Mateu Esteban¹, Charles Minero², Conxita Taberner³

(1) Carbonates International Iberia, S. L, E-07314 Mallorca, Spain
(2) ENRON Oil and Gas, Houston
(3) Institut de Ciencies de la Terra, C.S.I.C, Barcelona, Spain

ABSTRACT: Micropores, Vugs and Caverns formed by Late Burial Mixing Corrosion in Carbonates

Secondary porosity under unconformities is commonly taken as evidence of surface-related meteoric diagenesis. However, similar features may result from later diagenetic processes in burial conditions.

The Eocene-Oligocene carbonates from offshore India display a long history of early meteoric leaching, cementation, compaction, pressure dissolution and a variety of burial cements (calcite, dolomite, ankerite and siderite). These carbonates were essentially tight prior to major late corrosion, although, minor vuggy and intergranular porosity may have been locally preserved in the interior of nodular fabrics and some grainstone layers.

Corrosion post-dated pressure dissolution and all the carbonate cements. Minor dickite, quartz, pyrite, gypsum and fluorite precipitated after the corrosion event. Corrosive fluids were introduced into the carbonate units along a complex network of faults, fractures, unconformities, pre-existing pores, tension gashes, stylolites and dissolution seams. Corrosion was particularly intense under a regional unconformity, with micropores, vugs and caverns; but similar diagenetic products are observed above and below this unconformity.

Analytical data support a mixing corrosion model of variable amounts of two fluids. Formation fluids had up to 80oC and d18O around +1o/oo, with d13C from +3 to -20o/oo depending on the interaction with organically-derived CO2. These formation fluids mixed with foreign fluids from a deep-seated, evolved meteoric aquifer (up to 160oC, d18O up to +8o/oo and d13C from 0 to -20o/oo depending on the interaction with organic matter from the adjacent source-rock). The post-corrosion paragenesis suggests that acidic conditions could also have been influenced by maturation and migration of organic matter.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado