Study of reservoir properties can yield exploration clues

Frederick R. Haeberle
Consulting Geologist
Delaware, Ohio

Past exploration methods have been primarily concerned with, and concentrated on, the search for structural and stratigraphic traps by seismic or subsurface mapping. As many of these traps have contained only small amounts of oil or have been dry, this approach obviously does not answer all the questions about where to find oil reserves.

The study of reservoir properties, their relationship to oil reserves, and the distribution of reservoir properties in a basin is just as important as the location of potential traps. Studies involving porosity and permeability have been undertaken, but detailed studies of other reservoir properties are rare.

This concept of using reservoir properties promptly divides the study into two parts. First, what properties are associated with large reservoirs? Second, where in a basin are these properties located?

Area studied

Because of the large amount of data available, the chosen model is the Permian Basin of West Texas.

The area studied in the Permian Basin covers some 102,000 sq miles, or almost 66 million acres, and was limited to 107 reservoirs of Permian age with reserves of more than 10 million bbl each.

The importance of these reservoirs is that they contain over 70% of the oil in place in the basin. Although many more reservoirs with less than 10 million bbl are obviously present, only very limited data are available for most.

As a background, of the 107 reservoirs in this study, 71 (67%) were in dolomites, 27 (25%) in sandstones, and nine (8%) in limestones. The reserves were in dolomites (84%), sandstones (14%), and limestones (2%). Reserves, as used here, includes a combination of cumulative production plus remaining reserves (estimated ultimate recovery).

Reservoir properties

In an effort to determine what effect the various reservoir properties had on reserve size in this area, eight of the more commonly available reservoir properties were examined. These included:

Depth of production;
Porosity;
Permeability;
Reservoir oil column thickness;
Proved acres;
Acre-feet;
Reservoir temperature; and
Producing formation pressure.

Each property was plotted by intervals, such as depth in thousand feet, porosity in 1%, thickness in 50 ft and so on, against the percent of reserves for that interval. In different areas other properties such as hydrocarbon gravity, water saturation, lithology, type of traps, or age may be more important or worthy of study.

Table 1 [20,984 bytes] shows the average values calculated for each reservoir property, and Table 2 [39,392 bytes] shows the intervals with the largest reserves for each reservoir property, the number of reservoirs at that point, and the percent of total reserves for each property at that interval.

Permian reservoirs vary in depth from 1,250-9,100 ft with an average depth of 4,629 ft. Twenty-seven reservoirs are at depths of 4,000-5,000 ft and contain 32% of the Permian reserves (Fig. 1 [97,956 bytes]). Eighteen percent of the reserves are in seven reservoirs at depths between 1,000-2,000 ft.

Porosity in Permian reservoirs ranges from 5-25% with an average of 12%. Seventeen reservoirs had 10% porosity and contain 40% of the reserves (Fig. 2 [88,178 bytes]). Sixteen percent of the reserves are in 13 reservoirs with 12% porosity.

Permeability ranges from 1-118 md and averages 10 md. Twenty-eight percent of the reserves are in 30 reservoirs, with permeabilities of less than 5 md (Fig. 3 [89,130 bytes]). Eighteen reservoirs with permeabilities between 10-14 md contain 17% of the reserves.

Thickness of oil columns in Permian reservoirs ranges from 55 to 1,400 ft with an average of 317 ft. Seventeen percent of the reserves are in eight reservoirs with oil column thickness between 300-350 ft (Fig. 4 [89,130 bytes]). Fifteen percent of the reserves are in four reservoirs with thickness between 450-500 ft.

Proved acres in reservoirs range from 9,600-134,100 acres with an average of 28,666 acres. Ten reservoirs cover between 25,000-30,000 acres and contain 19% of the reserves (Fig. 5 [89,164 bytes]). Twelve percent of the reserves are in 31 reservoirs with 15,000-20,000 acres, and 12% of the reserves are in two reservoirs with 70,000-75,000 proved acres.

Acre-feet in these reservoirs range from 1.2-37.3 million with an average of 9.5 million. Only three reservoirs are in the 10-12.5 million acre-ft range, but have 24% of the reserves (Fig. 6 [90,443 bytes]). Thirteen percent of the reserves are in 35 reservoirs with 2.5- 5 million acre-ft.

Reservoir temperatures range from 80-162° F. with an average of 106°. Fourteen reservoirs have temperatures between 105-109° and contain 33% of the reserves (Fig. 7 [90,558 bytes]). Twenty-three percent of the reserves are in 13 reservoirs, 12% of the total, with temperatures between 80-84°.

Reservoir pressures range from 450-4,103 psi with an average of 1,925 psi. Thirty four reservoirs have pressures between 1,500-2,000 psi and contain 44% of the reserves (Fig. 8 [89,870 bytes]).

Important properties

The next question is, which reservoir properties were the most important in containing large oil reserves?

The graph in the upper part of Fig. 9 [86,634 bytes] shows the values of the properties for the reservoirs with the 10 largest and 10 smallest reserves. The 10 largest reservoirs had a total 7.8 billion bbl of reserves, 67 times larger than the 116 million bbl of reserves in the 10 small reservoirs. The reservoirs with the largest reserves had 12,783 bbl/acre, and the reservoirs with the smallest reserves had 576 bbl/acre.

The lower part of Fig. 9 shows the property values in the larger reservoirs divided by the same property values in the smaller reservoirs. The number 1 indicates the values are equal. Values smaller than 1 mean the reservoir property values for the smaller reserves are higher than the same reservoir property values for the larger reserves. Numbers above 1 show how many times greater the reservoir property values for larger reserves are than the same values for the smaller reserves. For example, proved acre values in the reservoirs with larger reserves are three times greater than the proved acre values in the reservoirs with smaller reserves.

On the upper part of Fig. 10 [82,973 bytes] Fig. 10 the property values for the reservoirs with the large and small reserves are plotted. In some cases the values for certain properties, such as depth of production, pressure, porosity, and temperature are larger for reservoirs with smaller reserves than they are for the reservoirs with larger reserves.

The lower part of Fig. 10 shows the data plotted by reservoir property values for the 10 largest and 10 smallest reserves. As the reservoir property values increase in the smaller reserves the values move to the left from top to bottom. The values for depth of production, pressure, temperature, and porosity all shift to the right in the reservoirs with smaller reserves indicating the values are higher. The values for oil column thickness, proved acres, acre-feet, and permeability all shift to the left indicating a decrease in the property values from the larger reserves to the smaller.

This raises the possibility that the great importance attached to porosity for so many years may not have been so great after all.

Distilling the results

The study indicates that:

In a given area the reservoir properties favorable to large reserves can be established.
In this area the most important factors in the size of reserves appear to be reservoir thickness, proved acres, acre-feet, and reservoir permeability. Although reservoirs with 10% porosity have the highest percentage of reserves, the porosity in reservoirs with smaller reserves generally increases as compared to the porosity in reservoirs with larger reserves.
By plotting areas where the most reserves are found for various reservoir properties, trends or fairways favorable for large reserves can be outlined to indicate areas favorable for exploration.

The concept of trends or fairways is not new, although it is rarely expressed or applied to reservoir properties. While values for individual reservoirs may vary, the concept of fairways for reservoir properties leading to large reserves is rarely used but certainly worth trying.

Bibliography

Galloway, W.W., Ewing, T.E., Garrett, C.M., Tyler, N., and Bebout, D.G., Atlas of major Texas oil reservoirs, Bureau of Economic Geology, Austin, Tex., 1983, pp. 71-132.
Landes, Kenneth K., Petroleum geology of the United States, Wiley InterScience, New York, 1970, pp. 323-351.

The Author

Fred Haeberle is a consulting geologist in Delaware, Ohio. His career has spanned more than 50 years. He was employed with Standard Oil Co. of California, J.J. Lynn Oil Division, Mayfair Minerals, Atlantic Refining Co., Mobil Oil, and as a consulting geologist. He holds BS and MS degrees in geology from Yale University and an MBA from Columbia University Graduate Business School. E-mail: [email protected]