Origin of Secondary Porosity and Cement Distribution in a Sandstone/Shale Sequence from the Frio Formation (Oligocene)
G. K. Moncure, R. W. Lahann, R. M. Siebert, 1984. "Origin of Secondary Porosity and Cement Distribution in a Sandstone/Shale Sequence from the Frio Formation (Oligocene)", Clastic Diagenesis, David A. McDonald, Ronald C. Surdam
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Petrographic, SEM, and chemical analyses of closely spaced samples from a core of sandstone and shale (Oligocene Frio Formation, Brazoria County, Texas) reveal a mechanism for secondary porosity development. Maturation of organic and inorganic materials in the shale produced a solvent solution which, upon expulsion, resulted in zoned reservoir quality in the adjacent sandstone. Framework grain dissolution (secondary porosity) originated at the sandstone/shale contact zone (near the solvent source). Aluminum in this zone was not conserved by the process but instead was removed by mobile, shale-derived organic complexers. The production of these complexers (ligands) appears to be essential to the process of framework grain dissolution. Aluminum removal elevated the silica activity and resulted in precipitation of authigenic quartz cement.
Secondary porosity was developed to a lesser extent farther away from the shale. Imported aluminum from the contact zone and a failure to complex aluminum adequately resulted in kaolinite precipitation. This sink for silica prohibited quartz precipitation.
This general process of framework grain dissolution is probably common in sandstone/shale sequences. In summary, secondary porosity development is accentuated by: (1) high initial permeability, (2) increased relative thickness of shale to sandstone, (3) increased organic content in the shale, and (4) abundant soluble grains (potential secondary pores).
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Clastic diagenesis has evolved from a very descriptive science to a much more process-oriented study. This evolution has been driven by the realization that many hydrocarbon reservoirs have significant diagenetic compotents directly affecting porosity and permeability characteristics. The prediction in time and space of reservoir characteristics affected by diagenesis can greatly reduce the risk in the search for hydrocarbon accumulations, particularly in subtle targets lacking pronounced structural expression. This publication contains three sections designed to increase understanding in the processes controlling clastic diagenesis: Conepts and Principles; Aspects of Porosity Modification; and Applications of Clastic Diagenesis in Exploration and Production. The first two sections deal with processes controlling various aspects of clastic diagenesis, and the third section applies these principles and observations to specific examples. Altogether, the three sections contain 22 chapters.