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          Zero Porosity Triggers   

Non-Porous Lithology Triggers
Some sedimentary rocks have apparent porosity indicated by various logs, but are truly non-porous. Such rocks as coal, anhydrite, gypsum and salt fall into this category. In order to discriminate these rocks, and set the porosity value to zero, specific trigger levels are for each log to help identify these minerals. The number of logs required to exceed their trigger level, before the zone is described as non-porous, is also needed. It is called the lithology trigger level.


The logic is as follows:

COALTRIG - Coal Trigger Levels
For example, to formulate a logical operator from the above rules for coal, select the individual trigger logic for each log type:

      1: E = (RESD > RTTRIG + PHIN > NTTRIG + PHID > DNTRIG + DELT > DTTRIG + GR < GRTRIG)
      2: IF E >= G
      3: AND G # 0
      4: THEN PHIe = 0
      5: AND CoalFrac = 1.00 - Vsh
      6: AND CoalCount = CoalCount + INCR
      7: IF CoalFrac > 0.0
      8: THEN Vmin1 = 0.0 AND Vmin2 = 0.0 AND Vmin3 = 0.0

Zone is defined as coal and PHIe = 0.00 if the positive triggers (E) equal or exceed the trigger level (G). Otherwise, PHIe = porosity from whichever porosity method was used in the calculation. The CoalCount term accumulates the coal thickness, which is useful in evaluating coal bed methane projects.

WHERE:
  CoalCount= coal thickness (feet or meters)
  CoalFrac = fraction of rock volume that is coal (for plotting on depth plots (fractional)
  DELT = sonic log reading (usec/ft or usec/m)
  DNTRIG = density coal trigger (fractional)
  DTTRIG = sonic coal trigger (usce/ft or usec/m)
  E = sum of triggers which are exceeded
  G = coal trigger level (0 = don’t check, 1 to 5 = number of triggers needed to indicate specified lithology)
  GR = gamma ray log reading (API)
  GRTRIG = gamma ray coal trigger (API)
  INCR = digitizing increment (feet or meters)
  NTTRIG = neutron coal trigger (fractional)
  PHID = density log reading (fractional)
  PHIe = effective porosity assigned to this zone
  PHIN = neutron log reading (fractional)
  RESD = resistivity log reading (ohm-m)
  RTTRIG = resistivity coal trigger (ohm-m)

COMMENTS:
The expected lithology in a zone will determine which zero porosity rocks should be triggered. The logic level is determined by how many logs are available and how well they resolve the lithologic problem.

Logical triggers can also be used when porosity is not zero. For example, in a limestone-dolomite-anhydrite sequence with only density and neutron data, the two mineral model could be used to obtain limestone and dolomite fractions, and an anhydrite trigger could be used to define pure anhydrite beds. This would eliminate the need for manual zoning.

NUMERICAL EXAMPLE:
1. Assume the trigger levels as for coal in the above table, and data as follows:
RESD = 350 ohm-m
PHIN = 0.45
PHID = 0.45
DELT = 328 ohm-m
GR = 15
LITH = COAL
RTTRIG = 200 ohm-m
NTTRIG = 0.40
DNTRIG = 0.40
DTTRIG = 300 ohm-m
GRTRIG = 50

Then all five tests are true and zone is coal.

2. If RESD = 180, then four tests are true and one is false. Zone is coal, if G = 4 and not coal if G = 5.

3. Assume instead:
RESD = 180
PHIN = 0.30
PHID = 0.45
DELT = 300
GR = 20

Then only two tests are true and three are false. If G > 2, then the zone is not coal, and is probably a zone with bad hole conditions.

4. If G = 0; then zone is not coal, regardless of the number of true tests.
 

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