ABSTRACT: GEOLOGY AND FUEL RESOURCES OF THE FRUITLAND FORMATION AND KIRTLAND SHALE OF THE SAN JUAN BASIN, NEW MEXICO.

The San Juan Basin is an asymmetric structural basin in northwestern New Mexico containing sedimentary rocks that range from Cambrian to Holocene in age and are as much as 15,000 feet thick. Upper Cretaceous rocks, which are more than 6,000 feet thick. are composed of intertounguing marine and nonmarine sedimentary rocks deposited during three basin-wide transgression and regression of an epicontinental sea. The final regression of the sea is represented by the marine Pictured Cliffs Sandstone. subsurface studies of the this unit, based primarily on well-log interpretation, and surface evidence indicate that the Pictured Cliffs regression was interrupted frequently by relatively minor transgressive episodes that resulted in vertical buildups and intertounguing of the Pictured Cliffs Sandstone with the overlying Fruitlland Formation. The strandlines of the Pictured Cliffs sea had northwest alinements, and the Pictured Cliffs Shale are younger in the northeastern part of the basin than in the southwestern part. Concomitant with the Pictured Cliffs regression, an area southeast of the basin was uplifted, resulting in either lack of deposition or possible erosion of the Pictured Cliffs; there the Fruitland Formation now rests on the Lewis Shale

After deposition of the Fruitland and Kirtland, the Upper Cretaceous McDermott Member of the Animas Formation was deposited in the northwestern part of the basin area. Next, the basin area was tilted toward the northwest and as much as 2,100 feet of rocks was eroded in the east. Following this erosion cycle, the fluvial Ojo Alamo Sandstone of Paleocene age was deposited. The source of the Ojo Alamo was primarily from the west, as indicated by a decrease in pebble size in the Ojo Alamo conglomerates, toward the east, although renewed uplift to the east may have furnished some Ojo Alamo sediment. In the northwestern part of the basin, the Ojo Alamo rests on Lewis Shale. After deposition of the Ojo Alamo, the Paleocene Nacimiento Formation and upper shale member of the Animas Formation were deposited, followed by deposition of the Eocene San Jose Formation. These units seemingly rest unconformably on older rocks in the central part of the basin.

In the San Juan Basin, significant resources of both coal and coalbed gas are in the Upper Cretaceous Fruitland Formation. The occurrence, thickness, and geometry of Fruitland coal deposits are strongly influenced by depositional environment. Coal deposits resulted from peat that accumulated on sandstone platforms of the underlying Pictured Cliffs Sandstone, which were deposited along the coast of a northeast-prograding shoreline. Individual coal beds are as much as 40 ft thick. The greatest net coal thickness, up to 100 ft, is in a northwest-trending belt in the northern part of the basin where thick coal deposits occur in both northwest- and northeast-trending deposits, with the thickest deposits in the northwest-trend. In the northwest-trend, individual coal beds average more than 9 ft in thickness, which resulted from standstills in the Pictured Cliffs Sandstone shoreline. Average thickness of coal beds in the northeast-trend is 6 ft. They occur in floodplain facies between channel-fill sandstone deposits. Depths of burial for the Fruitland coal beds are as much as 4,200 ft in the northeastern part of the basin. Coal beds in the Upper Cretaceous Menefee Formation are an additional target for gas. The Menefee is older and deeper than the Fruitland and is in the middle part of the Mesaverde Group. Although as much as 35 net ft of coal occur in the Menefee, the coals are generally thinner, more discontinuous, and dispersed over a greater stratigraphic interval than those of the overlying Fruitland Formation. The Menefee coals are as deep as 6,500 ft. Across the central basin, the rank of Fruitland coal increases northeastward from subbituminous C to medium-volatile bituminous. Coal rank generally conforms with the structural configuration of the basin and abruptly decreases along the steeply dipping north flank. Rank trends for Menefee coals are similar, but somewhat higher because of greater burial depth.