role of geology is fairly well-defined in conventional oil and gas
plays, but emerging unconventional gas plays have muddied the waters.
is often perceived as an engineering play, the question is raised:
In continuous-type gas plays, what is the role of the geologist?
answer, according to two geologists who presented a paper on the
topic at the annual meeting in Dallas, sounds a lot like "carpe
It is up
to geologists to carve out their responsibilities and prove how
geology can economically impact these plays, according to Jeffrey
Levine, a consulting geologist in Richardson, Texas, and Creties
Jenkins, with DeGolyer and MacNaughton in Dallas.
controls on production rates are reasonably well established for
conventional gas production, we pretty much know what to look for
and what to measure," Jenkins said. "However, with unconventional
continuous reservoirs it may require years before productive potential
is fully realized.
"As a result,
geologists too often end up working from a defensive position regarding
his or her contribution," he added, "and all too often with insufficient
data to back them up."
unconventional reservoirs are often geologically complex, leaving
geologists with multiple working hypotheses regarding controls on
sometimes regards this as being indecisive, which puts us in a very
difficult position," he said. "I have worked on projects where managers
cynically perceive geologists as problem finders rather than problem
unfair, but rather than complain we need to be able to clearly show
the importance of an accurate, thorough geological analysis," he
contend that finding and exploiting continuous-type gas accumulations
requires the simultaneous application of accurate geological interpretations
and effective engineering technology.
wrote in a paper presented at the AAPG Annual Meeting in Dallas:
is well established in conventional gas systems where reservoirs
are localized features with discrete boundaries. Continuous-type
gas accumulations, however, are regional in extent and lack obvious
seals or traps.
As a result,
the geological factors controlling gas-in-place and production rates
can be difficult to identify and quantify. This can lead to the
erroneous assumption that the reservoir is too complicated to characterize,
or that heterogeneities are uniformly distributed and, therefore,
can be approximated with simple models.
is the assumption that these reservoirs are 'statistical plays,'
which allows companies to rationalize and justify their lack of
knowledge regarding reservoir geology."
As a result
of these assumptions, many companies emphasize engineering and minimize
geology in exploration and development of continuous-type gas accumulations.
this, geologists must do a better job of defining their responsibilities,"
Jenkins said, "demonstrating why geological knowledge is so important
and delivering recommendations that will have a favorable impact
on the bottom line."
gas accumulations are reservoir systems where gas-bearing strata
extend over much larger stratigraphic thicknesses and lateral distances
than conventional gas reservoirs. These accumulations include coal
gas, shale gas and tight sandstones.
resource in continuous-type accumulations is distributed more or
less continuously through the reservoir, although the reservoir
quality can be highly variable, both in terms of the resource's
density and deliverability.
and Jenkins say that experience in the United States and elsewhere
shows that economic gas rates from sorbed gas reservoirs require
a favorable combination of reservoir properties, which tends to
occur only over a small portion of the entire basin.
Juan and Black Warrior basin coal gas plays and the Michigan Basin
shale gas play provide notable examples of narrowly constrained,
geologically controlled production fairways which have provided
most of the gas production," Levine said. "The challenge facing
geoscientists is to identify these trends as early as possible and
to work with engineering staff to appraise and develop them efficiently."
said there are a number of reasons why geologists tend to play a
secondary role in assessing and developing continuous-type gas accumulations.
a project manager acknowledges there are geologic questions concerning
a play, they may expect an algorithm or standardized form to determine
those answers," he said. "We need to recognize, however, that we
are still very much on a learning curve for this type of reservoir,
and that every prospect presents a unique combination of variables.
has to be an element of faith that spending money early in the project
to collect basic geologic data will make it possible for geologists
to draw inferences and see relationships that will be important
down the road."
cases geologists have not done an adequate job of generating answers
that challenge the status quo and confront misperceptions, both
men say — so companies often move forward with minimal geological
support and make decisions based primarily on short-term economics.
often good geological work is disregarded because companies are
entrenched in a culture that makes decisions based on non-geologic
issues," they said. These issues include:
understandably hesitate to burden projects with the up-front costs
of coring, logging, testing and analysis," Levine said. "This reluctance
to spend money is often shortsighted as the benefits can be realized
many times in the appraisal, pilot and development stages."
Focus on Engineering Technology
Successful development of continuous-type gas accumulations over
the past 30 years has been made possible by development of novel
engineering technologies. As a result, engineering has taken the
dominant role in many projects. But engineering technologies can
only affect the near well bore region while production rates and
reserves depend mainly on the quality and distribution of reservoir
properties in the interwell areas, Levine and Jenkins said.
Geologists must seek the same kind of support for tackling geological
problems as engineers receive to overcome drilling and completion
issues, they added.
Statistical Play Syndrome
Unconventional gas developments typically require hundreds of wells,
often on close spacings. The variation in production rates between
adjacent wells can be greater than those of conventional gas accumulations
due to subtle changes in lithogies, fracturing and well completion
effectiveness. Often companies conclude that these variations are
"statistical," meaning they are unpredictable and their distribution
can only be known by drilling up the entire reservoir at development
Complexity of Sedimentary Organic Matter
Sedimentary organic matter plays a critical role in the storage,
diffusivity and permeability of sorbed gas reservoirs, which includes
coal and shale gas, but "despite its importance, expertise in the
sampling, processing, analysis and interpretation of organic matter
remains a geological specialty that is not widely held," Levine
is the legitimate role of geologists in these continuous-type gas
and Jenkins agree that first and foremost geologists can:
reduce risk by anticipating trends in reservoir quality.
data for optimizing drilling and completion, remediation of production-related
problems, reservoir simulation and economic modeling.
assess the distribution of gas in place and improve the accuracy
of reserve estimates, target stratigraphic intervals for completion
or recompletion and determine optimal well spacing.
critical in guiding project expansion and acquisition.
have to carve out their place in the development of these unconventional
gas plays; challenging the perception that geology isn't important
is just one part of the hurdle. Often geologists completely unfamiliar
with coals or shales are asked work these plays, but they don't
have sufficient background.
intimidating and difficult to be thrust into this type of situation
and be expected to provide answers on something you aren't familiar
with," Levine said. "So, education is the first critical step for
any geologist asked to work continuous-type gas accumulations."
"I am relatively
new to unconventional gas," Jenkins added. "I started working these
plays in 1997, and the first thing I did was read papers and contact
key people with experience in the field. I knew it was up to me
to learn as much as I could so I could be an important part of the