The Apco field of north-central Pecos County, Texas, discovered June 1, 1939, has produced, from 46 wells in an area of 1,840 acres, more than 2,646,000 barrels of 42.6º gravity oil to January 1, 1947. The field is on the northwestern flank of the Fort Stockton granite ridge. Oil is produced from a buried hogback of weathered Ellenburger dolomite (lower Ordovician) at depths varying from 4,300 to 4,600 feet. The Ellenburger dolomite rests on a basal sandstone which overlies the pre-Cambrian complex of igneous and metamorphic rocks. The reservoir consists of fracture and honeycomb types of porosity, the latter of which was probably developed by ground-water action during pre-Permian exposure and weathering. The reservoir trap is formed by overlapping and practically flat-lying middle Permian shaly dolomite which envelopes the hogback. An area of pre-Cambrian granite, gabbro, schist, and gneiss, immediately below the Permian, is present on the south and east sides of production at elevations lower than the Ellenburger ridge. Insoluble-residue studies reveal a buried Ellenburger dip slope, held up by a resistant zone approximately 450 feet above the base of the Ellenburger. The restored Ellenburger structure dips from 600 to 1,000 feet per mile west and northwest. Shallow Permian structure, which is probably due to compaction, reflects the hogback as a small closure.
Figures & Tables
Structure of Typical American Oil Fields: A Symposium of the Relation of Oil Accumulation to Structure
Modern petroleum geology in the United States had its beginning in the first decade of the 20th Century when the U.S. Geological Survey began mapping the structure of the rocks in and near old fields in order to discover the various types of structural conditions under which oil and gas are trapped. Structural geology has evolved as a branch of the broader science far more rapidly than have methods of mapping the attitude of rocks at the surface. This volume, published in the late 1920s, was designed to afford authoritative and modern descriptions of the structure of typical oil fields in the United States. Each of the 39 fields contained here is described by an author who is intimately familiar with the available data. The relationship of structure at the surface and at depth for different terranes is clearly set forth wherever the strata are not parallel. The volume concludes with a summary paper on the role of geologic structure in the accumulation of petroleum. Fields include: Florence, Colorado; Stephens, Arkansas; Kevin-Sunburst, Montana; Bradford Pennsylvania; and Salt Creek, Wyoming.