Evaluating the Role of Meteoric Karst vs. Burial Corrosion in an Offshore Indian Carbonate Field

Oates, Michael; Chandra, Viswasanthi

A mismatch between horizontal and vertical permeability in the Eocene-Oligocene Bassein Limestone reservoir of the giant Panna Field, Bombay Basin, offshore India has long been observed from core studies and DST results. A recent study demonstrated just how dramatic this is by moving a small volume of tracer up to 2km in several directions in less than a day. Evident high permeability below a lowstand surface capping the Bassein "B" reservoir is believed to be the conduit. While connected phreatic meteoric karstic porosity development would seem the obvious permeability generator, no diagnostic petrographic evidence of meteoric porosity is seen, and where cored, the unconformable surface has a pervasive mudstone infill which acts as a fluid-flow barrier in some areas of the field.

Instead this permeability originates from high amplitude stylolites and associated small tension fractures which are more abundant in this stratigraphic level of clean, inner ramp facies which preceded the lowstand; flow takes place along stylolites, fractures and heavily corroded zones with high mouldic and microporosity adjacent to the stylolites. This has resulted from late stage hydrothermal corrosion along stylolite networks and dickite precipitation is common.

The stylolites and the highly corroded zones were mapped from core and image logs to obtain enhanced core description and were correlated with the available petrophysical data. The results indicated strong agreement between the density of distribution of stylolites and the extent of corroded zones. This concurs with the premise that the stylolites and associated fractures were the features of initial high permeability that conducted the invasion of the corrosive fluids into the surrounding matrix. Analysis of available RCA and minipermeameter measurements further substantiated this by indicating good correlation between the stylolites associated with solution enhanced porosity zones and the enhanced porosity-permeability zones in the Bassein B reservoir.

The link between good reservoir quality and a lowstand surface is therefore not related to meteoric dissolution but to facies-controlled pressure solution effects providing a conduit to later corrosive fluid flow. This was enabled by a change in stress regime which reduced the relative effect of overburden, and allowed relaxation of horizontal stylolite planes and certain basement faults up which acidic hydrothermal fluids could ascend.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013