Page 193 - Geology of Carbonate Reservoirs

174 DIAGENETIC CARBONATE RESERVOIRS

according to diagenetic origins. Purely diagenetic porosity in reservoir zones can be
traced with more certainty by following the pathways that diagenesis took during
burial history, such as pore reduction by cementation or pore enhancement by dis-
solution. Then, identifying times during which each diagenetic event occurred
enables one to establish the sequence of burial diagenetic events that shaped the

final pore system. Finally, reservoir flow units can be identified, mapped, and ranked

at field scale if the geological conditions that existed during key diagenetic episodes

can be identified. Did pore enhancement by dissolution influence reservoir porosity,
for example? Are the dissolution zones linked to some kind of marker like an
unconformity in the stratigraphic section? Careful observation will point the way
to identifying more of the dissolution porosity if the stratigraphic and geographic
coordinates of the unconformity, or other type of marker, are known. The geological

setting that existed in order for the marker event to happen in the first place may
have been tectonism associated with local or regional structural features, fl uctua-
tions of relative sea level, or dramatic climate change. Knowing the major causes of
geological change that set the stage for specific diagenetic environments and kinds

of diagenesis to occur provides the large - scale spatial reference to use in predicting

the locations of “ good ” and “ bad ” diagenetic changes at field or regional scale.
SUGGESTIONS FOR FURTHER READING

A good starting point for nonspecialists is the book by McIlreath and Morrow
(1990) , Diagenesis . This book is a compilation of papers on diagenesis in both car-
bonates and siliciclastics. It is well illustrated and has an extensive list of references
for that time period. Many of the illustrations are excellent line drawings that illus-
trate diagenetic features more vividly than photographs. A must - read reference is
the book by Tucker and Wright (1990) , Carbonate Sedimentology . Chapters 6 – 9
provide excellent discussions, illustrations, and references for carbonate mineralogy
and chemistry (Chapter 6), diagenetic processes, products, and environments
(Chapter 7 ), dolomites and dolomitization models (Chapter 8 ), and carbonate rocks
throughout geological time (Chapter 9 ). The geological history of carbonate rocks
through time includes discussions about temporal and secular changes in carbonate
mineralogy and chemistry over time as well as discussions on variations in deposi-
tional styles and facies through time. Those interested in the more academic aspects

of carbonate diagenesis will find a thorough review of carbonate geochemistry in
Morse and Mackenzie (1990) , Geochemistry of Sedimentary Carbonates . An older
but useful reference on carbonate sedimentology and diagenesis is Bathurst (1975) ,
Carbonate Sediments and Their Diagenesis , 2nd edition. A recent and comprehen-
sive review of topics on burial diagenesis is by Machel (2005) , “ Investigations of
Burial Diagenesis in Carbonate Hydrocarbon Reservoir Rocks. ” The most common
techniques, other than standard petrography, for identifying the types of diagenesis
and the environmental conditions under which the changes took place involve one
or more of (1) stable isotope geochemistry, (2) cathode luminescence microscopy,

(3) analyses of fluid inclusions, or (4) one of several types of microprobe analyses
to identify trace element concentrations. These techniques require extensive expla-
nations with examples — something beyond the scope and purpose of this book.
Those interested in a brief summary of cathode luminescence applied to carbonates