Applications of Geophysics to Geologic Models and to Reservoir Description

Abstract

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