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Influence of Facies, Fractures, and Weathering

on the Hydraulic Properties of the Monroe Gas Rock
 

Douglas Carlson and Thomas Van Biersel
 

Louisiana Geological Survey – Louisiana State University,

3079 Energy, Coast and Environment Bldg., Baton Rouge, Louisiana  70803
   

ABSTRACT

 

Although the Monroe Gas Rock (MGR) has been the studied for almost 90 years, this study is the first to consider in detail the hydraulic properties of this unit.  The examination of approximately 3500 well test results and 35 geophysical logs shows that the fractures-faults, facies, and weathered zones influence the hydraulic properties of the MGR.

 

Earlier studies have observed the presence of three carbonate facies within the MGR (in descending order, an upper packstone, a middle sandy-silty wackstone, and a lower grainstone).  These facies appear to yield different average porosities (measured from core samples).  However, the examination of geophysical logs reveals a fourth zone at the top of the MGR of somewhat greater and more variable porosity.  This is possibly a weathered zone that lies below the erosional unconformity that defines the top the unit.

 

The influence of facies not only impacts porosity but also impacts the unit’s permeability in a vertical direction and the spatial distribution of permeabilities.  Previous studies identified a facies change near the northwest-north edge of the MGR which acts as a flow boundary for the Monroe Gas Field.  This is corroborated by this study’s well test results, which show a general cluster of lower permeability values (one standard deviation below the geometric mean value) in this region.  Similarly, the higher permeability values (one standard deviation above the geometric mean value) are not randomly scattered, but generally fall along lines trending approximately east-west.  These lines have orientations that appear to be similar to known faults in northern Louisiana observed by previous workers.  In summary, the porosity and permeability of the MGR is typical of a carbonate rock where facies, fractures, and a weathered zone influences hydraulic properties. 

Carlson, D., and T. Van Biersel, 2009, Influence of facies, fractures, and weathering on the hydraulic properties of the Monroe Gas Rock:  Gulf Coast Association of Geological Societies Transactions, v. 59, p. 159-170.

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AAPG Search and Discover Article #90093 © 2009 GCAGS 59th Annual Meeting, Shreveport, Louisiana