relationships among formation electrical properties. A conductivity formulation permits the asking of questions about how a formation's conductivity should respond to changes not only in brine conductivity, but also in the fractional amount of brine in a formation, and its geometrical configuration. By answering these questions in an obvious way, and with some analysis of data taken in the laboratory, an intuitively obvious model explaining bulk formation conductivity emerges. The model is not the same as the Archie model. However, when certain parameters are taken to their limiting values, and the model is converted into resistivity space, Archie's power law model is revealed as an approximation to the limiting cases. Thus, from the conductivity formulation, an intuitive understanding of the Archie model emerges. Moreover, the conductivity model can be derived in at least three different ways, each yielding different insights into formation conductivity.

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology. Summary. Geophysics techniques, and especially seismic techniques, aresimply tools for developing a rock/fluid model. Like well logs, seismictechniques are physical measurements of the reservoir and its container. Theireffectiveness depends on a combination of careful design of the recording andprocessing systems and skilled interpretations. In processing systems andskilled interpretations. In recent years, improvements in the seismic systemhave enhanced its application to reservoir description. Of particular interestarc improved methods of interpretatione.g., seismic stratigraphy and thin-bedanalysis by use of computer techniques to interpret seismic waveforms; andimproved seismic acquisition and processing techniques, resulting inthree-dimensional (3D) surveys and improved resolution of reservoir beds andfault traces. This paper will briefly discuss these and other seismic benefits, all of which can make important contributions to reservoir description if theright questions are posed. Introduction The bread-and-butter application of geophysics is the interpretation ofpatterns of seismic reflections. Used here, "reflections" refers to theindividual cycles on the seismic waveforms:, seen as black-and-white lines onthe seismic section. A sonic pulse at the earth's surface generates reflectionslike an echo from a barn wall. A geophone planted at the surface records aseries of seismic reflection pulse or echoes from progressively deepersedimentary layers. The progressively deeper sedimentary layers. The geophonerecords these pulses as a composite waveform in real time. Each sedimentarylayer has a characteristic acoustic velocity and density, and the product ofthese quantities is the acoustic impedance. product of these quantities is theacoustic impedance. The degree of contrast in acoustic impendance across layersdetermines how much energy is reflected and how much continues to the nextdeeper layer. Sedimentary layers reflect surfaces basically because water andwind tend to spread similar sediment types in relatively thin sheets over abroad area during, periods of similar environmental conditions. Consequently, sediment types, and therefore velocity and density, are much more similarwithin layers than between layers. As a result, reflections show the patternsof strata and their contained fluids in the subsurface. They are like a crosssection of the earth, showing the thicknesses and configurations of bedsdeposited during common time spans. (Naturally, there are exceptions to thissimplified explanation. Most exceptions are not worth mentioning in thisdiscussion, however, an important one will be discussed later.) Strata Patterns in Reservoir Descriptions A primary use of strata patterns is in defining structural closures and thegeometry of the individual reservoir beds and their seals. Typically, severalstructure maps are made during the history of a field. To begin, prospect mapsare made on the basis of a relatively loose grid and with few or no well data. Then, refined maps are made during the evaluation and delineation phase withadditional well and seismic control. Today, 3D surveys are often used to refinethe structural control further. Fault geometry is another key use of seismicreflections. Fault interpretation typically involves the careful integration ofwell data (cores, cuttings, logs, and tests) with seismic correlation to definefault positions and to delineate individual fault drainage positions and todelineate individual fault drainage blocks. Gross interval thickness maps mayrequire seismic reflection input, in addition to log correlations, to completethe areal coverage. These maps assist in pay-thickness mapping and help definethe history of pay-thickness mapping and help define the history of the burialand possible late tilting of a structure. Aquifer extent and geometry can be animportant contribution because Well control is usually sparse away from thefield and because the flanks of the reservoir structure may be deeply buried. In thick reservoirs that encompass several siesmic cycles, seismic reflectionsshow the gross stratification of the field. Seismic examples will be discussedfurther in upcoming Technology Today Series papers. It is important torecognize that seismic resolution is rather limited compared with core and welllog data. Fig. 1 shows a comparison of the scale of the three data sources. Theperiod of a typical single seismic cycle (peak-to-peak, time) is about 30milliseconds. This can represent between 135 and 225 ft [41 and 69 m], depending on the velocity of the sediments transited. At shallow depths, withspecial high-resolution seismic methods, the period may be much shorter, perhaps on the order of period may be much shorter, perhaps on the order of 10milliseconds, with equivalent thicknesses of 45 to 75 ft [14 to 23 m]. Certaincombinations of fluids and rocks allow mapping of fluid contacts. JPT P. 753