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SPE 35742
Possible Sources of Gas from the 31S C/D Shale Reservoirs
Monterey Formation Elk
Hills Field California
Thomas J. Hampton, SPE, Stephen A. Reid, and Jana L. McIntyre,
Bechtel Petroleum Operations, Inc.; and E. Mark
Querin, U. S. Department of Energy
Tiwspapa was prepared for presentatmn at the Western Regmnal
Meeting held (nAnchorage,
A laska 22.24 May 1S96
Thospaper was sel~cted for presentatmn by the SPE Prqiram Cmmmee
f.allwwmgrevwvvof
Inform at Ion conta ined m an abstract submlfted by the
author(s) Contents of the paper as
presented, have not been rewevmd by the Soaety of Petroleum
Engineers and afe subject to
cnnet?.wnbytie alhcf (s) The material, as presented, does
notneces arlly reflect any pawtton
tfw wtyf Petro leum Engtneers or I ts members Papers
presented at SPE meetmgs are
subject to publ(catjon rwlew by Edltor al Ccmmmee of the
Sc-sIetyof Petroleum Engineers
Pwmusnn tocwpyISmstr cted to an abstract of not more than XXI
words Illustratmns may not
twcc d The absbact should contmn conspicuous acknowledgement of
where and bywiwrn
the paper was presented Wr te L(brarlan, SPE. P O 8333836, R
ichardson TX 75083.3836
USA,
filX
01.214.952+ 435
Abstract
Cumulative gas production from the C/D Shale Reservoir
slgniti-
cantlycxods expected gas production as estimated from
pressure
decline data. Reservoir studies indicate three possible
sources
leakage from other reservoirs, unconventional gas resources,
and
underestimation of original oil in place. Each of these
three
sources are discussed.
Introduction
Cumulative gas production from the C/D Shale Reservoir
signifi-
cantly exceeds expected gas production as estimated from
pressure
decline data,The original oil in place is estimated at 250
MMSTB
and the original gas in place is calculated at 238 BCF based
on
PVT analysis. Estimated total released gas from pressure
decline
data ismuch less than the actualproduction of 170.5 BCF
(through
Deeember, 1994) Therefore, other sources of gas must be
present
to provide additional gas production, Reservoir stud]es
indica[e
three possible sources: leakage from other reservoirs,
unconven-
tional gas resources} and underestimation of original oil in
place.
Figure 1is a schematic drawing of a conceptual model of the
three
possible sources.’
Reservoir Descriptionz
The C/D Reservoir is in the Monterey Formation and is
stratigraphically below the prolific Stevens sand reservoirs on
the
31S anticline,such as the 26R Sand Reservotr Although the C
and
D intervals have diverse Iithologies, both intervals are in
pressure
communication and are considered one reservoir. The reservoir
is
produced by depletion drive, locally assisted by a strong
gravity
component, with well spacing at 10 to 40 acres. The C interval
is
100 to 300 feetthick and consists of basin floor deposits of
organic
shale and thin, od saturated turbidite sandstone in multiple
upward-
finmg cycles. The upper D interval is about 200 feet thick
and
consists mostly of porous biogenic chert. Highly porous and
oil
saturated laminae alternate with less porous, orgamc nch
Iaminae.
The lower D interval is also about 200 feet thick and contains
a
more chaotic composition of oil-stained chert, sandstone and
dolomite. Permeability measured from core for the C interval
ranges from O.I md for organic shales to over 60 md for
thicker
turbidite sandstone beds. For the D interval chert, measured
matrix
permeability is consistently about O 1md.
Unlike most Monterey shale resemoirs, the most productive
intervals ofthe C/D Reservoir exhibit little evidence of large
scale,
widespread fracturing. However, small scale and microscopic
fractures are common and occur parallel and perpendicular to
laminations Lothe upper D Shale, core is commonly broken
along
bedding planes, which md]cates that bedding-plane fractures
are
open in the formation and help accommodate gravity drainage
Only the lower D interval exhibits classic Monterey fracture
characteristics, with a large scale permeable and
intercomected
fracture system
Investigation of Fluid Conductive Faults
Production and pressure evidence indicates gas enters the
C/D
from adjacent, higher pressure reservoirs most likely along
a
significant fault that connects the 26R secondary gas cap with
the
CID Reservoir. Gas injection surveys veri~ that gas enters the
fault
from the 26R Reservoir and establishes that the fault zone
is
permeable. The degree of this communication was recently
demonstrated with a blowout of a 26R well, which rapidly
lowered
26R pressures and also immediately affected C/D reservoir
pressure. An estimate of gas intlux from full field
simulation
studies3 shows an influx of 66. I BCF primarily from the 26R
Reservoir.
Shale Gas Desorption
A possible significant unconventional gas source in the
Monterey
Formation is the desorption of gas from organic material
Organic
material, such as unrnature kerogen, is common in the
Monterey
and may be as much as 10percent of the rock volume. As
pressure
is redueed inthe formation, gas may evolve from organic matter
in
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