Bob CluffThe Discovery Group, Denver, Colorado

Mike MillerCimarex , Tulsa, Oklahoma

April 2010 DWLS luncheon

The early years – shale evaluation from GR and density logsTrying to get quantitative – deltaLogR and other methods to estimate TOC from logsThe porosity problem – if TOC looks like porosity on density logs, how can you tell them apart?Geochemical logs finally find their marketModern multi-mineral solutions – reverse engineering the Shale Gas logsWhere are going next?

There was the gamma ray log,And it was good…………

Recognition that black shales are radioactive is not rocket science, we saw it in the 40’sEarly uses were just to separate hot shale from the background and map net thicknessBlack shale vs. Gray shale in Appalachian basin terms

Beers, 1945,AAPG Bull 29

Potter, Maynard & Pryor, 1980,fig 1.25

Mound Facility, 1980

Woodford Shale,Arkoma basin

Uranium in sea water drops out in reducing environments

Uranium forms organometallic complexes with organic matter at or just below the sediment-water interfaceRequires very low oxygen in seawater (anoxia)Slow process, residence time dependentAnaerobic bacteria are involved in reduction of U(VI) to U(IV)

Tetraethyl lead is a common example of an organometallic complex

Linear dependence of GR on TOCy = mx + bTOC = m GR + bm = slope of GR (API) vs TOC (wt %)b = GR intercept at 0% TOC (gray shale baseline)

requires normalized GR logs relationship varies between basins

no single set of coefficients workneed local calibration to core data

only approximate- the amount of uranium associated with TOC varies even within a formationSchmoker, 1981, AAPG 65 1285-1298

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16

0 50 100 150 200 250 300 350 400 450Gamma ray (API)

New Albany Shale, Illinois basinEGSP cores (1976-1979), all Big Blue logs

TOC

%

y = 0.0265x – 1.3161R2 = 0.5453

In the 1970’s, when bulk density logs first became common, it became apparent hot black shales also had low densityCross-plots of TOC vs. density quickly showed the story……..

Kerogen is light, like coal, so it “looks like” porosity on a density logVery, very common to pick a density porosity cutoff as “net pay” in gas shale plays

Schmoker, 1979, AAPG 63 (9) 1504-1537

New Albany Shale, Illinois Basin

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2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9Bulk density (g/cm3)

tota

l org

anic

car

bon

(wt %

)New Albany Shale, Illinois basinEGSP cores (1976-1979), all Blue logs

! slope varies if you plotTOC as v/v or as weight %

y = -30.28 x + 79.94R2 = 0.95

Passey et al, 1990, AAPG Bull 74 (12) 1777-1794The deltaLogR method was proposed as an improvement over simple GR and RhoBestimators for TOCRelied on two key observations:

∆t (and also ρb) vary with kerogen contentResistivity increases with increasing maturityLogarithmic separation between ∆t and Rdeep, appropriately scaled, can be related to TOC

It’s just an old-fashioned sonic F-overlay!!!

Passey et al, 1990,AAPG 74 (12)

Passey et al, 1990,AAPG 74 (12)

∆logR = log10(R/Rbaseline) + 0.02(∆t-∆tbaseline)∆t - resisitivity separation means likely source interval∆logR separation is linearly related to TOC if maturity is known or can be estimated

where:Rbaseline is the “gray shale” baseline resistivity

∆tbaseline is the “gray shale” baseline ∆t

important: at any given ∆logR, TOC decreases as LOM increases

TOC (wt %) = ∆logR * 10^(2.297 – 0.1688*LOM)

Passey et al, 1990,AAPG 74 (12)

Many people have noted, including Passey, that ∆logR has trouble at high maturities

TOC relationship was calibrated for LOM of 6-9 and low ∆logR, everything else was extrapolated LOM scale is only loosely related to common lab values, including Tmax and Ro.

Resistivity does not continually increase through the gas window, in fact it starts to fall back above Ro of ~1.1%

We’ve seen conductive shales at very, very high Ro’s

Mike Miller, BP, DWLS 2010 Spring Workshop

Ro ~1.8 – 2.0 Ro ~ > 4.5

XRD mineralogy ~ equivalent between wells

Low RhoB, density-neutron converge, high Rt (100’s of ohmm)

high RhoB, density-neutron separate, very very low Rt (< 1 ohmm)

Data provided by Brian Cardott, Okla. Geol. Survey

Vitrinite reflectance (%)

LOM

5th order polynomial fit

after Hood et al, 1975, AAPG 59 (6) 62

If kerogen has a density of ~1.15 g/c3 and water is ~1.0, and both contain hydrogen, how the heck do you separate them?Two separate, related issues:

Kerogen density is clearly low, but difficult to measure exactlyNeutron porosity of kerogen is unknown

So we just reverse the problemSolve for matrix values that work best to match both core measured TOC and core porosity

Dry shale point (50% qtz, 50% ill)

Type II kerogen

bad hole

porosity line

kerogen line

Salt Water?

Mike Miller, BP, DWLS 2010 Spring Workshop

↑Sg

Both GR and density correlations implicitly assume porosity is constant within the error of the measurementsSlope-intercept of the regression compensates for the average porosityScatter around the trend is presumably a measure of porosity that varies independently from TOC

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2.00 2.10 2.20 2.30 2.40 2.50 2.60 2.70 2.80 2.90

Bulk density (g/cc)

tota

l org

anic

car

bon

(%)

RhoMa ~ 2.72

Example: at any fixed TOC, RhoBVaries by +/- 0.1 g/c3 around the mean.At RhoMa = 2.72, this is +/- 6% porosity

The old standard answer – somebody will sell you another measurement.Pe doesn’t cut it, not a matrix/lithology problem

also heavy barite muds render it useless in several plays (e.g. Haynesville Sh)

Spectral GR doesn’t reduce uncertainty much, if any

Nice to know the clay breakdown, but not essential

Finally….geochemical spectroscopy logs find their purpose in life!

ECS tool uses AmBe chemical source, detectsSi, Ca, Fe, S, Mg, and “pseudo-Al” (+ Ti, Gd; not used)

SpectroLith processing is a DETERMINISTIC set of equations, based on a large sample database, that solves for the common minerals:

Fe, S → pyrite“excess Fe” (after making pyrite) → sideriteCa → total carbonate (calcite + dolomite)Si, Ca, Mg, pseudo-Al → clayLeft over → quartz, feldspar + mica undifferentiated

Another regression equation is used to solve for grain density (RHGE)

NOT a linear combination of mineral volumes and densities

ECS does NOT see kerogenSpectroLith grain density is assumed to be a kerogen-free value: RHGE – RHOM → Vkerogen

SpectroLith outputs → ELAN combine with density, neutron, Pe, NGS, etc.STOCHASTICALLY solve for lithology, porosity, Sw, Vkerogen

R. Lewis, Schlumberger

Raw logs RhoMa ECS ELAN Porosity, Sw, TOC, & GIP

First issue is adsorbed gas contentComplex function of TOC, pressure, temperature, maturity, and kerogen typeModel calibrated to core isothermsGenerally isotherms are run at reservoir temperature for the cored well, and T is ignored thereafter

Isotherms → set of Langmuir equationsLangmuir volumes ≈ a linear function of TOCThat plus pressures → adsorbed gas content

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0 200 400 600 800 1000 1200 1400 1600 1800Pressure (psi)

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con

tent

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/ton)

TOC %

15.50

13.70

12.64

10.44

8.808.217.25

3.55

Gab = VL * P / (P + PL)

GRI 91/0296

y = 14.217x - 32.974R² = 0.9733

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0 5 10 15 20Total organic carbon (%)

Lang

mui

r vol

ume

(scf

/ton)

75

25

12-15

5

Just like a tight sandstone or siltstoneNeed to determine porosity and saturation from a log model or core dataGIP (scf) = 43560 ft2/ac * A h φ Sg / Bg

where A is the area in acresh is the net thickness, in feetφ is porosity available for gas storage, v/vSg is the gas saturation, v/vBg is the gas formation volume factor, rcf/scf

“gas filled porosity”

R. Lewis, Schlumberger

Multimineral, stochastic solutions seem to work better than deterministic 2, 3 or 4 mineral models

Challenge is to incorporate ECS-type data directly in the models, bypassing the current regression based approaches

Geomechanical modelingVp-Vs data from cores suggest most properties can be predicted from DTC logs alone

Better analysis of limited and older log suitesHorizontal well evaluation with minimal data

Illinois Geological Survey for getting me started on black shales in the 70’sChevron USA for supporting our work on Barnett Shale and others in the early 90’sBP North America Gas for recent log examples and ongoing supportTodd Stephenson, BP Reserves & Renewal