A study of temperature, pressure, and salinity distribution in the Lirette field reveals important information concerning the effects of the hydrodynamic regime on sandstone diagenesis. Mineralogic examination of associated shales also aids in the understanding of diagenesis in these sands.

The Lirette field is a large domal structure related to deep-seated salt, approximately 20,000 ft (6,096 m), bounded to the north and south by major growth faults. Isothermal surfaces in the Lirette field closely follow the structure. Isotherms commonly drop in downthrown fault blocks. Along fault leakage zones, temperatures increase. Pressure distribution in the Lirette field is primarily related to structure, and the presence of a sufficient shale to sand ratio. Formation water salinities are lower (< 50,000 ppm) for wells that have been "flushed" by geopressured waters.

The well-documented decrease of smectite in mixed-layer illite-smectite is present in Lirette shales. A more detailed analysis indicates that some montmorillonite may be converted to beidellite before it is converted to illite.

Sandstone diagenesis in the Lirette field is complex and there are significant lateral and vertical variations. The relative sequence of diagenetic events in Lirette sands is as follows: (1) spherulitic calcite cement, probably formed at or near the sediment-water interface; (2) authigenic chlorite rims and platelets, which help to preserve primary porosity; (3) quartz and feldspar overgrowths, uncommon; (4) ferroan calcite cement, due to localized flushing of sandstones by waters released from clay diagenesis; and (6) authigenic kaolinite cement, which reduces porosity along fluid escape routes.

Extensive carbonate cement and orthomatrix are the primary contributors to decreased porosity. Late stage kaolinite cement in flushed zones also reduces porosity, but to a lesser extent.

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