Topic:
"Cotton Valley Reef Trend, East Texas Basin: Exploration and Diagenesis Model"
SpeakerDan Zeigler, SK Resources, Houston.
Abstract:
Exploration has exploded in the mature East Texas Basin for deep,
over-pressured "pinnacle" reefs in the Cotton Valley Limestone (Haynesville).
Reefs have been reported to be more than 800 ft high, 100 acres and contain
as much as 100 BCF in a single reef.
The reefs germinated during Cotton Valley Lime deposition and grew into the mixed siliciclastic/carbonate environment of the Bossier Shale along compactional features, salt-withdrawal-induced fault scarps, and salt-cored and basement structures. Productive reefs always have abundant delicate finger coral, and faunal diversity has been seen to increase with wave energy. Generally, wave energy was moderate and the reefs grew within wave base. Deepwater microbial mounds have not been seen to date. Reef poisoning and growth-inhibiting influences of nearshore influxes of terrigenous sediment and fresh water have been inferred locally to have resulted in tight reefs. Cumulative reef height was driven by vertical accomodation space provided by long period (third order) relative sea-level rise.
In the East Texas basin, local salt withdrawal and regional tectonics caused by post-rift thermal subsidence were the dominant processes affecting sea level. Recent studies of the Upper Jurassic eustacy curve indicate that sea level was probably static and did not provide accomodation space globally. The Upper Jurassic was a time of "greenhouse" climate where smaller polar ice caps induced low amplitude (10Ð20 ft?) fourth-and fifth-order sealevel cycles. This composite sealevel curve provided the opportunity for reef communities to accumulate vertically via the third order rise and provided a mechanism to create significant porosity via the higher frequency sealevel fluctuations. Drops in sea level exposed reefs (islands) where rain collected at the near surface, generating a freshwater lens (water table).
Gross recrystallization occurred over much of the most recent reef cycle construction into micro-rhombic calcite and micro- to fine porosity. Less stable calcite components (aragonite, Mg calcite) morphed into more stable calcite and vugular porosity. These reefs are rhythmically stacked patch reefs that look like pancakes with synoptic relief probably never exceeding 50ft. Each "pancake" grew vertically 20Ð50ft. during each fourth or fifth order sealevel rise and was then leached during the subsequent drop.
Exceptions to this process have been inferred locally where local freshwater influx depressed the tops of the reefs to slightly deeper, preventing their exposure to freshwater leaching. The reefs backstep in space and time away from the locus of thermal subsidence at the center of the basin - a failed rift. There is ample evidence for these hypotheses from well logs, Sr and C/O isotopes, fluid inclusions, SEM, and petrography.
Biographical Sketch:
Daniel G. Zeigler grew up in Philadelphia, Pa., and earned a B.S. in geology
in 1977 from Indiana University of Pennsylvania. He started in the industry
as a mudlogger with Baroid and independent geologist during 1977Ð1979.
Zeigler became a staff geologist with Southeastern Exploration and
Production Company (SEPCO) in 1980 and explored the Triassic rift graben
frontier on the eastern seaboard until 1988. He returned to graduate school
in 1988 at the University of Texas at Dallas Ph.D. program. He studied
deformation using global positioning systems at the Nevada Nuclear Test Site,
Baja California, and New Zealand.
In 1991, Zeigler joined SK Resources as a staff geologist. Since then, SK has become a major player in the Cotton Valley Reef Trend. Other areas of interest include the Mississippi Salt Trend, South Texas Sligo trend, and the Lodgepole Formation of North Dakota.
Figure caption:
First Stage: Reef grows in moderate to high energy environment. Fauna is
dominated by corals and sponges; facies by grainstones. Second stage:
Reef is exposed during sea-level drop. Much of the reef becomes finely porous
because of recrystallization of micro-rhombic calcite. Dissolution of coral
framework and grains enhance capacity and flow rates of fluids in the reef
complex. Figure by Dan Zeigler.
[Chairman's note: We would like to thank Faizur R. Khan, P.E., Project Manager for Laidlaw Environmental Services Columbia Engineering Department (Houston), for substituting at the last moment for October's speaker. His talk was entitled "Cut-Off Wall System for Subsurface Liquid Containment."]
Topic:
"Advances in Electromagnetic (EM) Surveys for Environmental Contaminants"
Speaker:
David G. Paton, President, PEMSERV, Inc.
Abstract:
Electromagnetic (EM) instruments have been used for the past 20 years to map
areas of elevated subsoil conductivity, but only very recently have there
been any changes to the design of the instrumentation or to the techniques of
its use. The biggest change to EM instrumentation occurred in September 1997
with the commercial introduction of a multi-frequency EM unit that can be
programmed from 300 to 20,000 Hz. This opens up a huge range of potential
uses, because now one person operating the equipment can detect changes in EM
responses from near surface to 40 m in depth. When combined with a real-time
global positioning system (GPS), the labor component is removed because the
GPS creates a virtual grid, thus eliminating the need for a physical grid.
This helps to reduce costs, while maintaining sub-meter accuracy of the survey.
Automated software processing can enable data reduction and mapping in the field,
thereby avoiding expensive delays waiting for the EM survey map. The advantages
of mapping a site with EM prior to sampling are better delineation of target
areas for drilling and reliable estimates of horizontal plume migration. With
the advent of the multi-frequency EM, a much better estimate of depth of
contamination can be obtained.
Biographical Sketch:
David G. Paton completed an M.S. in soil science from the University of Alberta
in 1981. He has been involved in environmental consulting since 1975 and has
an extensive background in soil chemistry, including commercial lab experience.
For the past four years, he has been developing techniques for the use of EM
surveys in the detection ofsubsoil conductivity and hydrocarbons. In 1996, he
formed Paton Environmental Services and concentrated solely on EM and GPS
applications. In 1997, he moved to Houston to escape the harsh Canadian winter
and is planning to stay here permanently. He is currently president of
PEMSERV, Inc.
Topic:
"Anadarko's Experience in Algeria: A Case History"
Speaker and Authors:
Richard Hook, James Emme, Robert Lunn, Brian Sunderland, Robert Daniels
and François Gauthier of Anadarko Petroleum.
Abstract:
In 1986 Anadarko began a regional evaluation of the Algerian Saharan basins at
a time when most of industry was exiting and new petroleum legislation was
being written. Senior management promoted a vision for a long term exploration
commitment commensurate with the opportunity for finding significant petroleum
reserves.
Technical efforts quickly focused on the Ghadames and Illizi Basins where two world-class source rocks were identified and multi-pay potential existed. A contract with Sonatrach was signed in 1989, committing Anadarko to a ten-well exploration program on four blocks totaling 5.2 million acres. Seismic reprocessing, acquisition, geochemical studies, and stratigraphic studies began in earnest in 1990.
During the early phase of new seismic acquisition, the lure of the "big trap" led to the initial wildcat drilled in 1991, resulting in a dry hole. Many companies might have sought an exit strategy at this point. With the unwavering support of management, Anadarko continued technical evaluation with increased emphasis on migration pathways, regional reservoir studies, and new seismic acquisition. As work continued, the second well was successful in 1993. Data from the drilling and ongoing seismic acquisition programs continued to be integrated into a more coherent picture.
New emphasis was given to seismic imaging, hydrocarbon charging, and the depositional setting of the Triassic sandstone reservoirs. With a better understanding of the overall petroleum system, Anadarko drilled five more new field discoveries in the period 1994 through 1996. Recoverable reserves for the discoveries total 1.5 billion barrels of oil.
Key elements in achieving this success include: management vision and support; A strong relationship with Sonatrach based on mutual respect which fostered their making significant contributions, including the benefit of ideas, experience and access to their extensive database; Initial block selection; improved seismic imaging; teamwork, tenacity and technical excellence on the part of the staff; and flexibility to learn from both our successes and failures to improve exploration models and drilling techniques.
Biographical Sketches:
Richard Hook received his B.S. in Geology from University of Oregon in 1977
and his MS from Oregon state in 1981. He joined Anadarko in 1980 in Oklahoma
City and worked on the Algeria project from 1989 to 1994. He is currently
exploration manager for Africa and the Middle East for Anadarko, based in
Houston.
Robert Daniels received his B.A. in economics from the Colorado College in 1981 and his M.S. in geology from Colorado School of Mines in 1985. He joined Anadarko in 1985 in Denver. He has worked on the Algeria project since 1992. Daniels is manager of geology for Anadarko Algeria, based in Uxbridge, England.
James Emme received his B.S. in geology from the University of California at Davis in 1978 and his M.S. in geology from Colorado School of Mines in 1981. He joined Anadarko in 1981 in Denver. He has been involved with the Algeria project from 1986 to 1992. Currently Emme is Anadarko's onshore exploration manager for North America, based in Houston.
François Gauthier, exploration manager for Anadarko Algeria Corporation in Uxbridge, U.K., joined Anadarko in 1982 in the Calgary office as Division Geologist. Mr. Gauthier received both his diploma in engineering and science in 1973, and his B.Sc. with honors in geology in 1976 from McGill University.
Brian Sunderland received his B.Sc. in physics from Reading University in 1968. Presently, he is manager of geophysics for Anadarko Algeria Corporation, in Uxbridge, U.K. He joined Anadarko in Houston in 1981. He has worked in various areas, including Asia, Europe, Africa, and North America.
Robert Lunn joined Anadarko in Houston in 1985 after five years with Gulf Oil in New Orleans. He earned his B.S. from the University of Texas at El Paso in 1980. Mr. Lunn is currently exploration manager for South America and the North Atlantic.
Topic:
"Confined-flow Turbidite Reservoirs: Plio-Pleistocene,
Ship Shoal - Ewing Banks - Green Canyon areas, Offshore Louisiana"
Speaker and Co-Authors:
John M. Armentrout, Steve J. Malecek, Vinod R. Mathur,
Gary L. Neuder, and Gerry M. Ragan, Mobil Oil Corporation, Dallas, Texas, USA
Abstract:
Seismic facies and high-resolution bio-stratigraphic analysis provide a
sequence stratigraphic framework for interpreting lateral distribution of
sand-prone facies and possible reservoir connectivity in the Ship Shoal 351-358
to Ewing Bank 988 minibasin, offshore Louisiana. The interval of interest is
an isochron thick interpreted as a lowstand systems tract deposited in
bathyal water depths within an intraslope-minibasin. This basin is
approximately 50 km from the age-equivalent shelf/slope break. The isochron
thick was deposited between the late Pliocene Discoaster pentaradiatus
and early Pleistocene Discoaster brouweri Condensed Sections, as
calibrated by data from five wells penetrating this interval within the
intraslope basin.
The D. pentaradiatus - D. brouweri sequence consists of the synclinal fill of a salt withdrawal basin forming an isochron thick that thins onto adjacent salt-cored structural highs. This isochron interval was subdivided into four seismic facies and each was calibrated with local well data. Mapped patterns of these seismic facies suggest a network of channel-form systems supplying sediment to depositional lobe-forms within a slope valley. Sandy sediments were transported by gravity-driven processes from the shelf, down slope through a valley and into this local intraslope-minibasin. Upon filling this minibasin, the gravity-driven sediments spilled farther south into the area of the Green Canyon 18 field, where these sandstones are the main reservoir. The mapping of channel-and levee-form patterns suggests that the transport process included turbid flow with consequent overbanking of the channel and levee formation.
Gulf of Mexico Gravity Flow Event; from Shanmugam (1993, AAPG Bulletin)
Art work by Mark Lindsey, Mobil Technology Company
(Colorizing experiment by HGS web-page staff)
Local abundance peaks of planktonic microfossils provide correlation control between the four depositional lobes of this lowstand deposit. The biostratigraphic data and paleogeographic interpretation suggest that each depositional lobe is separated from the others by mudstone drapes, resulting in discrete partitioning of reservoir sands.
Biographical Sketch:
John M. Armentrout has a Ph.D. from University of Washington (1973) and a M.S.
in geology from University of Oregon (1967). He has been an AAPG Distinguished
Lecturer and President of SEPM. His current assignment in integrated
stratigraphic interpretation at Mobil's Exploration and Producing Technical
Center in Dallas began in 1990.
Figure:
Gulf of Mexico Gravity Flow Event; from Shanmugam (1993, AAPG Bulletin)
artwork by Mark Lindsey, Mobil Technology Company.
