Sample Grading Method of Estimating Gas Reserves

A technique is presented by which well samples and core plugs of dolomiteformations are classified by microscopic examination into seven differentporosity grades. Quantitative values of porosity and permeability aredetermined for each grade by a statistical correlation of the core plug testdata with the porosity grading system. These quantitative values are applieddirectly to the grades exhibited in the well samples for the purpose ofestimating the reservoir void space for wells that were not cored. The procedure is described for estimating the gas reserves per unit area forthe South Hugoton gas field, but a reserve estimate for the field is notgiven. Introduction The miscroscopic examination of well samples and the graphic recording oftheir lithologic qualities and other distinguishing characteristics of variousgeologic formations drilled is both a science and an art of long standing andwide application. Usually the primary objective of a geologist who "sits onthe well" and examines the samples are: to identify the formation beingdrilled, determine the total depth, casing point, and completion interval. Inmost cases the porosity is described, if done at all, in general terms, suchas: trace, scattered, fine, poor, fair, medium, good, excellent, or in someother relative terms. In fields where various geologists have examined samplesand recorded observations on many wells considerable variations in lithologicterms and porosity descriptions occur unless there is primary effort toestablish uniformity of logging observations and standards of recordingobservable porosity. When an estimate of the pore volume of a reservoir is made a geologicconcept of the processes that control the magnitudes of porosity andpermeability is developed by microscopic examination of well samples. Thecharacteristics and appearances are then mentally related to rather generalquantitative units of porosity based on physical core data from the samereservoir or on such data or experience in other reservoirs that have similarqualities. The reliability of such estimates depends largely on the variationsof the lithology of the formations, the geometric properties of its voidsystem, the extent of comparisons of sample appearances with porosity data, aswell as the uniform recording of all relevant characteristics. This statementis particularly significant for dolomitized limestone formations of substantialthicknesses and heterogeneity such as the Permian Dolomites of the Hugoton gasfield. T.P. 3374