Question

Which of the following conditions is/are inferred using cross plots of neutron porosity (\(\phi_N\)) and density porosity (\(\phi_D\))?

• A. Gas zone
• B. Shale distribution
• C. Water zone
• D. Oil zone

Hint:

Gas lowers neutron porosity relative to density porosity (crossover); oil and water share a similar hydrogen index.

Answer 1

The Correct Option is A, C

Step 1: Understand what a neutron density cross plot shows:
The neutron log responds mainly to the hydrogen content of the fluid filling the pore space, while the density log responds to the bulk density of the rock plus fluid system. When the two porosities, \(\phi_N\) and \(\phi_D\), are plotted together on the same scale, the amount of separation between them, and the direction of that separation, tells a log analyst what kind of fluid is sitting in the pores. This is why the tool is used to infer fluid type rather than rock texture.
Step 2: Check option A, gas zone:
Gas has a hydrogen index far lower than oil or water, so in a gas filled zone the neutron tool underreads porosity while the density tool, which is not fooled by hydrogen index, still reads close to the true porosity. This mismatch produces the well known gas crossover, where the neutron curve falls below the density curve. This crossover is the standard signature used to flag a gas zone, so option A is correct.
Step 3: Check option B, shale distribution:
Shale volume and its distribution through a formation are normally read off the gamma ray log or the SP log, and sometimes cross checked using the neutron density separation seen specifically inside shale beds. The neutron density cross plot on its own, without a gamma ray curve to anchor it, is not the primary tool used to map how shale is distributed, so option B is not correct.
Step 4: Check option C, water zone:
Water has a hydrogen index close to 1, which is the value the neutron tool is calibrated against, so in a water bearing zone the neutron porosity and density porosity read close to each other and follow the expected lithology trend with little or no separation. This matched, non crossed response is exactly the pattern analysts use to recognize a water zone on the cross plot, so option C is correct.
Step 5: Check option D, oil zone:
Oil, like water, has a hydrogen index close to 1, so an oil bearing zone gives almost the same neutron density response as a water bearing zone. Because the cross plot cannot tell these two liquids apart, it cannot be used on its own to specifically flag an oil zone, so option D is not correct.
Final Answer:
\[ \boxed{\text{Gas zone and Water zone, options A and C}} \]