Delaware Basin near surface velocity issues

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This page is currently being authored by a student at the University of Houston. This page will be complete by July 2018.

Delaware Basin near surface velocity issues

The velocities of the of the Delaware Basin overburden can be complex laterally due to near-surface anhydrites which exhibit changing sonic velocities with depth,[1] and the possibility of karsting which can have a negative impact on deep seismic images.[2]

Contents
Shallow Velocity Heterogeneity
Karsting

Shallow Velocity Heterogeneity

The Delaware Basin is one of five Permian-aged structural basins that exhibit thick Permian anhydrites, with maximum thickness approximately 1900 feet thick. [3] In the late Permian, the Delaware basin was filled by the Castille formation, allowing the younger Salado and Rustler Formations to deposit well outside the Delaware Basin limits, onto the Eastern Shelf and beyond. Despite being dominated by salts and gypsum, the Ochoan series of the Delaware also contains layers of limestone, dolomite, shale and sandstone, creating sonic velocity heterogeneity.[4]

The Ochoan evaporites and anhydrites of the Delaware Basin lie on top of the Guadalupian marine sediments, creating a sharp lithological and velocity contrast and the contact.[5] The interbedded anyhydrites and salts create ray bending due to the shallow velocity inversion. The highly variable velocities make time migration challenging, often resulting in a poor image of the deeper Permian sections.[6]


Karsting

In addition to the inherent velocity changes in the Ochoan section due to lithology, the anhydrite-dominated section is prone to dissolution and karsting, which can further complicate imaging of the deeper section. Introduction of water into these shallow formations occurs both naturally, when artesian groundwater encounters the salts, and anthropogenically, either accidentally through poorly cased oil wells, or intentionally in the form of dissolution mining.[7] Sulfur mines, such as the one operated at the Rustler Hills deposit, were producing upward of 2 million mT of sulfur per year, all through dissolution mining.[8] These karsts need to be identified and accounted for in the velocity model in order to get a clear picture of the subsurface.

  1. Karr et al, 2017
  2. Schmelzbach et al, 2015
  3. Maley and Huffington, 1953
  4. Dean and Johnson, 1986
  5. Adams, 1944
  6. Karr et al, 2017
  7. Land, 2013
  8. Smith, 1980