Southwest Section AAPG Annual Convention

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In the log analysis of a Permian Wolfcamp well the Wolfcamp was subdivided into two zone labeled Wolfcamp 1 and Wolfcamp 2. Using GEOCHEM [ECS] data the total porosity [PHItotal] was calculated from the bulk density log using variable matrix analysis. Effective porosity [PHIe] was then determined [PHIe = PHItotal − CBW].

The OOIPstb for both the Wolfcamp 1 and Wolfcamp 2 are listed below:

  • Wolfcamp 1
  • OOIPstb 12.7mmbo
  • Wolfcamp 2
  • OOIPstb 5.1mmbo

The logging suite for this well also included a CMR Log therefore OOIPstb could be calculated at different T2 Relaxation Times. The results are listed below:

  • Wolfcamp 1 [T2 3ms Pore Size 76.5nm]
  • OOIPstb 8.8mmbo
  • Wolfcamp 1 [T2 10ms Pore Size 250nm]
  • OOIPstb 5.3mmbo
  • Wolfcamp 2 [T2 3ms Pore Size 76.5nm]
  • OOIPstb 5.5mmbo
  • Wolfcamp 2 [T2 10ms Pore Size 250nm]
  • OOIPstb 2.0mmbo

Note, in the above OOIPstb values Wolfcamp 1 has much greater OOIPstb values than Wolfcamp 2. An examination of the lithologies indicate that Wolfcamp 2 is more clay rich, and has a higher minimum closure stress [SHmin] and lower Brittleness Coefficient compared to Wolfcamp 1. Therefore the better reservoir with more hydrocarbons is Wolfcamp 1.

However, because the well was logged with a High Resolution Array Laterolog [HRLA] the author examined the log for invasion profiles [HRLA5>HRLA2>Rxo], which indicate zones of moveable hydrocarbons due to invasion. The better invasion profiles were located in Wolfcamp 2, not Wolfcamp 1 as I would have expected.

Next OOIPstb was calculated based on the degree of invasion (Tixier, 1956 and Asquith, 2015).

Y = (Rmf/Rxo)⁁0.5 − (Rw/Rt)⁁0.5

OOIPstb = (7758∗Y∗h∗A)/BOI

The results are listed below:

  • Wolfcamp 1 [Y Method : Trixier, 1956]
  • OOIPstb 1.5mmbo
  • Wolfcamp 2 [Y Method : Trixier, 1956]
  • OOIPstb 3.8mmbo

Unlike the other OOIPstb values the OOIPstb determined from the Y Method are just the reverse, indicating the Wolfcamp 2 is the better reservoir [i.e. greater invasion]. The author has used the Y Method for years in many reservoirs including the Wolfcamp, and found it to be reliable [Asquith, 2015: WTGS Fall Symposium]. So the question is what causing the Y Method to indicate that the better reservoir is Wolfcamp 2, when the other calculated OOIPstb values and Geomechanical properties indicated Wolfcamp 1 has the better reservoir potential?