Compositional Variability, Origins, and Migration of Pictured Cliffs Sandstone Gases, San Juan Basin, New Mexico

Andrew R. Scott, Walter B. Ayers Jr.

The Pictured Cliffs Sandstone is an important source of natural gas with cumulative production exceeding 3.2 Tcf. Because Pictured Cliffs Sandstone gas composition varies significantly across the San Juan Basin, a 216-mi² area transitional between regional trends of chemically dry and wet gases to the northwest and southeast, respectively, was selected for detailed study. Gas data from more than 500 Pictured Cliffs Sandstone wells and 75 Fruitland Formation coal gas wells were used to evaluate gas compositional variability. Other data analyzed to determine geologic factors affecting gas variability were 679 initial gas potentials, 544 shut-in pressures, and approximately 1,250 geophysical logs.

The Lewis Shale underlies and intertongues with the Pictured Cliffs Sandstone, which is overlain by the Fruitland Formation. Fruitland coals developed behind Pictured Cliffs shoreline sandstones and between fluvial axes, resulting in northeast-trending belts of thicker coal. Thick (>15 ft) basal coal beds are within 5 ft of the top of the Pictured Cliffs Sandstone except in the north-central part of the study area.

Northwest-trending areas of high initial potentials (>3,000 Mcfd) in Pictured Cliffs Sandstone wells coincide with the distribution of shoreline sandstones. Pictured Cliffs gases probably migrated from the underlying Lewis Shale and overlying Fruitland coal beds. In the north-central study area, chemically drier gases (C₁/C_1-5 > 0.92) are associated with northwest-trending, higher permeability shoreline sandstones, whereas chemically wetter gases are associated with lower permeability sandstones. This chemical variability, occurring in an area of upward-flow potential, may result from fractionation during migration. Fruitland coals show similar gas compositional trends, suggesting that gases in both formations have similar origins and migration pathways.

In the remainder of the study area, sandstone gas compositional trends correspond more to areas of thick basal Fruitland coal. Higher reservoir pressures in Fruitland coal beds during gas generation and/or pressure decline during Pictured Cliffs production resulted in downward migration of coal gases into the underlying sandstones.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995