INTRODUCTION

The report area includes all of Colorado except the southwest corner of the state. The major geologic provinces within the area include the Denver basin, the Las Animas arch, the northwestern part of the Anadarko basin, and the Raton basin in eastern Colorado; and North and South Park basins, the Sand Wash basin, and the Piceance basin in northwest Colorado.

Tables 1 and 2 summarize the 1975 exploratory and development drilling in Colorado. Table 3 lists the significant discoveries within the area for 1975 and several that were completed in late 1974 and not included in last year's development paper. These discoveries also are located on the regional map (Fig. 1) which shows existing production and approximate boundaries of the major geologic provinces.

The western part of the Denver basin was the center of exploratory activity where the Lower Cretaceous Muddy "D" and "J" sandstones and the Upper Cretaceous Sussex and Shannon sandstones are the primary stratigraphic objectives. With the exception of the Mississippian and Pennsylvanian in southeastern Colorado and the Tertiary in the Sand Wash basin, the Cretaceous remains the primary drilling objective.

DRILLING ACTIVITY

In the report area 1,131 wells were drilled in 1975, a 38% increase over the 817 wells drilled in 1974. The Denver basin remains the most active province with 855 wells drilled or 75.6% of the drilling activity. The total footage drilled in eastern and northwestern Colorado was 6,687,563 for an average depth per well of 5,913 ft.

EXPLORATORY DRILLING

Exploratory drilling in the report area increased 21.7% from 323 wells in 1974 to 393 wells drilled in 1975. Activity increased in the Denver basin (31%) and in southeastern Colorado (7.8%), although exploratory drilling decreased 6.1% in northwestern Colorado. The exploratory-well success ratio in 1975 was 13.7% for eastern and northwestern Colorado.

Exploratory-Well Success Ratio (%)

Table

Of the 54 exploratory-well successes drilled, 33 were in the Denver basin, 9 were in southeastern Colorado, and 12 were in northwestern Colorado.

In the Denver basin, exploratory drilling increased 31% from 221 wells drilled in 1974 to 290 wells drilled in 1975. The number of exploratory-well successes decreased from 36 to 33 with a decrease in the success ratio from 16.3% to 11.4%.

The only new-field discovery in southeastern Colorado was on the Las Animas arch where Mull Drilling completed the 1 Matthies, Lookout field, for 216 BOPD and 18 BWPD from the Mississippian Spergen. In northwestern Colorado there were only 2 significant discoveries. In Foundation Creek field, Rio Blanco County, Norris Oil established production from a new pool, the Mancos "B" zone. In North Park basin, Pauley Petroleum completed the 1 Allard, Carlstrom field, for 75 BOPD from the Lakota as a new-field discovery.

Amoco Production Co. completed the 1 Everette Dahlinger for 119 BOPD and 21 BWPD from the Muddy "D" sandstone in the Denver basin.

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This was the discovery well in Bugle field which had 9 completed producing wells by the end of 1975. In December the field production averaged 519 BOPD and cumulative was at 109,515 BO and 101,504 MCFG.

Amoco also completed 2 significant field extensions in the Denver basin. The 1 UPRR 67D, midway between Hawkeye and Chieftain fields, was completed for 2,273 MCFGD from the Muddy "J" sandstone. The 1 C. J. Schmidt, Hambert field, was completed at 1,534 MCFGD from the Sussex sandstone.

The Muddy "D" sandstone remains a difficult exploratory objective and is still secondary to the Muddy "J" objective. Development of a Muddy "D" reservoir generally is still unpredictable. Two significant Muddy "D" new-field discoveries, other than Bugle field, were made in 1975. Tom Vessels drilled the 1-A Schmidt (IP 116 BOPD, 10 BWPD) and Summit Oil and Burlington Northern drilled the A 1 Claycomb (IP 176 BOPD, 41 BWPD, 908 MCFGD).

DEVELOPMENT DRILLING

Development drilling (Table 2) increased 49.4% from 494 wells in 1974 to 738 wells in 1975. The development-well success ratio decreased from 74.3% to 73.3%. Drilling increased over the entire area.

Major development drilling was concentrated in the western part of the Denver basin as Amoco Production Co. and several independent operators continue to develop the fields which generally have been discovered within the last 5 years. Wattenberg field continues to be developed at an active pace with 554 wells producing in December 1975, compared to 327 wells at the end of 1974. The field produces an average of 2,304 BOPD and 106,801 MCFGD. Within the Wattenberg spaced area, the Upper Cretaceous Sussex and Shannon sandstone production in Spindle, Singletree, and Surrey fields continued to increase as a result of extensive development; 91 wells were drilled in these 3 fields in 1975. Development probably will increase in these areas as infield drilling is continued and well spacing is a justed depending on production.

Northwestern Colorado and southeastern Colorado showed increases in development drilling that were not concentrated in any specific areas.

EXPLORATORY TRENDS

Exploratory drilling for Cretaceous objectives should continue at a high level in the western and central parts of the Denver basin. Exploratory interest will increase in the sparsely drilled area north of Wattenberg to the Colorado-Wyoming state line.

Exploratory drilling in western Colorado for Cretaceous objectives should continue at or below the 1975 rate, depending on the political-economic climate. Deep drilling for older Mesozoic and Paleozoic targets along the Uinta Mountains-Cherokee Ridge trend on the Wyoming-Colorado border is active. Successful completions of these deep tests could spur activity in this area in 1976.

In summary, 1976 drilling will be stimulated by increases in prices for oil and gas, dependent on additional taxes which might be imposed. The increasing trend in drilling activity from 1974 to 1975 should continue in 1976.

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Table 1. EASTERN AND NORTHWESTERN COLORADO COMPLETION SUMMARY, 1975

Table 2. DEVELOPMENT WELLS COMPLETED

Table 3. SIGNIFICANT DISCOVERIES IN EASTERN AND NORTHWESTERN COLORADO, 1975

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Table 3. Continued. See caption on page 1236.

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Fig. 1. Exploratory drilling in eastern and northwestern Colorado in 1975.

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