Carbonate Petrographic Studies

Thin section photomicrograph taken with diffused plane-polarized light showing the distribution of blue secondary micromoldic porosity in Jurassic ooids. This microporosity is not obvious in a normal thin section. Then extensive grain suturing in this view implies this porosity formed under deep burial. Black material is bitumen admixed with pore-filling calcite cements (See Dravis, 1989).

Dravis Geological Services specializes in resolving problems in carbonate exploration or development geology by using rock-based observations tied to wireline log and seismic data. In carbonate sequences, a full understanding of depositional facies and associated reservoir quality can only by gleaned by “looking at the rocks,” preferably core studies supplemented with thin section petrography. In the absence of core data, well cuttings can be thin sectioned and provide useful data as well.

Many clients have come to realize that logs and seismic are not panaceas for delineating play relationships in a carbonate sequence. Rock data must be properly documented and tied to existing log and seismic data inorder to define a play or establish the location of the next well. Companies that integrate rock-based data into their seismically-based play concepts, before they drill their first well, often avoid serious pitfalls associated with exploring soley from seismic data.

The simple petrographic approaches outlined below, quite frankly, have revolutionized thin section petrography. They have the potential, in massive dolomites and highly recrystallized limestones, to reveal depositional and diagenetic fabrics previously undetectable with standard thin section petrography or more cumbersome cathodoluminescence.

This consultant pioneered the application of two Enhanced Petrographic Techniques: blue-light fluorescence microscopy (Dravis and Yurewicz, 1985) and diffused plane-polarized light (Dravis, 1991)*. These techniques help resolve depositional facies and cyclicity, and controls on reservoir quality, in massively dolomitized or highly recrystallized limestone reservoirs. When rigorously applied, they can resolve depositional grains and textures, and diagenetic fabrics, previously invisible in standard thin sections. This technique is much less cumbersome than cathodluminescence (CL), requires no sample prepartion, and often yields information not obtainable with CL. When deemed necessary, these petrographic observations can be supplemented with standard geochemical analyses, such as stable isotopes or trace element geochemistry. Enhanced petrography often results in a firmer understanding of stratigraphic controls on porosity development and distribution. When properly tied to log data, a more rigorous stratigraphic-facies framework can be developed to define play relationships and more efficiently exploit the reservoir.

When applied to massively altered carbonates, the potential benefits of these techniques are:

• Recognition of relict grains
• Determination of primary depositional texture
• Documentation of the origin of porosity
• Recognition of microporosity invisible in normal thin sections (works for unaltered limestones too)
• Delineation of relict diagenetic fabrics, such as healed fractures, microstylolites, sutured grain contacts, etc.
• Establishment of a facies control on reservoir quality (relationship of preserved porosity to relict texture)
• Timing of diagenetic alteration (dolomitization or recrystallization)
• Timing of secondary porosity evolution and its implications for fluid migration
• Easier identification of associated diagenetic minerals which are epifluorescent and whose presence may bear on the origin of diagenetic fluids and their passage through a sedimentary sequence (fluorite, sphalerite, for example).