Print this pagePetrographic Assessment of Lateral Patterns in the Petrophysics and Geochemistry of Madison (Mississippian, USA) Dolograinstones
Samples at 1-ft spacing along 500 and 250 foot-long lateral transects in Madison dolograinstones (Lysite Mt., WY) are known to exhibit lateral patterns in petrophysical properties - a large near-random component (50%-60% of all variance at 1-ft spacing), a dominant short-range structure measured in tens of feet, and a pronounced low magnitude, longer range pattern also at scales of tens of feet. The short and longer range structures combine to form lateral periodic oscillations in porosity and permeability that is superimposed on the near-random noise. Calcite-free bulk-rock trace element (Mn, Sr, Na, and Fe) and stable isotope values (carbon and oxygen) all show the same distinct lateral variability, and in some cases exhibit an even stronger dominance by the longer-range periodic effects. These dolograinstones consist of medium-crystalline dolomite rhombs with intercrystalline and moldic porosity. No geochemical or petrophysical parameter except porosity correlates to the abundance of those two pore types. 18O-depleted late calcite cement and a small amount of late quartz cement occur in minor (1%-10%) amounts in ~40% of the samples, and they can produce some lower porosities. “Late” dolomite is present as evidenced by Sr-isotopic compositions, a thin (<10 microns) orange (Mn-richer) luminescent dolomite cement rim on most crystals and the same luminescent phase as mottles within the dolomite rhombs (recrystallization).
The lack of significant covariance between porosity and Mn (R=-0.10), oxygen isotopes (R=0.34), or abundance of late calcite and quartz (R=0.31) suggests that late diagenetic overprinting is not the primary cause of the spatial patterns. Generation of all the lateral patterns by self-organizing processes during dolomitization remains an intriguing explanation. (Self-organization is the creation of patterns in a system from non-patterned states due to feedbacks between the evolving properties of the system and the processes affecting that system). A suite of initial reaction-transport models coupled to textural evolution during lime grainstone to dolograinstone alteration indicate that lateral porosity patterns, similar in spatial scale to those observed in the Madison, can form in dolostone beds less than 2 m thick, although the magnitude of the simulated patterns is less than the observed patterns. If this hypothesis holds up to further scrutiny, then self-organization is a phenomena to consider when assessing dolostones.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009