sequence strati graphic analysis using well cuttings

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Yang, 1998), and some of the faults underwent slight

inversion in the late Paleozoic (Shumaker and Wilson,

1996).

The study area consists of weakly deformed, flat-

lying strata beneath the Appalachian Plateau. Outcrop-

ping Mississippian rocks are restricted to the leading

edge of the overthrust belt in the east and to the distal

western edge of the basin bordering the Cincinnati arch.

Between these outcrop belts, the subsurface Mississip-

pian rocks are overlain by 0–4000 ft (0–1200 m) of

Pennsylvanian strata. The Mississippian interval is pen-

etrated by approximately 10,000 wells (K. L. Avery,

Figure 1. Geologic location map of West Virginia study area, showing wells used and distribution of exposed MississippianGreenbrier carbonate rocks (gray shading) in the Appalachian Basin. Isopach contours (nonpalinspastic, in feet) show the total

thickness of Greenbrier carbonates, which thicken into the Appalachian foredeep to the southeast (modified from Pryor and Sable,1974; MacQuown and Pear, 1983; Yeilding and Dennison, 1986; Dever et al., 1990; Sable and Dever, 1990; Dever, 1995). Crosssection BB0 is shown in Figure 5.

Wynn and Read 1871



2006, personal communication), 193 of which were

used for this study, along with one core and eight out-

crop sections (Figure 1), which are described in Al-

Tawil et al. (2003).

Regional Stratigraphic Framework and Facies

The stratigraphy and biostratigraphy of the Missis-

sippian system in Virginia and West Virginia are given

in Reger (1926), Butts (1940, 1941), Wells (1950),Flowers (1956), de Witt and McGrew (1979), Rice

et al. (1979), and Maples and Waters (1987). The rocks

in this area overlie the Price and Borden formations

(Kinderhookian–middle Osagean) in the northeast

(Branson, 1912; Butts, 1940, 1941; Bartlett, 1974; Bjer-

stedt and Kammer, 1988) and the Fort Payne and Salem

Formation (late Osagean– early Meramecian) (Bjerstedt

and Kammer, 1988; Sable and Dever, 1990; Khetani

and Read, 2002). The Greenbrier Group and the lower

Bluefield Formation (Lillydale Shale, Glenray Limestone

Member, and Reynolds Limestone Member) make upthe study interval and are 0–3000 ft (0–900 m) thick.

These units are overlain by upper Mississippian and

Pennsylvanian siliciclastic rocks.

Regionally, the Greenbrier units on the shallow

ramp consist of shallow-water carbonates and minor

siliciclastic units. They thicken to more than 2000 ft

(600 m) toward the southeast into the proximal fore-

land, where they are dominated by thick slope muds,

which are intercalated with thin, shallow-water units

of quartz sandstone and limestone.

Al-Tawil et al. (2003) provided the first detailed

sequence-stratigraphic framework for the region based

on the eastern outcrop belt and limited subsurface data.

The Mississippian facies of the eastern Appalachian Ba-

sin are shown schematically on an idealized ramp mod-

el (Figure 2) and resemble contemporaneous facies de-

scribed elsewhere in North America by Leonard (1968),

Ettensohn et al. (1984), Carney and Smosna (1989),

Smith and Read (1999, 2001), Al-Tawil and Read (2003),

and Al-Tawil et al. (2003). Over much of West Vir-ginia, facies consist of terrigeneous red beds, quartz

peloid eolianite, lagoonal muddy carbonate, ooid- and

skeletal grainstone-packstone shoal complexes, and on

the ramp slope, deeper water dark-gray wackestone-

mudstone, and dark-gray laminated argillaceous lime

mudstone. The facies and their environments of depo-

sition are summarized in Table 1.

METHODS

Data were collectedfrom193 wells with cuttings, along

with one core and seven outcrop sections from pre-

vious studies (Wray, 1980; Yeilding, 1984; Yeilding and

Dennison, 1986; Al-Tawil, 1998; Al-Tawil et al., 2003).

The coarse fraction (1–2 mm; 0.04–0.08 in.) of the

cuttings for each sample interval (typically 10 ft [3 m])

was washed, acid-etched (2.5% HCl) and, if dolomitic,

was stained with Alizarin Red S and examined under a

binocular microscope. For each sample interval, rock

Figure 2. Schematic facies profile for the Mississippian carbonates of the Appalachian Basin. Actual facies distributions are morecomplex, in that skeletal grainstone-packstone and ooid grainstone facies are not only developed on the ramp margin, but also inlocal areas far back in the ramp interior.

1872 Geohorizons



types were classified according to Dunham (1962) and

counted to determine the relative abundance of rock

types. Percentages were recorded on data sheets, en-

tered into a commercial spreadsheet program, and saved

as comma-delimited files. Undergraduates helped clas-

sify and count the well cuttings and were overseen for

quality control by the senior author. This helped to

obtain and process the large amount of data required.For each well, the comma-delimited files were imported

into a commercial log-plotting program and plotted

against depth to form a percent lithology log. Well-

cuttings data for the 193 wells were calibrated against

geophysical logs where available, with gamma-ray and

bulk-density logs being the most useful. Wire-line logs

were digitized and exported as Log Ascii Standard files

and imported into a commercial log-plotting program.

The logs were then plotted alongside the cuttings-

percent logs, after adjusting the logs by 10 ft (3 m) or so

for best match with diagnostic markers (fine siliciclas-tics, oolite) to consider drilling lag. The correlation of

gamma-ray and bulk-density logs with the cuttings data

helped remove errors caused by the drilling lag. The

combination of cuttings-percent logsand gamma-raylogs

wasused to produce lithologic columns with a resolution

of 10 ft (3 m), showing dominant lithology and gamma-

ray response. The gamma-ray and bulk-density logs,

when combined with well-cuttings data, also made it

possible to identify and locate depths and thicknesses

of siliciclastic units thinner than the well-cuttings sample

interval. Five to six distinctive gamma-ray marker hori-zons are associated with several regionally extensive silic-

iclastic units, which are mostly transgressive shale inter-

vals. These regionally extensivegamma-raymarkers were

used to help constrain correlations between the wells.

The cuttings-based well sections were used to pro-

duce three dip-oriented and two strike-oriented strati-

graphic cross sections. Using these five cross sections

as a framework, the sequence picks were extended to

nearby wells, guided by log signatures where distinc-

tive. For each sequence, the following data were re-

corded in a spreadsheet: county, permit number, latitudeand longitude, sequence number, sequence thickness,

lowstand-transgressive systems tract thickness, lowstand-

transgressive systems tract dominant facies, the high-

stand systems tract thickness, dominant highstand fa-

cies, aggregate grainstone thickness in the sequence,

dominant marine grainstone type (skeletal, ooid, or pe-

loid), aggregate sandstone thickness, caliche (present or

absent), and produced fluid (oil, gas, or oil and gas). The

data for each well were then imported into a geographic

information system (GIS) and plotted as point themes.

The well sections with their sequence-stratigraphic

picks were compiled into regional cross sections show-

ingthe vertical and lateral distribution of facies(Figure 3).

Sequence boundaries and maximum flooding surfaces

were traced from section to section, guided by distinc-

tive log markers and biostratigraphy. This generated

a high-resolution sequence-stratigraphic framework

(Figures 3, 4). With the well data in GIS, the succes-sion throughout the region of interest was then time

sliced into sequences and systems tracts (Figure 5).

Lowstand-transgressive and highstand dominant

facies maps, isopach maps, and isolith maps were pro-

duced in GIS for each sequence using the data from the

point themes. The isopachs and isolith maps were pro-

duced using computer contouring software, edited by

hand, and then imported into GIS. Isopach maps were

constructed for individual sequences and systems tracts

(Figure 5). In addition, maps showing the dominant fa-

cies could then be rapidly made for each systems tract todefine geographic facies distribution and major poten-

tial reservoir trends. Grainstone isolith maps (Figure 5)

were generated to show the location of the primary

reservoirs. The maps and cross sections (Figures 3, 5) il-

lustrate the power of using sequence analysis of well cut-

tings within GIS to generate a high-resolution sequence-

stratigraphic framework for carbonate successions.

FACIES STACKING IN WELLS ANDSEQUENCE ANALYSIS

The conversion of cuttings data into dominant lithol-

ogy for a sample interval is needed to consider effects

related to interbedding of different lithologies and the

relation of the sample interval to the lithologic bound-

aries. All of these effects could result in the mixing of

lithologies within a sample interval.

Problems Caused by Mixing of Interbedded Lithologies

Interbedding can give the appearance of mixing in well

cuttings and can be a major problem if not recog-

nized. Two types of interbedding cause problems when

working with well cuttings: thin interbedding of rock

types, and larger scale stratigraphic juxtaposition of

two or more lithologies within the sample interval

(Figure 6A, B). Thinly interbedded lithologies com-

monly occur as pairs of rock types (i.e., shale inter-

bedded with limestone) in the sample interval, with

individual beds being below the resolution of the logging

Wynn and Read 1873



Table 1. Greenbrier Group Facies

Lithofacies Description Biota Depositional Environment

Red beds Red, maroon, and green mottled

mudrocks and siltstones, massive,

to laminated, rare current- and

wave ripples on siltstone interbeds,

rare mudcracks.

Root and burrow

traces in paleosols.

Subaerial to marginal

marine

Gray shale Dark-gray to olive-green clay and

silt, poorly fissile to massive.

Poorly fossiliferous to very

fossiliferous. Mollusks,

ostracods, some echinoderms,

brachiopods, and bryozoa;

biota sparse and restricted

in updip shale.

Lagoonal

Quartz sandstone and

calcareous siltstones

White to light-gray, well-sorted

fine- to medium-grained shaly

sandstones and siltstones of quartz, lesser carbonate grains,

cross-bedded to structureless;

flaser, lenticular, and wavy

bedded locally.

Rare Shoreline clastic

complex and

barrier siltstones

Anhydrite White-glassy, bedded, sometimes

sandy with dolomite.

None Sabkha

Caliche Yellow to brown, cryptocrystalline

and fibrous calcite crusts and fracture

fills; patches of caliche-coated

peloids and pisolites. Variably

silicified.

None Subaerial

Quartz peloidal

grainstone

Light to dark gray. Rounded, and

abraded peloids and some ooids,

abraded skeletal fragments, and

subangular very fine to fine

quartz up to 50%.

Abraded, rounded skeletal

fragments. No in-situ biota.

Coastal eolianite,

minor marine

sand sheets

Dolomite Yellowish tan. Poorly fossiliferous

to unfossiliferous. Fine-grained

dolomite crystals and may

include quartz silt and clay.

None to sparse, small

mollusks, small crionoids,

and ostracods.

Tidal flat

Fine-grained lime

wackestone-mudstone

Light-gray to creamy white,

unfossiliferous to moderately fossiliferous fine wackestone and

mudstone and pellet packstone.

Skeletal debris fine grained

and may contain quartz silt

and clay. Locally cherty.

None to sparse and may

have mollusks, smallcrinoid columnals,

ostracods. Small

oncolites, rare corals,

and brachiopods.

Low-energy lagoon

Peloid and ooid

grainstone

Light-gray to white, well-sorted, rounded,

medium to coarse grainstone

of sand-size ooids, peloids, lesser

skeletal fragments, and minor

intraclasts.

Crinoid, brachiopod,

bryozoan, and mollusk

fragments, forams.

High-energy shoal

1874 Geohorizons



tool, which then gives an average value for the thinly

interbedded units. Distinguishing this interbedding from

the mixing of superimposed rock types in the well cut-

tings is difficult. Where one lithology has far greater

abundance than the others, then it can be designated as

the dominant lithology for the interval; but when they

are subequal, both were considered dominant litholo-

gies for the interval (Figure 6A).

Mixing of lithologies in a sample interval also can

result from two or more lithologic units stacked withinthe sample interval. In a typical 10-ft (3-m) sample in-

terval containing three beds of different rock types,

each approximately 3 ft (1 m) in thickness, it is difficult

to locate the lithologic units in the section unless there

is a distinctive wire-line-log signal (e.g., shale, evapo-

rates, clean oolite). Where the percentage of each li-

thology is similar, it is also difficult to assign a dominant

lithology to the interval. Lithologies can be assigned

to the correct depth where one or more had a distinc-

tive wire-line-log signature (gamma ray or bulk densi-

ty; Figure 6B), for example, shale or shaley limestone(gamma ray), porous limestone, or anhydrite (density).

When these could not be differentiated in the logs,

there was no way to determine the actual succession

of lithologic units in the sample interval.

Sample Interval-Induced Mixing

Mixing can also result from the sample interval bound-

aries not being the same as the lithologic interval bound-

aries. Minimum mixing caused by sample spacing exists,

where the sample interval boundaries roughly coincide

with the major lithologic unit boundaries (Figure 6C).

However, if the sample intervals are larger than the

spacing of lithologic unit boundaries, then mixing of

lithologies in the interval results (Figure 6D). Again, the

assignment of the cuttings to actual depth in the sample

interval required a distinctive wire-line-log signature for

one or more of the lithologies.

Drilling-Induced Mixing

Mixing of well cuttings and contamination by caving

are, in most cases, caused by improper mud viscosity

(Hills, 1949). Low-viscosity drilling fluids do not al-

low a good mud cake to form on the wellbore, thus

allowing contamination from beds higher up in the

section. Improper mud viscosity allows well cuttings

from different sample intervals to mix, thus distorting

the primary stratigraphic succession in the well. How-

ever, with proper mud viscosity, well cuttings may

be held in suspension even when drilling stops (Hills,1949; Rider, 1996). Wells with improper mud viscosi-

ty can be recognized by a poor correlation to geophysi-

cal logs and by familiarity with the stratigraphy of the

region.

Sequence Analysis Using Cuttings

Sequences in the wells were recognized on the basis

of major landward and basinward shifts in diagnostic

facies belts (cf. Sarg, 1988; Kerans and Tinker, 1997),

Skeletal

grainstone-packstone

Light to medium-gray, variably

fragmented echinoderms, brachiopod,

bryozoa, mollusks, and rare ooids.

Mud-free grainstones to grain-rich

packstones and minor wackestones,

some with argillaceous seams.

Abundant echinoderms,

brachiopods, and

bryozoas; and

lesser mollusks.

Midramp and lagoonal

skeletal sheets and

shoals

Argillaceous skeletal

wackestone

Medium-gray to dark-gray echinoderm,

brachiopod, bryozoa, and lesser

mollusks; include wackestone and

lesser packstone; abundant lime mud;

terrigenous clay disseminated or

in seams and stringers.

Abundant echinoderm,

common brachiopods

and bryozoas, rare

mollusk.

Deep subtidal ramp

Laminated shaly

lime mudstone

Typically dark-gray, carbonate and

siliciclastic clay and silt.

None to sparse small

skeletal fragments.

Ramp-slope and basin

Table 1. Continued

Lithofacies Description Biota Depositional Environment

Wynn and Read 1875



1 8 7 6

G e o h o r i z o n s



such as eolianites, red beds, siliciclastic units, and lime

grainstone units. The sequence boundaries (Van Wagoner

et al., 1988) in outcrop are disconformities developed

on shallowing-upward carbonate units and commonly

are veneered with a thin paleosol or with transgres-

sive siliciclastics (Al-Tawil and Read, 2003; Al-Tawil

et al., 2003). In the sections produced from the cut-

tings, the sequence boundaries were arbitrarily placedbeneath red beds, sandstone, and shale that rest on

the underlying (highstand) carbonate (Figures 2 –4).

Along the ramp margin and slope, correlative confor-

mities arbitrarily were placed beneath the shallow-

water tongues (lowstand systems tracts) extending into

the dominantly deep-water successions (Figures 2 –4).

In wire-line logs from the shallow ramp, many sequence

boundaries, and some parasequence boundaries, have

high gamma-ray response caused by the presence of

the overlying terrigeneous mudrocks, shale, or argil-

laceous silty dolomite above the sequence boundary(Figures 2–4). Over most of the shallow ramp in West

Virginia, these siliciclastic-prone units are considered

to be transgressive deposits, deposited during initial

flooding onto the platform. However, possible low-

stand units are preserved locally on the shallow ramp

in the form of eolianite and caliche. Carbonate-prone

units were placed in the highstand systems tract, with

the maximum flooding surface being placed at the

transition from siliciclastic to carbonate lithologies,

or beneath the deepest water carbonate unit in the

sequence.

DISCUSSION

In carbonate successions with interbedded siliciclastic

units, such as the Mississippian succession described

here, cuttings provide a valuable data set based on ac-

tual lithologies drilled within the sample interval. In

fact, they provide the only direct sampling of the rocks

in uncored intervals. The cuttings data, when tied intothe wire-line logs, can be used to generate reasonable

representations of the lithologic successions in the drilled

section. Wire-line logs alone cannot provide reliable,

definitive facies recognition in these carbonate-prone

units, but need to be integrated with cuttings and/or

core data.

In lithified units such as these Mississippian car-

bonates that lie at depths less than a few thousand feet,

mixing caused by caving appears to be much less impor-

tant than mixing where the sample interval spans lith-

ologic boundaries, interbedding of lithologies, and de-

velopment of parasequences of two or more lithologiesthat are at or below the scale of the sample intervals.

Many of the best cuttings suites in mature basins

are in the early wells. These wells should be incorpo-

rated into the data sets, even if their wire-line-log suites

are not as good as those in newer wells, especially if

wire-line-log suites are available from nearby wells.

The detailed cuttings-based lithologic logs of the

well section (Figure 4) and the resulting cross sections

(Figure 3) showing the high-resolution sequence stra-

tigraphy in this article could not have been generated

without the cuttings data because of the scarcity of core and because the wire-line-log data do not provide

unique lithologic discrimination in carbonate rocks.

This cuttings-based analysis has provided the first state-

wide subsurface picture of the Mississippian Green-

brier carbonate reservoirs in West Virginia. The region-

ally mappable sequences are fourth order instead of

third order (cf. Weber et al., 1995; Al-Tawil et al.,

2003). As such, the time-slice maps have a quite high

resolution and provide valuable information on differ-

ential subsidence of the foreland, apparently associated

with the numerous basement faults that dissect theforeland. The cross sections illustrate that these se-

quences are regionally mappable, and tectonic subsi-

dence determines regional thickness changes (Al-Tawil

et al., 2003). The generalized facies distribution for

each sequence defined by the cuttings, maps out the

midramp grainstone fairways bordering the basin, as well

as smaller grainstone areas farther updip. This is the

first time that these facies belts have been defined re-

gionally for the subsurface. Given the complexity of

the stratigraphic succession, with its numerous deposi-

tional sequences, rapid vertical and lateral facies changes,as well as rapid lateral thickness changes caused by syn-

sedimentary tectonics, it is no wonder that a detailed

regional picture and interval correlations had not been

developed previously for these rocks.

Figure 3. Using the cuttings and wire-line logs, it is possible to generate high-resolution sequence-stratigraphic cross sections, suchas this one (BB0 of Figure 1). The upper units were hung from the Lillydale Shale marker, but the lower sequences were hung fromthe base of the upper Taggard equivalent (quartz peloid eolianites and calcareous siltstones at the base of sequence C6). The crosssection clearly shows the likely distribution of potential oolitic and siliciclastic reservoir facies and potential flow barriers and seals.

Wynn and Read 1877



Figure 4. Comparison of core and cuttings data, showing sequence picks between Sun Oil 2 core and Greenbrier 22 (cuttings), theclosest well analyzed. Greenbrier 22 is updip of the Sun Oil well across a down-to-basin fault, hence, the thickness difference. Goodcorrelation of the sequences between the two exists. Parasequences are picked on the core, but are not able to be picked with thecuttings data generally.

1878 Geohorizons



Figure 5. Examples of maps of individual sequences and systems tracts generated using the cuttings. The high-resolution sequence frasequences to be generated, showing major flexures, highs, and lows at that time. The cuttings data can be used to generate maps showing the dlowstand-transgressive systems tract, or highstand systems tract, as well as isolith maps to illustrate the likely location of thick potential rese

Wy n n a n d R e a d

1 8 7 9



Outcrop analogs indicate that this cuttings-based,

high-resolution picture is still highly simplified, because

the parasequences that make up many of the sequences

are beyond the resolution of the cuttings (Figure 4).

This highlights the importance of using outcrops or

core where available to obtain a detailed picture of stacking patterns in the subsurface, as it is probably

the parasequence-scale changes that ultimately con-

trol the actual location of reservoir units and inter-

vening baffles and barriers to flow. In addition, the re-

gional scale of this study precluded closer well spacing;

thus, lateral porosity changes within a field associated

with, for example, mapping of individual tidal ooid

bars (Kelleher and Smosna, 1993), are far beyond the

resolution of the study, but obviously are of utmost

importance. Finally, the approach can provide a se-

quence framework for the interpretation of more lo-

cal, high-resolution 3-D seismic surveys, providing a

merging of old and new technologies.

CONCLUSIONS

This study analyzed the Mississippian Greenbrier car-

bonates of the Appalachian Basin in West Virginia to

show how well cuttings and wire-line logs can be used

to generate a sequence-stratigraphic framework of car-

bonates in the subsurface.

Well cuttings from 193 wells were classified ac-

cording to Dunham (1962), tied to the wire-line logs,

placed in GIS, and used to determine the vertical stack-

ing of lithologies in each well and to pick sequence

Figure 6. Diagram illustrating mixing of cuttings in wells through heavily lithified units. (A) Small-scale interbedding of two or morelithologies within the sample interval results in mixed cuttings, with no information as to whether they are interbedded or merely make up a part of the sample interval unless one lithology has distinctive log signature, e.g., shale. (B) Individual lithologic units thatare smaller than the sample interval result in mixed cuttings, which again require a distinctive log signature to relocate. This situation

makes picking parasequences difficult in cuttings. (C) Unusual case where boundaries of lithologic units coincide with samplinginterval boundaries; this is one of few cases where there is minimal mixing caused by stratigraphy. (D) Boundaries of lithologic unitsdo not coincide with sampling interval boundaries; this results in the lithology spanning the sampling interval boundary being mixedinto the overlying and underlying sample bags.

1880 Geohorizons



boundaries and flooding surfaces. The well sections were

used to construct regional cross sections with wells

correlated using regionally extensive gamma-ray mark-

ers, allowing sequence boundaries to be traced. Isopach

maps were generated for each sequence, along with

dominant facies maps of systems tracts, and isolith

maps were used to display the distribution of major

potential reservoir facies. The GIS time-slice maps pro-vide the first statewide view of potential Big Lime res-

ervoir trends and associated facies in West Virginia at

the fourth-order sequence scale.

Where seismic data are limited, well cuttings and

wire-line logs can provide a crucial data set for sequence-

stratigraphic analysis. In areas where seismic data are

available, well cuttings can provide the necessary li-

thologic data to aid in seismic interpretation where core

coverage is limited. Three-dimensional mapping of the

sequence-stratigraphic time slices and the resulting iso-

pach maps of the sequences can clarify subtle differen-tial subsidence patterns, help identify subtle regional

structures that are spatially too complex to be evaluated

by two-dimensional cross sections, and help better un-

derstand the complex interplay between tectonics and

glacioeustasy.

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