Wattenberg Field, Denver Basin, Colorado' R. A. MATUSZCZAK3 Abstract Wattenberg field produces from a large gas ac­ tion from the city of Boulder northward to a few cumulation (1.1 Tcf estimated recoverable reserves) in the miles south of Fort Collins. Haun (1963), Weimer "J" sandstone of Cretaceous age. The trap was formed in the delta-front environment of o northwesterly prograding (1970), and MacKenzie (1971) have provided ex­ delta. cellent studies that substantiate the presence of The gas is contained in a stratigraphic trap straddling the these delta lobes. A marine embayment crosses axis of the Denver basin. Reservoir characteristics are poor, the outcrop northwest of Denver, as indicated on and fracturing by artificial means is necessary to test a well. Figure 3. Large extensions to the field may be made in the future by following the trend of the delta-front environment. Industry Examination and evaluation of available data exploration efforts will be dependent on gas-price econom­ from old wells drilled on the Union Pacific Rail­ ics. road land (later optioned by Amoco Production Company) revealed the presence of a large area where all drill-stem tests or cores taken in the "J" INTRODUCTION sandstone had shows of gas. Core analyses of the Wattenberg gas field is located in the Colorado "J" sandstone were compared with those of the portion of the Denver basin. Its present area com­ Dakota Formation of the San Juan basin, which prises 978 sq mi (2,530 km2) lying between Den­ produces gas from very low-permeability sand­ ver and Greeley, Colorado (Fig. 1). The Denver stones of Cretaceous age. The "J" sandstone in basin is a Laramide feature oriented north-south the Wattenberg area is similar to the Lower Cre­ and paralleling the east flank of the Rocky taceous Dakota Sandstone reservoir of the San Mountain Front Range uplift. The basin is asym­ Juan basin (Table 1). metric, with a steep (10° dip) west flank and a Supporting data were provided by an earlier gentle (Vi° dip) east flank. Wattenberg field discovery at Roundup field, in T2N, R60W, straddles the basin axis. It produces from an ac­ northeast of Denver (Fig. 3). This small gas field, cumulation of gas stratigraphically trapped in the discovered in August 1967, has several unusual "J," or Muddy, sandstone of Cretaceous age (Fig. qualities. It produces from a stratigraphically 2). Estimated recoverable reserves from the "J" lower part of the "J" sandstone than is productive sandstone of Wattenberg field are estimated to be in the D-J basin "trend" area. The prospect also 1.1 Tcf of gas. In addition, the shallow Upper was located on the basis of minor shows of gas Cretaceous Hygiene sandstone is oil- and gas- recovered from drill-stem tests made in subse­ productive in local areas within the field. quently abandoned wildcat wells. The major risk The purpose of this paper is to present the his­ involved was the uncertainty of achieving a com­ tory of Wattenberg field from exploration con­ mercial completion by fracturing techniques. Res­ ception to date, tracing both evolution of geologic ervoir characteristics of the "J" sandstone are thought and working procedures used to develop similar to those of the Dakota Sandstone in the the field. San Juan basin. The discovery well in Roundup field was completed for 2,700 Mcf of gas per day EXPLORATION CONCEPT, "J" SANDSTONE after a fracture treatment. A regional and isolith map (Fig. 3) shows that a In the Wattenberg field area, gas shows suggest large lobe of the Cretaceous "J" sandstone was the presence of a sandstone of marine origin on deposited in the Denver-Greeley area by a wes­ the northeast edge of the marine reentrant. The terly to northwesterly prograding delta complex. A smaller lobe of sand was deposited in the area south of Denver by a northeasterly prograding 1 Manuscript received by editor, April 17, 1974. This paper was printed originally, in modified form, in The Mountain Geol­ delta. Study of outcrops on the west flank of the ogist, v. 10, no. 3 (July 1973), p. 99-107. basin provides further evidence of these two 2Amoco Production Company, Denver, Colorado 80219. lobes, inasmuch as the "J" sandstone is thin and This paper is printed with permission of Amoco Production exhibits characteristics of shallow-marine deposi- Company and The Mountain Geologist. Wattenberg Field, Colorado 137 SCALE rO l HYGIEME FIG. 1—Structural contour map, Denver basin. Datum is top of Precambrian. C.I.= 1,000 ft. transition from marine shale at the base of the "J" to sandstone at the top, as seen on electrical logs and in cores, implies the presence of a delta- front sandstone of a regressive marine sequence. The electrical logs run on several old wells in the GREEN HORN area are remarkably alike, indicating a similar en­ vironment of deposition. As a result of this sub­ surface evaluation, it was concluded that a delta- front sandstone was present in this area. Because delta-front sandstones tend to be sheet sand­ stones of possibly large areal extent, a prelimi­ nary prospect map was prepared (Fig. 4). FIRST STAGE OF DEVELOPMENT II The initial step was to outline the possible max­ imum areal extent of the "J" sandstone reservoir in the Wattenberg area. Of equal importance was FIG. 2—Type log, Denver basin. 138 R. A. Matuszczak MILES FIG. 3—Delta-sand isolith, "J" sandstone, Denver basin (modified from Haun, 1963). the need to prove that commercial completion production and shows in these intervals were was possible. To test these concepts, six wells present in nearby wells. However, to evaluate the were drilled close to the original control wells. Upper Cretaceous Hygiene sandstone (Parkman, Five were completed as small gas wells, and ac­ Sussex, Shannon sandstone equivalents?) proper­ quisition of additional leases began. ly by means of samples, cores, and drill-stem tests Subsequent to the acquisition of acreage by would have been very costly and would have con­ Amoco, a drilling program was undertaken in siderably slowed down the program for the deep­ partnership with Panhandle Western, a subsidiary er "J" sandstone objective. Denver basin wells of Panhandle Eastern Pipeline Company. Initial normally are drilled with native mud and water to plans were to drill 100 test wells spaced to outline at least the Niobrara Formation. This technique the limits of the "J" sandstone prospect as envi­ facilitates rapid drilling; penetration rates of 2-3 sioned at that time. ft/min (0.6-0.9 m/min) are not uncommon The evaluation of prospective shallow zones through the shallow formations. Under such con­ also was considered desirable, inasmuch as minor ditions, sample quality is poor, and proper core Wattenberg Field, Colorado 139 "~ o oo ,c Eo Vg Q t- 5 H » o & oo a. O OS \o co Q O £, <u Bt» o (3 * I ^H •* (N in i> *o r> 1, « VO ^5 io >/i v> 3 "* 3 9 13 O o en •31. O Q r< r- o> r- r- ^o Z r^ 00 4 > p z en d 7. 8. c B II I-. O > ID o & C —* in CO B0 S W IT) o 'O i—i 1% CO 3 1 -H -H V 9 d " en i-> o o A -i d !? 2 £• C II d d 3 t; a d d o o s < S V V V oo j: I 1 1 1 I § A rr o> tM vc m VO 00 o> C7\ <s ! H« J L •o _, .2 c O >. Q « '3 o o •s u CN : e ^ c Q ° 2 < 2 •B S o z O £ +i o c o "^ o ^ > fl «3 £*£ O os >. c Z § - OS II SI 2 ° 3 •H 2 <*» 6 2 tn H 0 m o SJS 2 c S • c E i» S E OS f- O So-a c^^r cT-o I < <N .5 < z^ m« ^§ 3 d C c -3d e CO <D CO CU V3 H JO £ O CO 1oS c8o <| ^ c»o : ?I PH *J CO CO 3 140 MILES FIG. 4—Isopach map of lower "J" sandstone. Preliminary prospect map, Wallenberg field. C.I. = 20 ft. points and drill-stem-test intervals are difficult to opment drilling is continuing so that production estimate. Therefore, it was decided to use mud- will be ready when equipment is installed. gas detecting units as an exploration method for the shallow formations above the "J" sandstone. PRESENT STATUS OF "J" SANDSTONE It was believed that gas-detection equipment, EXPLORATION combined with a suitable mechanical logging pro­ The discovery well for the field was the Tom gram, would provide sufficient data to make pos­ Vessels No. 1 Grenemeyer, SW >/», SW 'A, Sec. 26, sible the subsequent location of a series of shal­ TIN, R67W, Adams County, Colorado. Initial low wells to test the most prospective areas. potential, flowing, was 485 Mcf of gas plus 10 bbl Discoveries to December 1972 are shown on Fig­ of oil per day through a 2%4-in. choke. The well ure 5. was completed in July 1970 at a total depth of Cumulative production to the end of 1973 was 8,315 ft (2,537 m). 10 Bcf of gas and 260,000 bbl of oil. Most of the The drilling of 100 "J" sandstone wells by wells are shut in. Gas plants, gathering system, Amoco Production Company and Panhandle and other facilities are under construction. Devel­ Western resulted in the completion of 72 wells Wattenberg Field, Colorado 141 capable of gas production from the "J" sand­ stone. Other companies and individuals have drilled and completed over 30 additional wells in the field.