Estimation of paleo-depositional cycles in organic-rich chalk by NMR restricted diffusion
Estimating paleo-depositional cycles in a geological formation is accomplished by obtaining 1 H nuclear magnetic resonance (NMR) measurements of restricted diffusion of a protonated fluid in a direction perpendicular to a bedding plane in a core sample aligned in the perpendicular direction and removed from the geological formation. Diffusive tortuosity is estimated from the restricted diffusion measurements, and a diffusion length at which the restricted diffusion in the core sample reaches a tortuosity limit is ascertained. An average spacing between sedimentary boundaries of restricted diffusion can then be calculated.
1 . A method of estimating paleo-depositional cycles in a geological formation, the method comprising:
a. obtaining 1 H nuclear magnetic resonance (NMR) measurements of restricted diffusion of a protonated fluid in a direction perpendicular to a bedding plane in a core sample aligned in the perpendicular direction and removed from the geological formation;
b. estimating diffusive tortuosity from the restricted diffusion measurements;
c. ascertaining a diffusion length at which the restricted diffusion in the core sample reaches a tortuosity limit; and
d. calculating an average spacing between sedimentary boundaries of restricted diffusion.
2 . The method of claim 1 , additionally comprising: estimating a duration of an average paleo-depositional cycle based on the calculated average spacing between sedimentary boundaries of restricted diffusion and an estimated sedimentary depositional rate.
3 . The method of claim 2 wherein the sedimentary deposition rate is based on an age-depth plot of the geological formation.
4 . The method of claim 3 wherein the age-depth plot can be obtained from radioactivity, paleomagnetic and/or bio stratigraphic data obtained from core or well logs.
5 . The method of claim 2 , additionally comprising: comparing the estimated duration of the paleo-depositional cycles to present-day climate cycles.
6 . The method of claim 5 , additionally comprising: determining a change between the estimated duration of the paleo-depositional cycles to present-day climate cycles.
7 . The method of claim 5 , wherein the present-day climate cycles are an average of ENSO cycles dating back to 1850.
8 . The method of claim 1 , wherein the calculating of the average spacing between sedimentary boundaries of restricted diffusion includes multiplying, by a constant, the ascertained diffusion length at which the tortuosity limit is reached.
9 . The method of claim 8 , wherein the constant is between 1.5 and 2.5.
10 . The method of claim 8 , wherein the constant is between 1.8 and 2.2.
11 . The method of claim 1 , wherein the estimating of diffusive tortuosity includes using a Padé fit.
12 . The method of claim 1 , wherein the sedimentary boundaries of restricted diffusion comprise organic-rich layers and layers of accumulated shell fragments.