Changes

  | isbn    = 0891816607

  | isbn    = 0891816607

}}

}}

Careful examination and recording of information from continuous cores provide critical data for stratigraphic correlation, environmental interpretation, and wireline log calibration. Core plugs provide samples for analysis of [[porosity]], [[permeability]], fluid saturation, and a host of compositional and textural studies. Recommendations for the well site and laboratory handling of cores as well as sedimentological analyses are described by Siemers et al.<ref name=pt05r145>Siemers, C. T., Tillman, R. W., Williamson, C. R., eds., 1981, Deep-water clastic sediments—a core workshop: SEPM Core Workshop, n. 2, 416 p.</ref> and Miall.<ref name=pt05r109>Maness, M., Price, J. G. W., 1977, Well formation characterization by residual hydrocarbon analysis: Annual Meeting of the Society of Petroleum Engineers, Denver, CO, Oct. 9–12, SPE 6860.</ref>

[[File:FG18CoreSt63Appendix3.JPG|thumb|300px|Core slab photo.<ref name=Ruppel_2012>Ruppel, Stephen C., 2012, [http://archives.datapages.com/data/specpubs/study63/APPENDIX3/APPENDIX3.HTM Appendix 3], in S. C. Ruppel, ed., Anatomy of a giant carbonate reservoir: Fullerton Clear Fork (Lower Permian) field, Permian Basin, Texas: [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/sg63.htm AAPG Studies in Geology 63], unnumbered.</ref>]]

Continuous cores have some advantages over outcrop exposures for environmental interpretation.<ref name=pt05r166>Weimer, R. J., Tillman, R. W., 1980, Tectonic influence on deltaic shoreline facies, Fox Hills Sandstone, west-central Denver basin: Colorado School of Mines Professional Contribution, n. 10, 131 p.</ref> These advantages include the following:

 

Continuous cores have some advantages over [http://www.merriam-webster.com/dictionary/outcrop outcrop] exposures for environmental interpretation.<ref name=pt05r166>Weimer, R. J., and R. W. Tillman, 1980, Tectonic influence on deltaic shoreline facies, Fox Hills Sandstone, west-central Denver basin: Colorado School of Mines Professional Contribution, n. 10, 131 p.</ref> These advantages include the following:

* Not limited to stratigraphic unit outcrop positions

* Not limited to stratigraphic unit outcrop positions

* Often provide a more complete section of the stratigraphic unit

* Often provide a more complete section of the stratigraphic unit

* Better preservation of contacts between units having significantly different resistances to weathering

* Better preservation of contacts between units having significantly different resistances to weathering

* Better preservation of delicate primary and soft sediment deformation structures in shale and siltstone units

* Better preservation of delicate primary and soft sediment [[deformation]] structures in shale and siltstone units

* Better preservation of trace fossils

* Better preservation of trace fossils

* Ability to obtain material for petrographic study below the present groundwater table

* Ability to obtain material for petrographic study below the present groundwater table

* Allow comparison of lithologic properties with petrophysical properties and wireline log responses.

* Allow comparison of lithologic properties with petrophysical properties and wireline log responses.

These advantages, however, are offset by the lack of a three-dimensional view and the inability to observe lateral facies changes and large-scale sedimentary features directly.

These advantages, however, are offset by the lack of a three-dimensional view and the inability to observe [[lateral]] facies changes and large-scale sedimentary features directly.

 

 

A graphic log format, which visually expresses a stratigraphic succession, is strongly recommended for describing continuous cores. Such logs should reflect the following:

A graphic log format, which visually expresses a stratigraphic succession, is strongly recommended for describing continuous cores. Such logs should reflect the following:

* Textural maturity

* Textural maturity

* Oil staining

* Oil staining

* Fracturing

* [[Fracture|Fracturing]]

* [[Porosity]]

* [[Porosity]]

Recommended formats for graphic logging are given by Bebout and Loucks<ref name=pt05r24>Bebout, D. G., and R. G. Loucks, 1984, Handbook for Logging Carbonate Rocks: The University of Texas at Austin, Bureau of Economic Geology Handbook 5, 43 p.</ref> and Boyles et al.<ref name=pt05r28>Boyles, J. M., A. J. Scott, and J. M. Rine, 1986, A logging form for graphic descriptions of core and outcrop: Journal of Sedimentary Petrology, v. 56, p. 567–568., 10., 1306/212F89DB-2B24-11D7-8648000102C1865D</ref> A completed graphic log is shown in [[:file:core-description_fig1.png|Figure 1]]. Grain size, sedimentary structures, and accessories are shown in the left column. Let's look at how each type of data is handled in the graphic log.

 

Recommended formats for graphic logging are given by Bebout and Loucks<ref name=pt05r24>Bebout, D. G., Loucks, R. G., 1984, Handbook for Logging Carbonate Rocks: The University of Texas at Austin, Bureau of Economic Geology Handbook 5, 43 p.</ref> and Boyles et al.<ref name=pt05r28>Boyles, J. M., Scott, A. J., Rine, J. M., 1986, A logging form for graphic descriptions of core and outcrop: Journal of Sedimentary Petrology, v. 56, p. 567–568., 10., 1306/212F89DB-2B24-11D7-8648000102C1865D</ref> A completed graphic log is shown in Figure 1. Grain size, sedimentary structures, and accessories are shown in the left column. Let's look at how each type of data is handled in the graphic log.

 

===Grain size===

===Grain size===

Terrigenous clastic grain sizes and carbonate rock types are recorded as a continuous vertical curve reflecting changes in depositional energy. Note that grain size increases to the left, as is inferred in the case of spontaneous potential (SP) and gamma ray curves. Grain size is determined by comparison with a chart made of sized sedimentary material or a visual pattern such as that produced by Amstrat. A good qualify handlens or reflecting light binocular microscope are essential for accurate grain size determination.

Terrigenous clastic grain sizes and carbonate rock types are recorded as a continuous vertical curve reflecting changes in depositional energy. Note that grain size increases to the left, as is inferred in the case of [[Basic open hole tools#Spontaneous potential|spontaneous potential (SP)]] and [[Basic open hole tools#Gamma ray|gamma ray]] curves. Grain size is determined by comparison with a chart made of sized sedimentary material or a visual pattern such as that produced by Amstrat. A good qualify handlens or reflecting light binocular microscope are essential for accurate grain size determination.

===Sedimentary structures===

===Sedimentary structures===

Sedimentary structures are recorded to the right of and just inside the grain size curve. There is no single set of standardized symbols for sedimentary structures. Examples are given in Swanson<ref name=pt05r150>Swanson, B. F., 1981, A simple correlation between permeabilities and mercury [[capillary pressure]]s: Journal Petroleum Technology, v. 33, p. 2498–2504., 10., 2118/8234-PA</ref> and Bebout and Loucks<ref name=pt05r24 />. Realistic sketches of observed structures may be more useful than standard symbols. The vertical succession of sedimentary structures should be recorded as accurately as possible because this succession is often the key to successful environmental interpretation. Description, classification, and interpretation of sedimentary structures are discussed by Collinson and Thompson<ref name=pt05r35>Collinson, J. D., Thompson, D. B., 1989, Sedimentary Structures: Boston, MA, Unwin Hyman, 207 p.</ref> and Lindholm<ref name=pt05r103>Lindholm, R. C., 1987, A Practical Approach to Sedimentology: Boston, MA, Allen and Unwin, 276 p.</ref>.

Sedimentary structures are recorded to the right of and just inside the grain size curve. There is no single set of standardized symbols for sedimentary structures. Examples are given in Swanson<ref name=pt05r150>Swanson, B. F., 1981, A simple correlation between permeabilities and mercury [[capillary pressure]]s: Journal Petroleum Technology, v. 33, p. 2498–2504., 10., 2118/8234-PA</ref> and Bebout and Loucks.<ref name=pt05r24 /> Realistic sketches of observed structures may be more useful than standard symbols. The vertical succession of sedimentary structures should be recorded as accurately as possible because this succession is often the key to successful environmental interpretation. Description, classification, and interpretation of sedimentary structures are discussed by Collinson and Thompson<ref name=pt05r35>Collinson, J. D., and B. D. Thompson, 1989, Sedimentary Structures: Boston, MA, Unwin Hyman, 207 p.</ref> and Lindholm.<ref name=pt05r103>Lindholm, R. C., 1987, A Practical Approach to Sedimentology: Boston, MA, Allen and Unwin, 276 p.</ref>

===Accessories===

===Accessories===

===Lithology===

===Lithology===

Dominant lithologies and the nature of the contact between lithologic units are recorded in the “Rock Type and Contacts” column near the middle of a graphic log (Figure 1). Standardized symbols for illustrating lithologies are available and are presented by Tucker.<ref name=pt05r157>Tucker, M. E., 1982, The Field Description of Sedimentary Rocks: United Kingdom, The Open University Press, 112 p.</ref> and Lindholm<ref name=pt05r103 /> If more than one lithology or interbedded lithologies exist within an interval, lithologies should be logged by estimating the percent volume of each lithology and noting the terrigenous clastic lithologies to the left of the chemical rocks. Contacts between lithologies are recognized in core as gradational, interbedded, or sharp. Some sharp contacts are probably erosional and may represent unconformity surfaces. Additional information such as the presence of root structures and early diagenetic cements may aid in determining the true nature of sharp contacts.

Dominant lithologies and the nature of the contact between lithologic units are recorded in the “Rock Type and Contacts” column near the middle of a graphic log ([[:file:core-description_fig1.png|Figure 1]]). Standardized symbols for illustrating lithologies are available and are presented by Tucker<ref name=pt05r157>Tucker, M. E., 1982, The Field Description of Sedimentary Rocks: United Kingdom, The Open University Press, 112 p.</ref> and Lindholm.<ref name=pt05r103 /> If more than one lithology or interbedded lithologies exist within an interval, lithologies should be logged by estimating the percent volume of each lithology and noting the terrigenous clastic lithologies to the left of the chemical rocks. Contacts between lithologies are recognized in core as gradational, interbedded, or sharp. Some sharp contacts are probably erosional and may represent [[unconformity]] surfaces. Additional information such as the presence of root structures and early diagenetic cements may aid in determining the true nature of sharp contacts.

===Maturity===

===Maturity===

Textural maturity of sandstone units is recorded in the next column to the right (“Textural Maturity” in Figure 1) and is described following the method outlined by Folk.<ref name=pt05r57>Folk, R. L., 1974, Petrology of Sedimentary Rocks: Austin, TX, Hemphill Publishers, 159 p.</ref> Detrital clay (matrix) content and the sorting and roundness of grains are all considered in the determination of textural maturity. Because it is difficult to estimate the relative abundance of detrital versus authigenic clay without thin section or scanning electron microscope data, care must be taken in making estimates of detrital clay content. Image charts can be used for estimating sorting and roundness (Figure 2).

[[file:core-description_fig2.png|thumb|300px|{{figure number|2}}(a) Comparison chart for sorting and sorting classes. (Modified from Pettijohn & Siever.<ref name=pt05r125>Pettijohn, F. J., P. E. Porter, and R. Siever, 1987, Sand and Sandstone, 2nd. ed.: New York, Springer-Verlag, 553 p.</ref>) (b) Comparison chart for roundness and sphericity. (Modified from Powers.<ref name=pt05r130>Powers, M. C., 1953, A new roundness scale for sedimentary particles: Journal Sedimentary Petrology, v. 23, p. 117–119.</ref>)]]

[[file:core-description_fig2.png|thumb|{{figure number|2}}(a) Comparison chart for sorting and sorting classes. (Modified from <ref name=pt05r125>Pettijohn, F. J., Porter, P. E., Siever, R., 1987, Sand and Sandstone, 2nd. ed.: New York, Springer-Verlag, 553 p.</ref>.) (b) Comparison chart for roundness and sphericity. (Modified from <ref name=pt05r130>Powers, M. C., 1953, A new roundness scale for sedimentary particles: Journal Sedimentary Petrology, v. 23, p. 117–119.</ref>.)]]

Textural maturity of sandstone units is recorded in the next column to the right (“Textural Maturity” in [[:file:core-description_fig1.png|Figure 1]]) and is described following the method outlined by Folk.<ref name=pt05r57>Folk, R. L., 1974, Petrology of Sedimentary Rocks: Austin, TX, Hemphill Publishers, 159 p.</ref> Detrital clay (matrix) content and the sorting and roundness of grains are all considered in the determination of textural maturity. Because it is difficult to estimate the relative abundance of detrital versus authigenic clay without thin section or scanning electron microscope data, care must be taken in making estimates of detrital clay content. Image charts can be used for estimating sorting and roundness ([[:file:core-description_fig2.png|Figure 2]]).

===Porosity===

===Porosity===

Identification of porosity type and a qualitative estimate of porosity abundance should be made in the column labeled “Porosity.” For carbonate rocks, the porosity classification scheme discussed by Choquette and Pray<ref name=pt05r34>Choquette, P. W., Pray, L. C., 1970, Geological nomenclature and classification of porosity in sedimentary carbonates: AAPG Bulletin, v. 54, p. 207–250.</ref> is recommended. For siliciclastic rocks, four types of porosity are common:

Identification of porosity type and a qualitative estimate of porosity abundance should be made in the column labeled “Porosity.” For carbonate rocks, the porosity classification scheme discussed by Choquette and Pray<ref name=pt05r34>Choquette, P. W., and L. C. Pray, 1970, [http://archives.datapages.com/data/bulletns/1968-70/data/pg/0054/0002/0200/0207.htm Geological nomenclature and classification of porosity in sedimentary carbonates]: AAPG Bulletin, v. 54, p. 207–250.</ref> is recommended. For siliciclastic rocks, four types of porosity are common:

* Intergranular

* Intergranular

* Intragranular or moldic

* Intragranular or moldic

* Microporosity

* Microporosity

* Fracture (Pittman, 1979)

* [[Fracture]] (Pittman<ref name=Pittman>Pitman, E. D., 1979, Porosity, diagenesis, and productive capability of sandstone reservoirs, in P. A. Scholle, and P. R. Schluger, eds., Aspects of Diagenesis: Society Economic Paleontologists and Mineralogists Special Publication 26, p. 159-173</ref>)

Of these, microporosity is the most difficult to recognize with a binocular microscope. The existence of microporosity is suggested by the presence of detrital or authigenic clays in sandstones. Accurate laboratory or thin section determinations of porosity types and percentages should always augment the estimates made during core logging. (For more on porosity classification schemes, see chapter on “Porosity” in Part 5.)

Of these, microporosity is the most difficult to recognize with a binocular microscope. The existence of microporosity is suggested by the presence of detrital or authigenic clays in sandstones. Accurate laboratory or thin section determinations of porosity types and percentages should always augment the estimates made during core logging. (For more on porosity classification schemes, see [[Porosity]].)

===Comments===

===Comments===

Information not recorded elsewhere on the form can be included in the “Comments” column on the far right (Figure 1). Such items as color, presence of hydrocarbon residue, and inferred depositional environment can be placed here. Color should be determined using the standard color chart distributed by the Geological Society of America.<ref name=pt05r67>Goddard, E. N., 1979, Rock-Color Chart: Boulder, CO, Geological Society of America.</ref>.

Information not recorded elsewhere on the form can be included in the “Comments” column on the far right ([[:file:core-description_fig1.png|Figure 1]]). Such items as color, presence of hydrocarbon residue, and inferred depositional environment can be placed here. Color should be determined using the standard color chart distributed by the Geological Society of America.<ref name=pt05r67>Goddard, E. N., 1979, Rock-Color Chart: Boulder, CO, Geological Society of America.</ref>

==Rock-log calibration==

==Rock-log calibration==

In most basins, few intervals in only a small number of wells are cored. Thus, calibrating of rock information (from continuous cores, closely spaced sidewall cores, or cuttings) to wireline log responses is essential to interpretation of depositional environments from logs alone. (For details of correlating cores to logs, see the chapters on “Preprocessing of Logging Data” in Part 4 and “Core-Log Transformations and Porosity-[[Permeability]] Relationships” in Part 5. For information on interpreting depositional environments from logs, see the chapter on “Lithology from Logs” in Part 4.)

In most basins, few intervals in only a small number of wells are cored. Thus, calibrating of rock information (from continuous cores, closely spaced sidewall cores, or cuttings) to wireline log responses is essential to interpretation of depositional environments from logs alone. (For details of correlating cores to logs, see [[Preprocessing of logging data]] and [[Core-log transformations and porosity-permeability relationships]]. For information on interpreting depositional environments from logs, see [[Lithology from logs]].)

==See also==

==See also==

* [[Porosity]]

* [[Overview of routine core analysis]]

* [[Oil and condensate analysis]]

* [[SEM, XRD, CL, and XF Methods]]

* [[SEM, XRD, CL, and XF methods]]

* [[SEM, XRD, CL, and XF methods]]

* [[Thin section analysis]]

* [[Thin section analysis]]

* [[Rock-water reaction: Formation damage]]

* [[Conventional coring]]

* [[Overview of routine core analysis]]

* [[Core alteration and preservation]]

* [[Core-log transformations and porosity-permeability relationships]]

* [[Core handling]]

* [[Core orientation]]

==References==

==References==

[[Category:Laboratory methods]]

[[Category:Laboratory methods]]