Robbins Gas Field: A Low BTU, Nitrogen Rich Gas Accumulation

Hector, Scott; Grenfell, Conrad; and Blake, Karen E.
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Robbins gas field is located in the northern part of the Sacramento Basin, within the Forbes trend. This trend is characterized by a series of fields that exhibit natural gas trapped stratigraphically in lenticular sands deposited in a deep-sea turbidite environment. The Forbes Formation is Upper Cretaceous in age.

Robbins field is unique in the basin, having the lowest Btu value. The natural gas produced here has 140 Btu value. The gas is composed of 86% nitrogen and 14% methane. The field itself has produced almost 33 billion cubic feet of gas, of which approximately 4.6 Bcf was methane. The onset of field production in 1982 required the construction of a nitrogen rejection plant to extract pipeline-quality methane gas. Robbins gas field produces gas from the “Magoon” sand, a local member of the Forbes formation. The pay depth is at 6600’-6800’. Several wells encountered thin pay sections, and were re-drilled to find thicker pay. This additional drilling data confirmed the lenticular nature of the Magoon sand. Well data shows that this field is primarily a stratigraphic trap, but that it may be assisted by minor faulting. The Robbins field is only part of a fairly large area (1-2 townships) that is characterized by low-Btu accumulations within the Forbes formation.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013