ABSTRACT

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. The Lirette Field is a large domal structure related to deep-seated salt (approximately 20,000 feet), bounded to the north and south by major growth faults. Isothermal surfaces in the field closely follow the structure. Isotherms commonly drop in down-thrown 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-sandstone ratio. Formation water salinities are lower ((50,000 ppm) in wells that have been "flushed" by geopressure waters.

Sandstone diagenesis in the Lirette Field is complex and there are significant lateral and vertical variations. The relative sequence of diagenetic events in Lirette sandstones is as follows: 1) formation of spherulitic calcite cement, probably at or near the sediment-water interface; 2) deposition of authigenic chlorite rims and platelets, which help to preserve primary porosity; 3) development of quartz and feldspar overgrowths, uncommon; 4) cementation by ferroan calcite cement; 5) dissolution of carbonate and formation of secondary porosity; and 6) precipitation of authigenic kaolinite cement, which reduces porosity along fluid escape routes.

Late stage kaolinite cement is more extensively developed in flushed zones near faults.