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AAPG Bulletin, V.
Meteoric versus burial control on porosity evolution of the Smackover Formation Ezat Heydari
Department of Physics, Atmospheric, and General Science, Jackson State University, P.O. Box 17660, Jackson, Mississippi 39217; email: [email protected]
Ezat Heydari finished his undergraduate studies in geology at the University of Tehran in Iran. His graduate education in geology includes a master's degree from Pennsylvania State University and a Ph.D. from Louisiana State University (LSU). He has worked as a research scientist at LSU and at the Mississippi Office of Geology. He is currently an assistant professor at Jackson State University. He has conducted research on sedimentology and diagenesis of Mesozoic formations of northern United States Gulf Coast and Permian and Triassic strata of Iran. His interests revolve around depositional environment, diagenesis, and geochemistry of carbonate rocks to solve issues related to fluid-rock interactions and to the Earth's history.
I am indebted to Clyde Moore for countless hours of memorable exchange of ideas on this topic. I thank Phillips Petroleum for releasing the cores used in this study. W. J. Wade reviewed the manuscript and made valuable suggestions. Critical comments by AAPG reviewers Dave Eby, Jerry Lucia, and an anonymous reviewer clarified many aspects of the study.
Whole-rock 13C, 18O, and Sr compositions of the uppermost ooid grainstones of the Smackover Formation at Black Creek field in Mississippi reveal characteristics of a meteoric system in this area. The trend in 13C signatures indicates that meteoric water acquired its low carbon values at a soil zone and was buffered completely with the host rock before entering the meteoric lens. Identical 18O data suggest that allochems stabilized at the same temperatures in waters of similar 18O compositions during early meteoric diagenesis; that is, calcitization continued to completion before the onset of burial. Sr concentrations reveal differential transformation mechanisms. Low Sr values in the phreatic zones are attributed to slow stabilization by exchange with the bulk solution. High Sr compositions in the vadose zone resulted from rapid, incremental transformation.
Meteoric cementation was sparse; therefore, grainstones entered the burial realm with all of their porosity intact. Modification of porosity was a burial process and was directly related to the intensity of chemical compaction, which itself was controlled by three factors: (1) the degree of early cementation, (2) grain type, and (3) grain size. Early-cemented intervals experienced low to moderate compaction regardless of grain size or grain type. In the absence of early cementation, grain type and grain size were the dominant factors of porosity control. Coarse-grained intervals that were composed of micritic grains experienced less compaction than fine-grained, oolitic intervals.
Average volumetric percentages of cements as determined by point counting of 63 samples are as follows: (1) marine and meteoric cements = 1.5%, (2) prebitumen calcite = 6.5%, (3) saddle dolomite = 1%, (4) postbitumen calcite = 4.3%, (5) pyrobitumen = 1%. Total cement (sum of 1–5), total burial cement (sum of 2–5), and total burial carbonate cement volumes (sum of 2–4) average 14, 13, and 12%, respectively. Out of an original porosity of 40%, 13 porosity units were lost by cementation, and 27 porosity units were reduced by compaction (mechanical and chemical).
Present-day intergranular porosity of these grainstones is zero, whether they belonged to the vadose zone or the phreatic interval. Burial cementation and compaction destroyed all intergranular porosity, regardless of early diagenetic overprint or textural variables. The reservoir became compartmentalized during burial. Some intervals were active producers of calcium carbonate; others were passive recipients.
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