The Kemnitz field in Lea County, New Mexico, is a typical example of stratigraphic entrapment of oil in a carbonate reservoir rock. To understand the reason for entrapment of oil in fields such as Kemnitz, one should analyze geological factors in terms of depositional environment, diagenetic history, and structural history.

Environments favorable to deposition of reservoir-type carbonate formations include reefs, bioherms, oolite bars, and porous skeletal calcarenites. Production of organic material in such environments (with the exception of oolite bars) is prolific; however, under normal conditions most of the organic soft parts are destroyed by bacteria and scavengers, so that only skeletal parts are preserved.

Hydrocarbons are found in cyclic carbonates which were deposited on unstable shelves which were subject to recurrent sea-level fluctuations and periodic influxes of terrigenous clastic sediments. A reef bank or oolite bar can be covered by transgression of basin sediments, suffocated by regressive evaporites, or smothered by influx of terrigenous clastic sediments. Biogenic carbonates which are overlain by evaporites, black sapropelic shale, or basin sediments reflect an early diagenetic history which was favorable for the preservation of animal and plant remains. Early diagenesis is also important in dolomitization and modification of primary porosity of limestone.

Petrologic studies of the Kemnitz reef indicate that this stratigraphic trap is caused by a barrier reef crossing a plunging structural nose. The lower Wolfcamp (Permian) is slightly transgressive and the reef top is covered by basin sediments. Thus this reef retained its porosity and organic source material.

In the North Anderson Ranch field, Lea County, New Mexico, the upper Cisco (Pennsylvanian) is a reef-type porous carbonate but is non-productive because of unfavorable diagenetic history. The younger lower Wolfcamp also is of reefoid nature and occupies a position along the flank of the structure lower than the porous Cisco reef at the crest. The Wolfcamp had a favorable early diagenetic history and therefore contains commercial accumulations of oil.

Elusive stratigraphic traps in carbonate rocks can be explored effectively only after thorough subsurface and structural analyses are supplemented with studies of the environment and diagenetic history of prospective carbonate beds.

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