Recent exploration activity in the San Juan basin of northwestern New Mexico has resulted in the discovery of new oil fields in the Entrada Sandstone of Jurassic age. The major trapping element is provided by topographic relief in excess of 100 ft (30 m) on top of the Entrada. Preliminary analyses indicate that the topographic relief was created by preserved eolian sand dunes which were formed in a topographic basin which then became the sight of a large lake in which limestones and anhydrites of the Todilto Formation were deposited over the Entrada sands. The organic-rich limestones of the Todilto provide the most likely source for the oil found in the underlying Entrada.

Analysis of stratigraphy, oil shows, source-rock potential, and porosity distribution led to the selection of an initial exploration area located along the southeastern flank of the San Juan basin. Seismic model studies, confirmed by an experimental seismic program, indicated that the topographic relief on top of the Entrada could be mapped seismically. An extensive seismic and drilling program has resulted in the discovery of six new oil pools.

In addition to the topographic relief on top of the Entrada, other factors which control the oil accumulations include local structural conditions, hydrodynamics, source-rock and oil-migration history, and porosity-permeability relations in the Entrada. The knowledge gained from this exploration program should aid in future exploration for Entrada oil fields in the San Juan basin, and encourage exploration for similar stratigraphic traps in other basins.

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