Velocity-Porosity Logging in Volcanic Rocks
- Roderick D. Carroll (U. S. Geological Survey)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- December 1968
- Document Type
- Journal Paper
- 1,371 - 1,374
- 1968. Society of Petroleum Engineers
- 5.6.1 Open hole/cased hole log analysis
- 2 in the last 30 days
- 573 since 2007
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Volcanic rocks exhibit a wide range in porosity and velocity and are an excellent medium for checking the time-average formula. An analysis of core porosity data and velocity logs from five wells at the Atomic Energy Commission's Nevada Test Site indicates that the time-average formula applies in these rocks. An equation with the constants tma = 54 sec/ft; tf = 189 sec/ft best fits the data. The close agreement of this equation with that generally assumed for sandstones suggests that in volcanic rock areas, where porosity-velocity data are lacking, existing interpretation charts for sandstones, relating sonic log travel time to porosity, may be applied to these rocks.
The U. S. Geological Survey for several years has assisted the U. S. Atomic Energy Commission in evaluating rock media in the vicinity of underground nuclear detonations. The greater portion of this work has been concerned with volcanic rock sequences at the AEC's Nevada Test Site located in Nye County, Nev. In describing these media, physical properties play a large role, and a logical and necessary adjunct to these investigations has been the extensive use of wireline logs in exploratory wells. This paper concerns the use of velocity logs as a porosity-measuring tool in these volcanic rocks. porosity-measuring tool in these volcanic rocks. The use of velocity logs to determine porosity has generally been restricted to sandstones and carbonate rocks. This arises as a natural consequence of the favorable environment presented by these rocks for the accumulation of oil and gas. In the study of these rocks, Wyllie's time-average equation has been used widely.
Although volcanic rocks in general are not considered a favorable environment for petroleum, some instances have been reported of the location of oil in such rocks. Oil production has been reported from both a volcanic tuff and a welded tuff in the Eagle Springs field, Nye County, Nev., about 100 miles north of the Nevada Test Site.
Notwithstanding the poor suitability of volcanic rocks as a petroleum-producing medium, these rocks offer an excellent opportunity for testing the general applicability of Wyllie's equation because continuous sequences of saturated volcanic rock several thousand feet thick can be found in which porosity and velocity vary over broad ranges.
Geology and Environment
The Nevada Test Site lies in Nye County, Nev., in the southern part of the Basin and Range province (Fig. 1). Exposed rocks in the area consist of a wide variety of Paleozoic sedimentary rocks (mainly carbonates) and Paleozoic sedimentary rocks (mainly carbonates) and Tertiary volcanic rocks. In the valleys, Quaternary fill is present to thicknesses locally exceeding 2,000 ft. Tertiary volcanic rocks vary greatly in their thicknesses, from zero where they lap out on paleotopographic highs to more than 13,000 ft in Well 10 at Pahute Mesa (Fig. 1). Although nuclear testing has occurred in all of these media, most of the tests have been performed in ash-flow tuff and volcanic sediments.
In the wells involved in the testing program the regional water table in the area is often present at depths between 1,000 to 2,000 ft below the land surface. However, the tuffaceous rocks in the area exhibit extremely low interstitial permeability and high capillarity. Consequently, water-saturated rock may be found for hundreds of feet above the regional water table. The presence of the static water level is not evident on electric logs when the mud column is above the water table. Produced water in these rocks is very fresh and generally exceeds 20 ohm-m in resistivity. Therefore, the standard techniques utilizing electric log data to determine porosity cannot be applied.
The volcanic rocks in the area range from low porosity (less than 2 or 3 percent)-high velocity (greater than 18,000 ft/sec) rhyolitic lava flows and densely welded tuffs to the high porosity (less than 35 percent)-low velocity (greater than 7,000 ft/sec) ash-flow and ash-fall tuffs.
When rocks exhibit velocities on the velocity log at the upper end of this velocity range, the rock type is usually a lava flow or a densely welded tuff. However, in the intermediate and low range of velocity one cannot generally ascribe a rock type to a particular velocity on the velocity log. In general, the determination of rock type on the basis of the response of all wireline logs is extremely difficult, and in the final analysis detailed lithologic examination of core is often required to ensure proper identification of the volcanic rock type penetrated proper identification of the volcanic rock type penetrated in the hole. Consequently, in the analysis of the sonic log-porosity relationship reported here, no attempt has been made to separate rock types.
Method of Analysis
In five geologic exploration wells at the test site, both core analyses of porosities and velocity logs were available. The locations of these holes are shown on Fig. 1. The pertinent lithologic and environmental features of the various holes are listed in Table 1.
Travel times were obtained from the sonic velocity log at depths in the holes for which core porosity data were available. Plots of porosity as the independent variable were made against velocity log travel time.
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