The lower part of the San Andres Formation in east-central New Mexico consists of three cyclical zones, commonly known as the P-3, P-4, and P-5 porosity zones. Each of these, typically, consists of a thin, widespread evaporite at the top, followed by carbonates in the middle and a thin shaly carbonate zone at the base. Locally, halite may replace anhydrite. The rest of each zone consists of carbonates; dolomite normally underlies the upper anhydrite and may be as much as 100 ft or more thick. The dolomite may contain one or more layers of anhydrite and is underlain by limestone.

Lithofacies studies of the lower San Andres indicate that where the carbonate consists entirely of dolomite, there is an above-average amount of evaporites. Lithofacies studies also indicate that porosity trends are mappable and help to determine favorable areas for petroleum exploration.

The lower San Andres of east-central New Mexico was deposited along a north to northeast-trending coast that prograded southward and eastward except during the deeper water beginning of each cycle when shale or limestone typically was deposited. The north and west limits of traceable San Andres zones seem to be gradual limits where the San Andres becomes more evaporitic. Porosity in the P-4 and P-5 zones persists farther north than in the P-1, P-2, and P-3 zones, demonstrating the general southward shift of facies with time.

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