Investigation Of The Background Porosity Of Unconsolidated Sandstone Sediments

Gas hydrates are solid phase composed of water and low-molecular-weightgases ( predominantly methane ) which may become a new type energy sourcesfollowing petroleum and natural gas. At the same time, its concentration decideswhether it's valuable or not to practice, and it's very important to estimatethe concentration of gas hydrates precisely. One crucial parameter is theporosity of gas hydrate sediments. When there no well log measurements, we canonly use seismic data to inverse porosity. One direct and effective method isusing seismic velocity to estimate porosity. But gas hydrates usually depositin the stratum which is high-porosity unconsolidated sediments. The formertheories are not very suitable for these sediments. There is an advancedmethod which provided by Ecker.In his method, Ecker transforms theRoot-mean-square velocity to Interval velocity by DIX equation, then uses atheoretical rock physics model to calculate the interval velocity underassumpation that sediment is 100% water saturated.This interval velocity iscalled "water saturated interval velocity". At last, contrasts the "watersaturated interval velocity" and the interval velocity which comes from theseismic data in order to use the difference to count the porosity and saturation.In this paper, a new way to estimate the porosity of terrigenous sedimentsis presented. Root-mean-square velocity is used to inverse and estimate thebackground porosity of Gas hydrate sediments directly. The detailed processincludes five steps: At first, we build a theoretical rock physics model whichis the same as a horizontal sediment structure from deep sea. Secondly, we reviewall the factors which infect the porosity basing on this model to make clearwhich factor is the major one. Thirdly, we review all the factors which infectthe seismic-wave velocity. Fourthly, we review the Hole 995 and the stackvelocity beside it in Blake Ridge to research the relationship betweenbackground porosity and "water saturated interval velocity", the relationshipbetween Root-mean-square velocity and "water saturated interval velocity",and the relationship between Root-mean-square velocity and background porosity.Finally, we use the relationship between Root-mean-square velocity andbackground porosity to inverse and estimate background porosity. We also obtaina profile of background porosity in Blake Ridge. In a word, the method is simpleand applicable.Finally, we make a summary of the paper and draw some conclusions.