The characteristic vertical variation in petrophysical properties across thin (0.6-1.5m) reservoir units was determined at 5 cm intervals from CT-scan and minipermeameter measurements of core samples from the Upper Cretaceous UA-SB sandstone at Patrick Draw Field, Wyoming and the Pennsylvanian Bartlesville sandstone of Oklahoma. This information was used to determine their expected responses on wireline logs. The correlation with this data of a highly permeable interval at the base of the producing shoreline sandstone (UA-5B) indicated that the distribution of this interval significantly affected waterflood production from the field. Introduction The most productive of the shoreline barrier island reservoirs at Patrick Draw field, Wyoming, is the Upper Cretaceous UA-SB sandstone, which produces from both high-permeability (consisting of the tidal inlet, tidal channel, and tidal delta facies) and low-permeability (consisting of tidal creek, tidal flat, swamp, and lagoonal facies) rocks. Within the high-permeability facies, there are low-permeability intervals resulting from detrital clay, carbonate and clay cementation, and also zones with permeabilities as high as 400-500 mD compared to an average permeability of 20-40 mD for the rest of the sandstone. The high-permeability streaks are predominantly from the tidal channel facies and are believed to be an important source of heterogeneity contributing to poor waterflood recovery from certain parts of the field. For optimum planning of waterflood projects, techniques are needed to map the distribution of the highly permeable streaks from wireline logs. To determine the expected log responses of these streaks, the vertical variation in petrophysical properties across these zones was obtained from CT-scan and minipermeameter measurements of one outcrop and three subsurface cores from the Patrick Draw Field. In addition, two more cores from the Pennsylvanian Bartlesville sandstone obtained from a roadcut near the city of Claremore, Oklahoma, were also studied. The Bartlesville (bluejacket) sandstone was deposited in a delta plain setting at the location of the study. For the purposes of this investigation, only cores from the fluvial channel-fill facies were examined. In a general way, the channel-fill facies of the Bartlesville sandstone can be used to gain some insights into the tidal channel deposits from Patrick Draw Field. All cores were described in detail for vertical distribution of lithology, mineralogy, grain size, sedimentary structures, etc. The characteristic vertical variation of petrophysical properties within and at the bounding surfaces of each producing facies was studied to determine if highly permeable zones could be identified and correlated on logs from the diagnostic changes in petrophysical properties. Measurements of Porosity and Permeability Distributions A minipermeameter (initially calibrated with permeability measurements on standard core plugs) was used to measure permeability of the core samples at every 5 cm depth interval. These measurements gave good estimates of permeability within about 1-2 cm3 of rock for average rock types. The CT density gave an accurate measure of bulk densities over about 50 cm3 of rock volume. P. 973^

A new technique is presented for a more effective description of the vertical variability of reservoir rock properties that are typically obtained or derived from wireline logs or determined from laboratory measurements of core samples. Rock properties and other data available for this study were the geological description of cores, XRD analysis of core samples for mineral percentages, SEM and CT-scan data from a few selected wells in two fields, namely Patrick Draw (WY) and Bell Creek (MT), laboratory measured porosity, air permeability, and primary and waterflood oil production data from the wells studied. In this investigation, first the vertical distribution of reservoir rock properties available as a function of depth is transformed into functions of frequency (cycles per foot) by the application of standard time series analysis techniques. A statistical parameter called the "power spectrum" is calculated which gives the variance of the time series for each frequency. In this way a continuous description of variances is obtained for each frequency into which the original series has been transformed. Estimation of vertical variability of rock properties was initiated by computation of "power spectra" of core measured permeability, and digitized gamma ray, sonic, density and neutron logs. The power decay patterns with frequency were correlated with reservoir rock properties and oil production, and it was observed that different logs had different decay patterns in sandstones having different distributions of reservoir properties. The power spectra of permeability, sonic, and gamma ray logs for a clean sandstone or for a sandstone with uniform properties (low variance) decayed very rapidly at higher frequencies if there was no thin bed stratification and assumed very low powers at the high frequency end. The sandstone with variable reservoir properties had power spectra that decayed more gently, and the slope of spectral decay was a measure of the vertical distribution of reservoir properties, in the sandstone. For a few wells, the spectra of density logs in kaolinite clay filled sandstones showed very low amplitudes at higher frequencies because the matrix rocks (predominantly quartz) and the clays (dominantly kaolinites) both had nearly the same density (around 2.64 g/cm3). From minimal geological information, studies of highly automated spectral decay curves of different types of distributions could provide quantitative information on vertical variability of reservoir rock properties in different parts of a field. Under favorable conditions the study of decay spectra of a combination of logs may provide quantitative information on the variations in cement content and the types and volumes of clays in reservoir rocks. Introduction Information about the vertical variability of reservoir rock properties such as permeability, porosity, fluid saturation, lithology, and type and volumes of clay, is of critical importance for exploitation of hydrocarbon resources and effective planning of enhanced recovery projects. The vertical sweep efficiency of a waterflood depends primarily upon the distribution of permeability within the reservoir and also upon the mobility ratio. As a consequence of non-uniform permeability, any injected fluid will move through the reservoir as an irregular front. In a miscible slug process, effective design of a slug required to maintain miscibility throughout the displacement process requires knowledge about the degree of stratification of the reservoirs Froning and Treiber reported how the distribution of clays in reservoir pore surfaces may affect micellar design in several ways but primarily by higher adsorption of surfactants in high surface area clays compared to that on silica. They concluded that the amount of clay and the nature of cation present (i.e., Na+ and Ca++) will affect the ionic environment contacted by micellar fluids as it moves through the reservoir. P. 637^