Changes

==Sample collection==

==Sample collection==

Petrographic analysis can be performed on samples collected from conventional cores, rotary sidewall cores, percussion sidewall cores, or cuttings, but sample quality varies considerably.

Petrographic analysis can be performed on samples collected from [[Overview of routine core analysis|conventional cores]], [[Sidewall_coring#Rotary_sidewall_coring|rotary sidewall cores]], [[Sidewall_coring#Percussion_sidewall_coring|percussion sidewall cores]], or [[Mudlogging: drill cuttings analysis|cuttings]], but sample quality varies considerably.

Conventional cores are most desirable because samples can be collected from specific intervals of interest determined by a combination of direct observation of physical characteristics (such as facies), measured properties (such as [[porosity]] and [[permeability]]), and wireline log responses. They are also better because sample damage tends to be minimal. Thin sections are commonly made from porosity-permeability plugs cut from conventional cores because petrographic observations can be compared directly to porosity and permeability measurements.

Conventional cores are most desirable because samples can be collected from specific intervals of interest determined by a combination of direct observation of physical characteristics (such as [[facies]]), measured properties (such as [[porosity]] and [[permeability]]), and [[Basic open hole tools|wireline log]] responses. They are also better because sample damage tends to be minimal. Thin sections are commonly made from porosity-permeability plugs cut from conventional cores because petrographic observations can be compared directly to porosity and permeability measurements.

Sidewall cores are less desirable because sampling sites are remotely selected based on log responses and because sample size is small. This introduces the possibility that the sidewall cores are not representative of the interval of interest. Rotary sidewall cores are preferred because they have better recovery in the most porous parts of a reservoir and because they usually display relatively little damage induced by coring. Percussion [[sidewall coring]] often recovers little sample, particularly in the most porous parts of a reservoir, and commonly induces [[fracture]]s and disrupted fabrics within samples.

Sidewall cores are less desirable because sampling sites are remotely selected based on log responses and because sample size is small. This introduces the possibility that the sidewall cores are not representative of the interval of interest. Rotary sidewall cores are preferred because they have better recovery in the most porous parts of a reservoir and because they usually display relatively little damage induced by coring. Percussion sidewall coring often recovers little sample, particularly in the most porous parts of a [[reservoir]], and commonly induces [[fracture]]s and disrupted fabrics within samples.

Cuttings are the least desirable because they are the least site specific of the sample types and because they disproportionately represent the least porous parts of a reservoir. The most porous parts commonly disintegrate into constituent grains. Moreover, it is difficult to evaluate the homogeneity of reservoir characteristics because of the small size of cuttings.

Cuttings are the least desirable because they are the least site specific of the sample types and because they disproportionately represent the least porous parts of a reservoir. The most porous parts commonly disintegrate into constituent grains. Moreover, it is difficult to evaluate the homogeneity of reservoir characteristics because of the small size of cuttings.