7/25/2019 0311 Tgs Marcellus Petrophysical Analysis

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STORY NUMBER: 3-3M , T1, A2

STORY SLUG: Shales/Marcellus 3-D

AUTHOR: CC

Logs Reveal Marcellus Sweet Spots

Appalachian Basin Wells with Marcellus Shale Log Curve Data

Marcellus Project Wells with Tops Picked - 3,193

Marcellus Project Wells with both Resistivity and Porosity Log Data - 9,013

Marcellus Project Wells - 16,894

A shale formation that was described in the very first geologicalsurvey conducted in Pennsylvania in 1836 is driving boomingupstream and midstream activity in the same basin where thenation’s first oil and natural gas wells were drilled. Stretching600 miles across the Appalachian Basin and covering millionsof prospective acres from New York to Virginia, the Marcellus

Shale formation is estimated to contain as much as 500 trillion

cubic feet of natural gas–enough to supply the entire U.S. mar-ket for nearly two decades! Log curve data are available fornearly 17,000 wells that have penetrated the Devonian-ageshale. The well locations with tops are shown here, along withthe measured depth contours to the top of the interval based onmapping the Marcellus Shale’s areal extent and thickness in

3,193 wells.

Ohio 

Kentuck! 

West 

Virginia 

Penns!lvania 

Ne York 

Delaare District of Columbia 

Mar!land  

Virginia 

Ne Jerse! 

Connecticut 

Massachusetts 

Vermont 

Indiana 

By Pete Dotsey

HOUSTON–Sweet spots in the Mar-

cellus gas shale trend are being mappedby performing geologic and petrophysicalevaluation and interpretation of well logdata. Log data for more than 18,000 wellswithin the trend are being used.

Many authors have indicated that totalorganic carbon (TOC) and porosity aregood indicators of gas shale potential.TOC sample data and log curve data frompreviously drilled wells are needed to cal-culate a TOC-calibrated model. Publicdomain TOC sample data are availablefrom state and federal geologic agencies

for many wells within the Marcellus trend.TOC analytical results are needed to cal-culate porosity.

The gas shale thickness can be deter-mined by interpreting the log curve datafor each well, and the areal extent andthickness of the gas shale can be mappedby using all the interpreted wells. Sweetspots can be delineated by mapping cal-culated TOC and porosity with respectto gas shale area and thickness. The sweetspots are evaluated with respect to gasshale production. This approach has beenimplemented in several steps:

· Identify the Marcellus interval inMarcellus trend wells;

· Digitize well curve data for the iden-

tified wells;· Source available Marcellus well

TOC sample data;

· Develop a calibrated TOC modelto calculate TOC curves;

· Calculate TOC and porosity for theMarcellus interval in trend wells;

· Correlate and map the geologicaland petrophysical results throughout thetrend; and

· Utilize released production data tovalidate the geological and petrophysicalmaps.

Identify The Marcellus Interval

The first step was to identify the Mar-cellus interval in wells located in the

FIGURE 1

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Marcellus trend. Marcellus regional struc-ture and trend area maps generated bythe United States Geological Survey wereused at project startup. Within the trend,275,000 wells have been drilled, but itwas not known how many of these wells

had penetrated the Marcellus Shale. Wellcurve data for an initial list of 9,349wells were loaded into the interpretationproject. These wells were geologicallyevaluated to determine which ones pene-trated the Marcellus Shale.

As a result of the geologic evaluation,tops were interpreted in 3,193 wells. Topswere interpreted for the Marcellus Shale,the underlying Onondaga Limestone andthe overlying units, which include theRhinestreet and Genesee shales. The Mar-cellus Shale is composed of three members:an upper and lower shale member and a

middle Cherry Valley Limestone member.All three units are not present throughoutthe entire trend. Of the 3,193 wells thatwere interpreted, the upper MarcellusShale member was present in 1,644 wells,and the lower Marcellus Shale was presentin 2,921 wells.

During the well top interpretation ex-ercise, 3,529 wells were eliminated fromthe initial list of project wells becausethe Marcellus interval was either notpresent or not logged in those wells.

Next, a measured depth map to the top

of the Marcellus was generated based onwells for which tops were interpreted.After that, the measured-depth contouredintervals were cross-referenced with thetotal drilled depth of the 275,000 wellsfrom the database that fell within the trend.

An additional 11,074 wells were iden-tified that should have the Marcellus Shaleinterval present. These additional wellswill be used in the geological and petro-physical evaluation of the trend. This isan ongoing and iterative process. As theMarcellus interval is picked in more wellswithin the trend, more detailed maps are

being made to further cross-reference thedatabase and eliminate wells with insuffi-cient well log interval coverage.

At the time of writing, 16,894 wellshad log curve data to map the area andthickness of the Marcellus. These wellsare shown in Figure 1. Also shown inFigure 1 are the locations of the wellswith tops and the measured depth contoursto the top of the Marcellus interval, whichare based on mapping the initial 3,193wells.

Following the interpretation of tops

for wells in the Marcellus Shale trend,

geological structure maps and bore holethickness or isopach maps were madefor the Marcellus Shale, the MarcellusShale members, the underlying Onondagaformation and the overlying formations,which include the Rhinestreet and Genesee

shales. By utilizing the tops and mapsthat have been generated, geoscientistswill know from anywhere within thetrend how far they need to drill to intersectthe top of the Marcellus and how muchfarther they will have to drill to penetratethe entire Marcellus section.

Digitize Curve Data

Once a well is identified that penetratesthe entire Marcellus section, the log curvedata are digitized. The additional digitalwell log data then are used for geological

and petrophysical evaluation.The next step in the process, and thefirst step in generating a calibrated TOCmodel, is to obtain TOC sample data forthe Marcellus interval. As stated above,TOC analytical results are considered toclosely correspond with gas shale pro-ductivity, or sweet spots, in the trend.

There are several ways to obtain TOCsample data. One is to drill wells and an-alyze the core and/or cuttings from thewell bore. Another way is to review theliterature and the Web for TOC sampledata results. A third way is to have oper-

ators contribute their sample data for re-

view and evaluation. The latter two meth-ods are being used.

After extensive research, TOC sampledata were obtained for the Marcellus in-terval for 93 wells, and 67 of those wellsfell within the active Marcellus trend.

Develop A TOC Model

The next step was to develop a TOCmodel that could be used to calculate aTOC curve for the Marcellus interval.Several approaches for calculating a TOCcurve for the Marcellus interval wereevaluated. After reviewing the literatureand evaluating the available data for rel-evant wells–that is wells that had bothTOC sample data and porosity log curvedata–the Passey approach was selected.

The Passey method has four distinct

advantages for use in the Marcellus:• It worked well; that is, the calculatedTOC curves pass through the TOC samplepoints in the well.

• It allows TOC curves to be generatedfor the wells with the requisite porosityand resistivity log data.

• There are 9,013 wells identified inour database that have porosity and re-sistivity log data for the Marcellus intervalin the Marcellus trend.

• Analyzing these wells should pro-vide a sufficient number of data points toaccurately map the sweet spots.

The wells with both porosity and re-

 Δ Log R Correlation between Geochem TOC (reddots) and calculated TOC (blue line) isexcellent. Note brown curve is TOC %vol.

Total organic carbon can help map gas shale sweet spots. Using TOC sample data andporosity log curve data, TOC curves have been developed for wells in the Marcellus Shaletrend. As data from one well show, the curves accurately predict actual TOC values.

FIGURE 2

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sistivity log curve data are shown onFigure 1.

When utilizing the Passey method, aterm  Δ  log R is calculated that representsthe separation between the deep resistivitycurve and the available porosity log curve(i.e. sonic, density or neutron log). Thiscalculation can be done for each of theporosity log types (Table 1, equations 1-3).

Next, for wells with TOC sample data,

an equation is solved to determine levelof organic maturity (LOM) (Table 1, equa-tion 4).

Figure 2 shows the close agreementbetween a calculated TOC curve andTOC sample data points for the MarcellusShale interval.

Perform Petrophysical Analysis

As noted above, the Passey approachis used to calculate LOM for each of the65 wells within the trend that have Mar-cellus interval TOC sample data. The

LOM calculated values then can be con-toured. Each of the wells within the trendthat have porosity and resistivity log datafor the Marcellus Shale interval will fallbetween or on an LOM contour line.

The LOM contour line values thenare applied to the equation for a wellwithout TOC data, and the equation issolved for TOC at each logged samplepoint (wells commonly are logged on a0.5-foot sample depth interval). A TOCcurve is drawn through the calculatedTOC sample points for the Marcellus in-terval in the well using petrophysical

software. Also, statistical information,such as the average or maximum TOCover the upper, lower or total Marcellusinterval, is calculated.

Once the TOC curve is calculated, aporosity curve can be created (since TOCconcentration must be taken into accountwhen calculating porosity in gas shaleintervals). Porosity curves can be calcu-lated for wells with sonic, density orneutron porosity log curve data.

Besides TOC and porosity calculations,a few additional petrophysical steps are

taken during the analysis. First and fore-

most, the curves for a well are mergedand put on depth, since there may be twoor more logging runs that need to becompiled. Suspect log data are flagged.Such data can result from a myriad of problems, typically from washed-out borehole intervals that result from drilling orfrom logging tools that get out of cali-bration. Basic environmental correctionsare applied to the data where necessary.

Lithology is calculated for the entirelogged interval.

Also, potential pyritic zones are flaggedby using the response from the photo-electric log if a Pe log is available. Pyriticzones are relatively common in the Mar-cellus Shale interval, and adversely impactthe results of the TOC and porosity cal-culations. Because there are several thou-sand wells with Pe log curve data andpyrite impact on the calculation of TOCis so important, pyritic zones are beingmapped. These will be noted, since they

can mask the presence of sweet spots.The processing steps to flag suspect logdata and pyritic zones, and to performenvironmental corrections are done foreach well analyzed prior to calculatingTOC and porosity curves.

In addition, available shear sonic datacan be used to evaluate brittleness of theshale, and spectral gamma ray log datacan be used to carry out a more detailedlithologic analysis. The Marcellus Shalecommonly contains vertical joints or frac-tures and is known to be brittle, so itshould fracture readily during comple-

tion.Some emphasis has been placed on

completing more detailed lithologic analy-sis to identify the more permeable siltyzones in gas shales. Fundamentally, shaleis composed of silt and clay-sized sedi-ments. The coarser silt-sized materialtypically results in more effective matrixpermeability. The more permeable siltyzones have been noted to be more attractivetarget intervals for implementing fracturing jobs when completing a well. More recentlogging tools have been designed to more

accurately determine the silt- and clay-

sized shale fractions. X-ray diffractionanalysis and CT scanning of bore holecores and/or cuttings also can be used toassess shale sediment size distribution.

There is also conjecture that makingthe distinction may not be a critical

concern for three reasons:• While drilling laterals to completea well, new logging tools are designed toidentify the more permeable silty zones,and the results are used to keep the drillbit situated within these zones.

• The fracturing jobs impact a rela-tively large area around the laterals andshould effectively reach the more siltypermeable zones.

• The core analysis results from re-cently cored wells indicate that the higherporosity zones tend to coincide with themore permeable zones.

A more rigorous approach can betaken for mapping geologically distinctzones within the upper and lower Mar-cellus Shale members. Detailed deposi-tional environment and facies maps canbe generated while evaluating the wellsand mapping the sweet spots. Lithologiczones of interest that are discernable fromlog analysis and are contained within thegross depositional environment and faciesmaps can be mapped within the contextof a sequence stratigraphic framework.

Map The Results

The next step is to map the geologic(gas shale area and thickness) and petro-physical (TOC and porosity) results. Struc-tural and isopach maps of the MarcellusShale, the upper and lower members of the Marcellus Shale, and the overlyingformations, including the Genesee andRhinestreet shales, have been generatedbased on the initial 3,150 wells that havebeen interpreted. The maps will be revisedas the additional wells with Marcellus in-terval log data within the Marcellus trendare interpreted.

In addition, TOC and porosity resultshave been mapped for a handful of wells,and again, the maps will be revised afteradditional wells with resistivity and poros-ity log data are processed and evaluated.Also, the data associated with the flaggedpyritic zones will be evaluated, and thesezones will be mapped if the data permits.Next, interpretating and mapping thegross depositional environments andfacies associations within a sequence-constrained stratigraphic framework canbe completed.

The final step of this process will be

EQUATION 1:  Δ  log R = Log10 (R/RBaseline) + 0.02 * (  Δ t –  Δ tBaseline

EQUATION 2:  Δ  log R = Log10 (R/RBaseline) + 4 * (NPHI – NPHIBaseline)

EQUATION 3:  Δ  log R = Log10 (R/RBaseline) – 2.5 * (RHOB – RHOBBaseline)

EQUATION 4: TOC = ( Δ  log R) * 10 (2.297 – 0.1688 * LOM)

TABLE 1

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to evaluate production data with respectto the geologic and petrophysical mappeddata. Production data for the Marcellustrend are now available, and regulatorychanges in Pennsylvania will make Mar-cellus gas production data accessible in

a shorter time following successful wellcompletions. This step will be used toconfirm and/or refine the Marcellus gasshale evaluation approach detailed here.

Log data are available for 2 millionU.S. oil and gas wells. More than 500,000of these wells have been digitized, andby year end, the number of digitizedwells will exceed 750,000. Productiondata are available for the entire UnitedStates. Geoscience evaluation already hascommenced for a number of active trends,such as the Eagle Ford, Bakken and Nio-brara formations.

PETE DOTSEY is the North and South American business development manager  for the geological products division withTGS. He attained an M.S. in geology from Stephen F. Austin State Universityin 1983. He worked for three years in ex- ploration with Sohio Petroleum Co., for nine years as a hydro-geologist and  project manager in the environmental in-dustry, and for four years as a softwareapplication consultant and technical salesrepresentative with Landmark. He has

been with TGS for 11 years.