An older algal-bryozoan bioherm has contributed to the deposition of high-porosity reservoir rock and the creation of a hydrocarbon trap in the San Andres formation at Hanford field, Gaines County, Texas. The bioherm (mound) is nonporous, unproductive, and lies almost 1,000 ft (300 m) below the producing zone. Starting with almost 300 ft (90 m) of relief, this mound was first "drowned" during a transgression and was later gradually buried by prograding, cyclic, subtidal to supratidal carbonates and anhydrites. Approximately 80 ft (25 m) of relief presently exists on the base of the producing interval. Relief on the top of the producing interval is approximately 140 ft (43 m). Lithology and porosity distribution within the producing interval suggest that compaction drape a counts for only part of this relief. The reservoir is thickest and most porous over the crest of the mound. Grainstones and packstones are more common within the reservoir over the crest of the mound than away from it. These observations indicate that the mound was reflected as a shoal in the subtidal zone during the deposition of the producing interval. Relief in this shoal was maintained by higher depositional rates and by the deposition of relatively uncompactible, mud-free sediments in the crestal portion of the mound. Sediments deposited within the subtidal zone, but away from the crest, were deposited at lower rates and contained more mud. Although this model is different from models for almost all other San Andres fields, we are confident that other fields of this type will be dis overed in the Permian basin.