Facies and Structural Controls on Carbonate Cementation in a Siliciclastic Reservoir

Steven J. Johansen

The three-dimensional distribution of carbonate cements in Lockhart Crossing field, east Louisiana, reflects localized sources for carbonate anions and cations, paleohydrology, and the processes that trigger carbonate precipitation. Siderite, ankerite, and calcite cements are concentrated at depositional facies boundaries near the crest of the structural trap.

The reservoir is primarily a littoral sandstone, underlain by marine shales and overlain by coastal-plain deposits. The structural trap is a rollover anticline with associated faults. Bicarbonate was supplied by both carbonate bioclasts and oxidation of organic matter, while the required cations were probably derived from carbonate allochems (Ca and Mg), detrital limonite (Fe), and marine clays (adsorbed Mg). These sources were concentrated in the underlying marine shales and lower parts of the littoral sandstone. Shale compaction flushed acidic waters bearing cations and bicarbonate into the littoral sandstone. Carbonates precipitated when these fluids mixed with waters in the sands that were buffered by framework grain dissolution and other processes. Consequently, carbonates are co centrated at a facies-controlled permeability discontinuity within the lower part of the sandstone. A second precipitation trigger was probably the loss of carbon dioxide gas as fluids leaked across fault zones and lost confining pressure. This is supported by the concentration of carbonates adjacent to faults, especially on the updip side of the structure.

Prediction of carbonate distribution in other reservoirs will require models that incorporate the distribution of bicarbonate and cation sources, ion transport in acidic waters, and the alteration of these waters by mixing or by carbon dioxide degassing.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.