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Carbonate Petrography 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).

APPROACH

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:

A FEW CASE STUDIES

All petrographic projects conducted by Dravis Geological Services utilize these enhanced petrographic techniques. Any exploration or development geology project which involves massively dolomitized or recrystallized carbonates must be studied using these techniques, if one is to successfully develop a reservoir or enhance exploration strategies.

Blue-Light Fluorescence

A. Thin section photomicrograph of Jurassic dolostone from southern France taken with standard plane-polarized light. Darker areas are secondary pores filled with epoxy resin. In this view, the primary depositional composition and texture, the timing of dolomitization, the origin or the porosity, and the timing of porosity development cannot be resolved. Compare to B. For scale, field of view is about 5.5 mm.

B. Thin section photomicrograph and same view as in A but taken with blue fluorescent light. This view now reveals well sorted relict ooids and peloids, a grainstone texture, and shows that the secondary porosity (bright areas) is fabric-selective moldic porosity and not intercrystalline porosity. This view reveals pressure solution contacts between grains. This relationship establishes that the replacement dolomitization occurred during burial and that the secondary porosity was also burial in origin and facies-controlled.

Diffused Plane-Polarized Light

A. Thin section photomicrograph of a Devonian dolostone sample from western Canada showing typical tightly interlocking crystals of dolomite. Nothing definitive can be said about the depositional texture and the likely environment of deposition (facies). Compare to B. Plane-polarized light; for scale, the field of view is about 3 mm.

B. Thin section photomicrograph and same view as in A but taken with diffused plane-polarized light . This view now reveals numerous small fragments of stromatoporoids, peloids and calcispheres in a grainstone or packstone texture. This sample reflects finer-grained debris shed from a stromatoporoid reef.

REFERENCES CITED:

Dravis, J.J. and Yurewicz, D.A., 1985, Enhanced Carbonate Petrography Using Fluorescence Microscopy, J. Sed. Petrology, v. 55, p. 795-804.

Dravis, J.J., 1989, Deep Burial Microporosity in Upper Jurassic Haynesville Oolitic Grainstones, East Texas, Sedimentary Geology, v. 63, p. 325-341.

Dravis, J.J., 1991, Discussion: Update on New Carbonate Petrographic Techniques and Applications, J. Sed. Petrology, v. 61, p. 626-628.


Dravis Interests and Dravis Geological Services can be contacted by email, by phone at (713) 667-9844, or by mail at 4133 Tennyson St., Houston, Tx 77005-2749.