LIMITATIONS ON THE LATERAL CONTINUITY OF TONSTEINS, SAN JUAN, LA PLATA AND NAVAJO COAL MINES, LATE CRETACEOUS FRUITLAND FORMATION; NORTHWESTERN SAN JUAN BASIN, NEW MEXICO

The lateral continuity of coals containing ash, or tonstein layers in Cretaceous coal deposits of the western interior seaway has been a topic of much debate and has economic importance for both the coal, and the oil and gas industry. These facies variations have been mapped from the coal bed methane producing areas of the northwestern San Juan Basin into several active coal mines situated on the western edge of the basin. The correlation of these ashes was achieved by using outcrop, core, and wire line logs. Based on these high resolution correlations of several tonsteins located within the Cretaceous Fruitland Formation it appears that there are two principle sources of lateral facies variability: 1) strike-parallel channel to interfluvial facies transitions, and 2) dip-parallel tonstein bearing coal terminations due to the stratigraphic rise of the Pictured Cliffs Sandstone shoreface. The lateral variation of the tonsteins within the Fruitland was found to be dependant on the orientation of the cross-section. Within strike-parallel (NW-SE) cross sections, the tonsteins could not be found for more then a 1-4 km once they entered the fluvial dominated channels. In contrast, in dip-parallel (SW-NE) oriented cross sections, an ash layer was traced for over 25 km before it left the southwestern edge study area. This study quantifies the scale of lateral variation in coal horizons of the Lower Fruitland Formation on the western and northwestern flank of the San Juan Basin. These reservoir scale data, and those of previous studies, are significant in that this basin has served as an important analog to other coal bearing regions of western North America.