Delaware Basin near surface velocity issues

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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] [Permian Basin Structure]

Contents
Shallow Velocity Heterogeneity
Karsting[1]

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 time, the Delaware basin was filled by the three seperate Ochaoan aged formations: the Castille, the Salado and the Rustler. The basin was initially 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 at 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]


Cenozoic basin fill isopach, Maley and Huffington Pl 1.png Ochoan Paleodepositional Map.png

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.

Sinkhole Map.png Sinkhold Cube Model.png .200px

  1. Karr, B., Tiefenthaler, S., Schneider, J., Li, D., 2017,Delaware Basin PSDM Seismic Data: Why near surface velocity inversion affects image integrity, Tulsa GS
  2. Schmelzbach, C., 2015, Understanding the Impact of Karst on Seismic Wave Propagation - A Multi-method Geophysical Study, EAGE 2015
  3. Maley, V., Huffington, R, 1953, CENOZOIC FILL AND EVAPORATE SOLUTION IN THE DELAWARE BASIN, TEXAS AND NEW MEXICO, GSA Bulletin (1953) 64 (5): 539-546
  4. Dean, W.,Johnson, K., 1986, Anhydrite Deposits of the United States and Characteristics of Anhydrite Important for Storage of Radioactive Wastes, U.S. GEOLOGICAL SURVEY BULLETIN 1794
  5. Adams, J.E. Upper Permian Ochoa Series of Delaware Basin, West Texas and Southeastern New Mexico in American Association of Petroleum Geologists -- Bulletin; 28, n 11; p 1596-1625 American Association of Petroleum Geologists ; 1944
  6. Karr, B., Tiefenthaler, S., Schneider, J., Li, D., 2017,Delaware Basin PSDM Seismic Data: Why near surface velocity inversion affects image integrity, Tulsa GS
  7. Land, L., 2012, Geophysical records of anthropogenic sinkhole formation in the Delaware Basin region, southeast New Mexico and west Texas, USA: Carbonates and Evaporites, v. 27, no. 1.
  8. Smith, R., 1980, SULFUR DEPOSITS IN OCHOAN ROCKS OF THE GYPSUM PLAIN, SOUTHEAST NEW MEXICO AND WEST TEXAS,New Mexico Geological Society Guidebook, 31st Field Conference, Trans-Pecos Region