Reservoir Property Implications of Pore Geometry Modification Accompanying Sand Diagenesis: Anahuac Formation, Louisiana
Michael T. Holland, 1984. "Reservoir Property Implications of Pore Geometry Modification Accompanying Sand Diagenesis: Anahuac Formation, Louisiana", Clastic Diagenesis, David A. McDonald, Ronald C. Surdam
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This paper represents a study of reservoir pore modification accompanying diagenetic secondary porosity development within a deep (13,400 ft) overpressured Anahuac Formation sandstone in southern Louisiana. Secondary porosity formed by dissolution of carbonate cement, detrital grains, and other soluble minerals comprises a significant portion of porosity formed in U.S. Gulf Coast Tertiary reservoir sands. The primary pore system within this reservoir is believed to have been significantly enlarged (by up to 32% porosity) by acidic fluids generated during hydrocarbon maturation and dewatering of adjacent shales. Subsurface secondary porosity development within sandstones is significant in influencing the development of potential reservoir porosity after much of the primary porosity has been destroyed by mechanical and chemical compaction. Properties of the reservoir pore system that affect fluid flow and mechanical resistance of the reservoir to compaction accompanying production will also be influenced.
Characteristics of the reservoir pore system were established by study of whole core samples using scanning electron microscopy, petrographic examination, mercury injection, and simulated in-situ reservoir condition core testing. Secondary pore size and distribution was found to be influenced by sandstone mineralogy, grain size, sorting and angularity, the pore matrix content, and by sedimentary structures and resulting textural components that may hinder fluid flow.
Changes in the mechanical resistance to compaction caused by the development of secondary porosity in sandstone reservoirs is important when considering reservoir stress sensitivity. Keystone bridging relationships between grains can be established during the initial phases of compaction so that when leaching of cement and soluble grains occurs, a less soluble quartz grain matrix is left to support porosity development. Special core tests were performed at simulated in-situ reservoir conditions of pressure and temperature to examine porosity and permeability reduction as a function of effective stress generated by pore pressure reduction (simulated fluid production). Observed volumetric strain to uniaxial compaction at reservoir conditions was determined within portions of the sand containing high (25–30%) porosity. Test results exhibited less than 1% reduction in total bulk volumes accompanying a 60% reduction in pore pressure. Permeabilities measured at in-situ conditions were commonly an order of magnitude less than those measured at ambient conditions. However, with increased effective stress applied to the rock fabric, data suggest that permeabilities decrease at a much slower rate, reflecting constriction of pore throats rather than constriction of stress-induced microfactures thought to exist in core samples at ambient conditions.
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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.