DEPOSITIONAL PALEOENVIRONMENT AND ELEMENTAL ANALYSIS OF THE UPPER MANCOS FORMATION IN THE SAN JUAN BASIN, NEW MEXICO, USA

The San Juan Basin has been a major producer of petroleum for over a century. Most of its production is gas and natural gas liquids from Upper Cretaceous blanket sandstones and coal beds. These sandstone reservoirs are primarily sourced from the organic rich and thermally mature Mancos Formation. Twenty-seven core samples from the Upper Mancos Formation representing the interval between 315.5–188.7 m at the Melrich-Shaft-Site drilled in Smith Lake Field, McKinley County, New Mexico have been previously studied to evaluate their hydrocarbon source potential. Kerogen types II, mixed II/III, and III were reported at various intervals. Although this gives some insight into the depositional paleoenvironment of the Mancos Formation in this field. More data is still needed to fully understand the intricate nature of factors controlling the deposition of the Mancos shales at this location. The present study introduces new elemental analysis data that paint an interesting picture of the interplay between marine water chemistry and terrigenous input during the deposition of the Mancos Formation.