23 Geometry and Depositional Environment of Fruitland Formation Coal Beds, San Juan Basin, New Mexico and Colorado: Anatomy |ot^^fn^|NER CATANA Coal-Bed Methane Deposit 0IL CONSERVATION Divisior James E. Fassett1 MAN EXHIBIT NO. ABSTRACT CASE NO. Coal-bed methane resources in Fruitland Formation coal beds, San Juan Basin, New Mexico and Colorado, have recently been estimated to exceed 50 trillion cubic feet of gas. This enor­ mous resource is only beginning to be tapped in the northern part of the basin, principally on the Southern Ute Indian Reservation in southwest Colorado. In order to develop exploration plans for the production of this resource in the most efficient and orderly manner, the detailed geometry of individual Fruitland coal beds must be carefully mapped. Previously published studies relate environments of deposition of the Fruitland and associated rocks to Fruitland coal- bed geometry throughout the San Juan Basin and provide guidelines that should make such coal-bed mapping easier and more accurate. The principal guidelines are: 1) Coal beds can only be correlated by using the Huerfanito Bentonite Bed of the Lewis Shale as a datum. 2) Deposi­ tional strike of Fruitland coals trended northwest. 3) Fruitland coals occur in an en echelon arrangement and rise 1,200 feet stratigraphically from southwest to northeast across the basin. 4) Coals generally thicken and terminate abruptly northeastward. 5) Coals generally thin and split south west ward. 6) Along depositional strike, coals may terminate abruptly against or be cut out by stream-channel deposits. ' INTRODUCTION GENERAL GEOLOGY The San Juan Basin of northwest New Mexico and south­ The central part of the structural San Juan Basin is outlined west Colorado (fig. 1) contains large tonnages of coal in sever­ in figure 1 by the outcrop of the Fruitland Formation. The al Upper Cretaceous rock units. One of these units, the basin outline is roughly circular in shape, except for the linear Fruitland Formation, has been described in detail in a paper north-trending east side. The basin structure is asymmetric by Fassett and Hinds (1971). That study showed that the dis­ (fig. 2) with its northwest-trending axis located near, and tribution of the thickest total coal in the Fruitland seemed to parallel to, the northeast rim of the basin. Dips around the coincide with the locations of large vertical buildups of the northwest, north, and east parts of the basin are 20-40 underlying regressive-marine Pictured Cliffs Sandstone. degrees, whereas dips on the west and south sides are quite Because of this relationship, it was possible to develop a gentle and average less than a degree. The diameter of the model describing how the distribution of coal in the Fruitland basin is about 100 mi and its surface area is about 7,500 miJ. Formation was closely tied to the depositional history of the Upper Cretaceous rocks have produced most of the oil and Pictured Cliffs Sandstone. gas in the San Juan Basin; they range from 0 to 6,000 feet In this report I present a summary of the Fruitland coal thick and consist of cyclically interbedded marine and non- depositional model of Fassett and Hinds (1971) and discuss the marine rocks. These rocks represent a remarkably complete basinwide distribution and geometry of the Fruitland coal beds record of a series of transgressions and regressions of the to illustrate some of the problems in coal-bed correlation and western shoreline of the Western Interior Upper Cretaceous present apparent physical constraints on the movement of seaway into and out of the San Juan Basin area (the location coal-bed methane through Fruitland coal beds and associated of the basin relative to this seaway is shown on fig. 3). The rocks throughout geologic time. Most of the data, all of the Fruitland Formation and its coal beds are intimately associated illustrations, and some of the text in this report are from Fas­ with the last regression of the Western Interior seaway from sett and Hinds, 1971 and more recently, Fassett, 1986. (This the San Juan Basin area. report is a slightly revised version of a paper by Fassett pub­ lished in 1987.) GAS AND OIL RESOURCES The San Juan Basin is a vast storehouse of gas, oil, and coal. In the conterminous United States the San Juan Basin gas 1 U.S. Geological Survey field is second in size only to the Hugoton field of Oklahoma, Box 25046, MS 939, DFC Texas, and Kansas. Total cumulative gas production in the Lakewood, Colorado 80225 basin (through 1982 for the New Mexico part and through 1988-COAL-BED METHANE, SAN JUAN BASIN ROCKY MOUNTAIN ASSOCW/ON OF GEOLOGISTS Envirdnment of Fruitland Formation Coal Beds 27 SPONTANEOUS CONDUCTIVITY POTENTIAL (Mlllimhos/m) (Millivolts) 400 200 <? -tt* 50 ipb. ~ 300 50b 10'OQ OJO ALAMO RESISTIVITY (Ohms m 2/m) SANDSTONE EXPLANATION FARMINGTON SANDSTONE MEMBER AND UPPER SHALE MEMBER Sandstone OF KIRTLAND SHALE Shale LOWER SHALE MEMBER OF KIRTLAND SHALE UJ u. Coal a. LU a FRUITLAND FORMATION Siltstone, sandy shale PICTURED CLIFFS SANDSTONE Sandy limestone LEWIS SHALE HUERFANITO BENTONITE BED LEWIS SHALE Figure 4. Induction-electric log and lithologic column of the type well of the Huerfanito Bentonite Bed of the Lewis Shale showing the interval from below the Huerfanito through the lower part of the Ojo Alamo Sandstone. Lithologies are based on an interpretation of the three geophysical logs shown. 1988-COAL-BED METHANE. SAN JUAN BASIN ROCKY MOUNTAIN ASSOCIATION OF GEOLOGISTS rn ro ' .:• '.'.V>"° z • ••'•.'-t^ifi-: v. i »-c C TJ •• I:*/...'- OJ •X »Q TJ >V\A/vV C 6 3 OJ 13 5? E in O ^ (0 tl § £ ^ e • \v... - -t, •: . V..:.-. .. O U OJ ^ J-* -*-> TJ tn OJ <D O >A DQ >-i _ w H « IS tV,',-.-C'>r' CD © 13 » • • 1*^4 o U O 0J O MM CO *C (J 3 " TJ o CO O OJ TJ c 13 TJ 02 £ CQ c OJ J? 13 i? 00 Id £ 5 « « O 1* »-• rt O "TJ «*-» o 05 « ft U c vS OJ S oc •/.:': .v" c (0 % » v o O o t c *g U S Wl <D UM CC TJ (0 OJ •d 2 .v'.:.-!-Hv S C T3 0J " ii ** — / 3 TJ o Z> OJ 0J TJ OJ G .Sd « OJ C $s s c £ u £ J J5 3 V} **4 U-> ' '': 'v. 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