Showing papers on "Effective porosity published in 1983"

High-Porosity Cenozoic Carbonate Rocks of South Florida: Progressive Loss of Porosity with Depth

Abstract: Porosity measurements by borehole gravity meter in subsurface Cenozoic carbonates of south Florida reveal an extremely porous mass of limestone and dolomite which is transitional in total pore volume between typical porosity values for modern carbonate sediments and ancient carbonate rocks. A persistent decrease of porosity with depth, similar to that of chalks of the Gulf Coast, occurs in these rocks. We make no attempt to differentiate depositional or diagenetic facies which produce scatter in the porosity-depth relationship; the dominant data trends thus are functions of carbonate rocks in general rather than of particular carbonate facies. Carbonate strata with less than 20% porosity are absent from the rocks studied here. Aquifers and aquicludes cannot be distinguished on the basis of porosity. Although aquifers are characterized by great permeability and well-developed vuggy and even cavernous porosity in some intervals, they are not exceptionally porous when compared to other Tertiary carbonate rocks in south Florida. Permeability in these strata is governed more by the spacial distribution of pore space and matrix than by the total volume of porosity present. Dolomite is as porous as, or slightly less porous than, limestones in these rocks. This observation places limits on any model proposed for dolomitization and suggests that dolomitization does not take place by a simple ion-for-ion replacement of magnesium for calcium. Dolomitization may be selective for less porous limestone, or it may involve the incorporation of significant amounts of carbonate as well as magnesium into the rock. The great volume of pore space in these rocks serves to highlight the inefficiency of early diagenesis in reducing carbonate porosity and to emphasize the importance of later porosity reduction which occurs during the burial or late near-surface history of limestones and dolomites.

Porosity reduction by pressure solution: A theoretical model for quartz arenites

Abstract: Pressure solution and the subsequent precipitation of the dissolved material may play an important role in the lithification of sandstones and limestones. This paper presents a one-dimensional model for a well-sorted quartz arenite sequence undergoing diagenesis. The sedimentary sequence is treated as a saturated porous medium in which pressure solution occurs at individual grain contacts. Quartz dissolved at grain contacts precipitates on free surfaces of adjacent grains. This solution-and-precipitation process reduces porosity and forces the migration of pore fluid. Permeability decreases with porosity, hindering fluid migration, and excess fluid pressures are generated. Distributions of porosity, pore-pressure, and fluid-flow rates are given for several sedimentation rates and thermal gradients. Complete reduction of the primary, intergranular porosity was accomplished, in all cases examined, by a depth of 3.5 km.

Effect of alteration on pore structure of crystalline rocks; core samples from Atikokan, Ontario

Abstract: Recent studic indicate that the extensive microDore network within the unfractured rock around major iractures may have a significant effect in reducing the rate of radionuclide migration. The micropore stmcture of granite samples from Atikokan, Ontario has been studied bv analyzing the results of porosity, electrical and permeability mq$urements. The effective porosity, connecting porosity, tortuosity and permeability are calculated from these measurements; their values for these samples are in the order of 0. 1 9-0. 6490, 0.06-0.3 4t10, 1.7 4. 5 and 0. l -54.9 microdarcies, respectively. Results indicate that alteration reduces the connecting porosity and permeability, and increases the tortuosity. This is explained by reduction of the aperture of pore pathways due to dissolution of certain minerals followed by deposition of secondary minerals. The results also suggest that laboratory data for permeability may be extended to field conditions below a certain depth (abour 400 metres).

Porosity Reduction in Sandstone

Abstract: Porosity reduction in siliceous sedimentary rocks is worthy of detailed study because the processes involved control the final porosity and permeability observed in all oil and gas reservoirs. In addition, porosity reduction and petroleum generation and migration may also be related. The effects on porosity reduction of the major porosity-controlling variables are virtually unknown and have been the subject of new experimental studies. Cores of St. Peter Sandstone have been subjected to high effective stress at room temperature and with fluid flow occurring at various rates. Relatively rapid pressure solution occurs even at room temperature in a flowing system.

Secondary porosity in sandstones

Abstract: Secondary porosity in sandstones is caused by dissolution and fracturing, and is common in the sedimentary record. Secondary porosity commonly develops in the deep subsurface and thus provides an opportunity to extend exploration to depths traditionally considered unsuitable for exploration. Two contrasting routes of diagenesis exist in nature: porosity reduction and porosity enhancement. Porosity reduction is commonly caused by compaction and cementation, whereas porosity enhancement is primarily caused by dissolution of carbonate minerals. Two basic types of primary pores (intergranular and intragranular) and four basic types of secondary pores (grain fractures, rock fractures, intergranular, and intragranular) can be differentiated on the basis of (1) position of pores, (2) timing of origin, and (3) processes of origin. The proposed classification system is useful in inferring reservoir quality. Various types of secondary porosity are recognized using a comprehensive set of 20 criteria. The various criteria are based on manner of breakage, pore geometry, grain geometry, products of dissolution, and sediment packing. New evidence suggests that silicate minerals, including quartz, dissolve more commonly than have been reported. The abundant occurrence of secondary porosity in reservoir sandstones emphasizes the importance of secondary porosity in evaluating deep reservoirs.

Secondary Porosity in Sandstones: ABSTRACT

Abstract: Secondary porosity in sandstones is caused by dissolution and fracturing, and is common in the sedimentary record. Secondary porosity commonly develops in the deep subsurface and thus provides an opportunity to extend exploration to depths traditionally considered unsuitable for exploration. Two contrasting routes of diagenesis exist in nature: porosity reduction and porosity enhancement. Porosity reduction is commonly caused by compaction and cementation, whereas porosity enhancement is primarily caused by dissolution of carbonate minerals. The dissolution of carbonate minerals in the deep subsurface can be attributed to release of carbon dioxide during thermal maturation of kerogen. Clay-carbonate reaction and flushing of undersaturated meteoric water from erosional unconformity are equally important mechanisms in generating secondary porosity at various depths. Two basic types of primary pores (intergranular and intragranular) and four basic types of secondary pores (grain fractures, rock fractures, intergranular, and intragranular) can be differentiated on the basis of (1) position of pores, (2) timing of origin, and (3) processes of origin. The proposed classification system is useful in inferring reservoir quality. Various types of secondary porosity are recognized using a comprehensive set of 20 criteria. The various criteria are based on manner of breakage, pore geometry, grain geometry, products of dissolution, and sediment packing. Examples of secondary porosity gathered in part from investigation of 2,000 thin sections will serve as a guide for recognizing secondary porosity in outcrop, in hand specimen, under the scanning electron microscope, and most importantly, under the petrographic microscope. New evidence suggests that silicate minerals, including quartz, dissolve more commonly than have been reported. The abundant occurrence of secondary porosity in reservoir sandstones emphasizes the importance of secondary porosity in evaluating deep reservoirs. Since the first recognition of secondary porosity by Nutting in 1934, Chepikov in 1959 and 1961 not only developed the first set of criteria for recognition of secondary porosity but also introduced the concept in which development of secondary porosity is related to arrival of oil. End_of_Article - Last_Page 547------------

Permeability of selected sediments in the vicinity of five salt domes in the Gulf Interior Region

Abstract: The purpose of this investigation was to determine what relationships might exist between permeability and parameters available from geophysical logs of sediments within the Gulf Interior Region. The parameters that have received the most attention in the literature are formation resistivity factor and porosity. Empirical relationships between permeability and porosity, gamma-ray bulk density, the fraction of intergranular pore space occupied by clay or silt, and porosity differences were investigated. Porosity appears to be the log parameter that best correlates with permeability.