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The Pros and Cons of Various Dolomite Models:
Some Work, Many Don’t*
Hans G. Machel1
Search and Discovery Article #50103 (2008)
Posted December 9, 2008
*Adapted from oral presentation at AAPG Annual Convention, San Antonio, Texas, April 20-23, 2008
1EAS, Univ. of Alberta, Edmonton, AB, Canada ([email protected])
The so-called “dolomite problem” has several facets: (a) dolomites can form in many different diagenetic and hydrothermal settings; (b) data often permit more than one genetic interpretation; (c) dolomite is rare in Holocene sediments, yet abundant in older rocks; and (d) well-ordered, stoichiometric dolomite has never been successfully grown inorganically in laboratory experiments at near-surface conditions. These aspects lead to practical problems plaguing the petroleum industry, most notably: how much dolomite can be formed by any given process (or model), and what are the resulting geometry and porosity-permeability patterns in dolostone reservoirs, which are notoriously difficult to interpret and predict? ---
From a practical point of view, the “dolomite problem” can be solved to a degree that would enhance exploration success, and thus allow for more efficient development of dolostone reservoirs. Attaining these goals requires a comprehensive understanding and application of the involved disciplines, most notably crystallography, geochemistry, and hydrogeology. Many ambiguities in previous studies arose from a superficial understanding and lack of rigor in the application of these disciplines. A common result is that some dolomite models, such as the mixing zone and the hydrothermal dolomite models, are essentially useless in the prediction of dolostone reservoirs.
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Thermodynamic requirements Most subsurface fluids CANNOT produce dolomite 2. Mass balance requirements Seawater or evaporated seawater High fluxes, long-lasting fluid flow – not just a “squirt up a fault” 3. Dolomitization model useful for exploration ? Reflux model yes Thermal convection model yes Mixing zone model Snake Oil Hydrothermal model Snake Oil
Is the HTD Model “Useful” as an Exploration Tool?
It has been claimed that the HYDROTHERMAL DOLOMITE (HTD) MODEL is useful, even highly successful as a hydrocarbon exploration tool. Multiple discoveries of dolomitized fault traps in carbonates, such as in the Trenton (Albion Scipio field and others), are often cited as examples.
This, however, is a hollow claim. Almost all traps in the Trenton were discovered using seismic (as well as other exploration tools, and a good portion of luck), over a period of more than 100 years, dating back to the 19th century, long before the HTD model was even proposed. Not a single one of these or similar exploration successes is due to application of the HTD model. The Trenton is a structural play, and the traps owe their existence to faulting, not to dolomite.
In fact, most fault traps in the world are devoid of dolomite.
We thus fail to accept the notion that the HTD model (even if it were free of its many original and current errors) is useful for exploration. Our assertion does not mean that there are no dolomitized fault traps (of course there are), nor does it deny the fact that dolomite, if and where it forms around faults, enhances the reservoir properties significantly (increased porosity, storage capacity, and permeability) in some localities. None of these aspects, however, are part of exploration, which commonly uses only seismic to delineate the geometry of a trap, dolomitized or not.
Lastly, if there is hydrothermal dolomite, it often is a CEMENT, thus DETRIMENTAL to the reservoir quality. Hydrothermal fluids often recrystallize older matrix dolomites, a common source of misinterpretation.
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