West Delta Block 107 Field: Hidden under a Mudflow
Abstract:
West Delta Block 107 Field, located eight miles seaward of the
Southwest Pass of the Mississippi River in 235 feet of water (Fig.1a),
contains approximately 18 million barrels of oil (MMBO) and 70
billion cubic feet of gas (BCFG) (recoverable) in nineteen Pliocene sands.
Hydrocarbons are trapped in a three-way structural closure downthrown to a large
down-to-the-southeast growth fault. Stratigraphic variation plays
a minor role in entrapment in five sands.
Walter Oil & Gas Corporation discovered the field in 1993 with its OCS-G 8736 No. 1 well, located within a large, active mudflow lobe that has caused major difficulties in seismic and drilling exploration over the years. Due to effects of the mudflow, pre-1983 2D seismic data did not adequately define the amplitude response or the structure of the prospect. Only after 1988 3D seismic data was reprocessed with refraction statics corrections by Shell Offshore, Inc. in 1993, was it possible to interpret structure and amplitude correctly enough to discover the field. A 1996 3D survey has further improved data quality and has refined our understanding of the field.
The field has been developed by seven wells. Five of those wells were drilled from a conventional platform site (West Delta 106 "A" platform) outside of the mudflow and two miles west of the field. The two other wells (including the discovery well) are active subsea completions. Daily field production rate since February 1995 has averaged 7200 barrels of oil (BO) and 17 million cubic feet of gas (MMCFG) daily.
The Block 107 Field was nearly found in 1968 by Humble Oil & Refining Company's OCS-G 1591 No. 1 well and in 1983 by McMoran's OCS-G 4244 No. 2 well, each of which missed the main accumulation by less than 200 feet horizontally.
Biographical Sketch
Carl Kuhnen prospects for Walter Oil & Gas Corporation in the
Gulf of Mexico and onshore Gulf Coast. From 1981 to 1990 he
explored the same areas for Prairie Producing Company following earlier work in
the Gulf Coast and U.S. frontier basins for Union Texas Petroleum and
Amoco.
He received his B.S. in physics from M.I.T. in 1969 and his M.S. in geophysics from the University of Houston in 1974. This paper was published in July 1998 in GCAGS Special Publication 3-D Seismic Case Histories from the Gulf Coast Basin.
"Environmental Architecture" - Discussion of philosophy and design considerations of environmental impact on architecture.
Abstract:
Geology Based Design
Understanding the value of geology from a Sustainable Design perspective
Architectural design has long been influenced by the realities of geology. Mountains and hills were chosen as sites to take advantage of inspiring views and to gain military advantage. Local geological conditions often dictated what construction materials were used. Certain soils were suitable for rammed earth or cob construction methods. Stone quarries offered durable materials but were difficult and expensive to transport over great distances.
The advent of mechanization, power of internal combustion engines, new materials, and other technologies obviated most of the problems and limitations of the past and has led us to our current process. Architects of today are typically concerned only with load bearing capacity of soils on site, and especially in Houston, how much water will the soils hold when designing retention ponds. Stone quarried in Italy is readily available product for use anywhere in the world. Rammed earth, cob construction, and stone carving are technologies that have become nearly lost to western man.
Architecture is moving towards a new awareness and is beginning to question our current means and methods for developing the planet. Will the resources we use for building materials be available for future generations? What are the long-term consequences of our siting and development practices? To truly develop a sustainable design, the architect must synthesize a vast body of knowledge from an army of specialists into a set of plans and specifications. Geologists will play a critical role in the spread of this new design process, and could become a pivotal influence in what materials are used and could serve to prevent the loss of land areas that are crucial to our survival.
About the Author:
An aviculturist and biologist with 14 years experience in physical plant operations at the
University of St. Thomas and University of Houston. Stephen entered the graduate program
at University of Houston’s College of Architecture, where he graduated with honors in
1992. He was invited to offer a special topics course on Sustainable Architecture at
the college, which he has taught for the past 6 years. Serving on the AIA - Houston
Chapter’s Environment Committee, he has helped organize a series of workshops to raise
awareness of ecological issues amongst Houston area architects and designers, and as a
member of the Rice Design Alliance’s Program Committee, he organized three fireside chats
on Sustainable Design. Stephen continues to guest lecture and writes about sustainability
issues, with an emphasis on raising awareness about the ecological consequences of our
built environment, while serving as the Chief Operations Officer for Heights Venture
Architects.
Technology Driven Economic Growth
Presently she is Managing Member for Venture Capital Investors L.L.C., Little Rock, AR, where she continues to seek new ways to blend fundamental academic discoveries, public funds, commercial entrepreneurship, and enlightened management decisions in technology-driven economic growth in her home state. This year Dr. Good became the second recipient of the Othmer Gold Medal which is awarded by the Chemical Heritage Foundation, American Chemical Society, and the American Institute of Chemical Engineers.
Abnormal Pressure Evaluation of the Recent Pliocene and Miocene Gas Discoveries from the Eastern Nile Delta, Egypt, Using 2D and 3D Seismic Data
Summary:
Interval velocities derived from surface seismic have been an important tool in
predicting pore pressure at drilling locations in the Nile Delta, and delineating
overpressure in the basin. Large accumulations of gas are trapped in Pliocene
and Miocene sandstones. Combined with pressure data from exploration wells, a
petroleum system can be postulated for both shallow and deep objectives. Both
stacking velocities supplied by contractors from regional 2D lines, a 3D survey,
and interval velocities derived from depth migration before stack (MBS)
processing have been used to estimate pore pressure in the basin. The success
of estimating pore pressure from either source of seismic velocity information
depends upon reflection quality, acquisition parameters including the cable
length, velocity anisotropy within the rock section, and geologic structures
that may cause out of plane reflections. Despite many sources of possible
error, reasonable pressure predictions have been derived from surface seismic
data using proprietary computer programs. Pore pressure is calculated and
presented as a color overlay at every trace along a seismic line, or in a 3D
velocity volume. The pore pressure calculation is based on a normal compaction
trend and an equation relating the normal, or expected velocity, and the
observed velocity. Both the normal compaction trend and the pressure equation
are exponential relationships written in terms of effective stress.
Introduction:
Since 1993, renewed exploration of the offshore, eastern Nile Delta of Egypt
has been dramatically successful. Significant new gas fields have been found
in Pliocene and Miocene sandstone reservoirs which have been tested with flow
rates of 25 to 36 million cubic feet per day, with 500 to 2000 barrels of
condensate. Significant volumes of gas are an overpressured section. Gas
reservoirs in the Pliocene Kafr el Sheikh formation form many separate
pressure compartments. Compartments range from normal pressure at about
1400 m at the top of the formation to over 4000 psi above normal
(near 16 pounds per gallon equivalent) at the base of the Kafr el Sheikh
formation near 3600 m. In the underlying Miocene section, Serravallian age
sandstones are the primary gas reservoirs. Pressures are about 3800 psi
above normal (near 14 PPG equivalent) at about 4000 m. Some Serravallian
reservoirs are lower in pore pressure than the overlying shale, suggesting
lateral fluid leakage from the overpressured compartment. Higher pressure in
the overlying shale creates an almost ideal petroleum seal. The underlying
Oligocene section is extremely overpressured in much of the area of current
exploration. Where the Oligocene section is at very high pore pressure, it
is thought to be incapable of retaining large accumulations of hydrocarbons
due to loss of seal integrity from naturally induced hydraulic fracture. High
abnormal pressure has caused significant drilling difficulties, increased
exploration costs, and is a significant factor in hydrocarbon migration and
trapping.
Work Methods:
Evaluation of abnormal pressure from well log measurements and seismic interval
velocity is based on empirical relationships comparing observed resistivity or
velocity of clay rocks to a normal compaction trend. Most of these techniques
were developed in the Gulf Coast of the United States in the 1960's where it was
observed that prior to significant pressure increases, shale resistivity
and density decreased while acoustic travel time increased (Hottman and Johnson,
1965). Shales had higher than normal porosity and were undercompacted for
their depth, being sealed such that pore waters could not escape or could
only escape slowly, effectively sealing underlying porous and permeable units.
Calculation of pore pressure from well logs and seismic velocities is
accomplished using two proprietary programs for the PC and workstation,
using very similar techniques. A normal trend line is based on equations
that describe the compaction of clay rocks and follows the form of an
equation by Hubbert and Rubey (1959), where compaction is an exponential
reduction in porosity with increasing effective stress, which is dependent
on overburden stress and pore pressure. The normal compaction trend is
converted from terms of porosity into terms of velocity
(Figure 2) using
a relationship developed by Eberhardt-Phillips et al (1989). Pore pressure
is calculated from well log and seismic data by Eaton's equation (1975)
which relates pore pressure to an exponential function of the ratio of
the observed velocity versus the expected or normal trend velocity.
The same techniques can be applied to interval velocity derived from seismic data. Despite new techniques and software, extracting good quality velocity information from seismic data is rarely an automated procedure. Poor reflection quality, conflicting events, noisy data, poor velocity sensitivity, velocity anisotropy with the rock system, highly dipping beds, and limitations imposed during acquisition such as cable length together make velocity analysis an interpretive procedure. Velocities derived from seismic are inherently low resolution. Seismic pressure predictions should not be able to resolve large changes in pore pressure over short distances. In addition, the seismic velocity field represents all rock types while the equations used to convert the velocity field to pore pressure presumes that all values are from clay rocks. Some variations in the velocity field are likely due to changes in the rock type and not changes in pore pressure.
Despite the many sources of possible error, successful predictions of pore pressure can be made prior to drilling using seismic velocities. The utility of seismic-based predictions is that they allow an interpreter to evaluate the lateral variation of a pressure field or volume away from well control, and predict how pore pressure is likely to vary from a point of control.
Our evaluation, of the eastern Nile Delta pressure was calculated based on conditioned stacking velocities from long regional lines and the Ras El Barr 3D survey. A spatial median filter and heavy smoothing were used to clean up the DMO stacking velocities prior to conversion to interval velocity using the Dix equation. Since seismically derived velocities may contain inaccuracies, a calibration process is required. Conditioned contractor velocity data sets were compared with more accurate seismic interval velocities derived using 2D, iterative depth migration before stack (MBS) processing as part of the calibration process as well as well control.
A comparison of the 3D velocity field to the Akhen #1 and Ha'py #1 wells showed that, in general, the seismic velocities compared well with the average velocity measured by sonic tools (Figure 2). Normal compaction trend for clay rocks in the basin was determined from the normally pressured shale of the Kafr el Sheikh fm. of Pliocene age. The overlying Plio-Pleistocene Mit Ghamr and El Wastani formations, which are dominantly sand and silt with lesser clay appear to have separate compaction trend s than the underlying formations.
The top of overpressure is clearly reflected by slower than expected velocities in the Kafr el Sheikh, but the underlying A30 sand interval appears to be normally compacted and normally pressured (Figure 2). In reality the A30 is mildly overpressured but the abundance of sand in this interval controls the velocity rather than undercompacted, slow shale. Also, within the Miocene section the 3D seismic velocity is too fast in comparison to the well velocity. This could lead to an under prediction of pore pressure. In this case it was found that the most expedient approach is to increase the sensitivity of the Eaton equation to make up for the inaccuracies of the seismic data. For the 3D survey, the exponent used in the Eaton equation was increased from 3 to 5 (Figure 3).
Discussion
Abnormally high pore pressure is widespread within the Nile Delta and
significant gas accumulations are present in the overpressured section
(Figure 4).
Overpressure is confined to the clastic dominated Tertiary
section, at least onshore where the older section has been drilled or
crops out. It is postulated that a similar section of older Tertiary
and Mesozoic sandstone and carbonates extends northward offshore, and
that a lack of shale pressure seals within the section does not permit
the retention of any overpressured fluids. Towards the south, the
clastic Tertiary section thins and overpressure ends. Top of pressure
is within the Pliocene Kafr El Sheikh formation, which is dominantly
comprised of shale. Abnormal pressure usually increases rapidly from
the base of the Kafr El Sheikh down into the Miocene shale-dominated
section. Extremely overpressured shale has been found in deep wells
that reached the Miocene and Oligo-Miocene section. The absolute magnitude
of the pressure gradient decreases to the north into the deep water region
of the delta which is just now being explored. Here the magnitude of
overpressure is controlled by the relative decrease in the overburden
pressure gradient due to the depth of water.
Significant reserves of natural gas have been discovered in overpressured sandstones within the Miocene section. Deeper Miocene objectives were the first targets for exploration offshore, but more recently, shallow and largely normally pressured gas of the uppermost Kafr El Sheikh formation are exploration objectives (Figure 4). Miocene discoveries include the Tineh, Port Fouad, Wakar, Temsah fields, and most recently the Akhen #1 discovery (Figure 1).
Of exploration significance is that the majority of the Miocene reserves are in overpressured sandstones that, however, are lower in pressure than the over- and under-lying shales. Over-pressured shales are ideal seals for trapping fluids in the lower pressured sandstones which act as pressure sinks for the accumulation generated within the Tertiary section. Relatively lower pore pressure in the sandstone suggests either that the sandstone has bled off some fluid laterally or that the fields are well down the flank of the true distribution of the sandstone. Rarely some sandstone beds within the Miocene are extremely overpressured and have higher pressures than the bounding shales. It is likely that these have been penetrated near the highest depth of their distribution, and that the top seal is breached possibly by hydraulic fracture.
In the Ras El Barr area, a model for the generation and migration of gas has been developed which takes pressure conditionsinto consideration. Figure 5 illustrates likely migration pathways to traps from possible sources in the Miocene and Oligocene section. Primary migration from organic rich shale to sandstone conduits is aided by pressure gradients where the sandstone pressure is lower than the bounding shale. Gas charged Miocene sandstone in the NN5 paleontological zone of Serravallian age is lower in pressure than the bounding shale, which creates large pressure gradients toward the sand, enhancing primary migration. Similarly, in the synclines, lower pressure than the bounding shale is likely for other sandstones, enhancing primary migration.
Secondary migration of gas and other fluids has probably followed several paths. The NN5 sand possibly leaked fluid laterally to the south of the Ha'py graben (Figure 5) where the Messinian unconformity truncates the Miocene section and prograding Pliocene sands onlap the unconformity. Along the unconformity, other eroded Miocene sands may have fed gas up the rotated Pliocene sands into the Ha'py graben. A third migration pathway for gas is through hydraulic fracture of the shale seals at the crest of dipping and highly pressured, Miocene sands that, however, can not leak off sufficient fluid laterally to bleed down their pore pressure like the NN5 Sand.
Conclusions
Reasonably accurate pressure estimations can be made from interval velocities
derived from seismic data where data quality is good, care is taken in
conditioning the data, and the pore pressure calculation is calibrated to
well control or seismically derived velocities from depth migration before
stack processing. Seismic pressure predictions are, by nature, low
resolution but can add great value by providing information on variations
in the velocity and pressure fields beyond well control. Integrating pressure
estimates from 2D and 3D seismic with well data can provide greater
understanding of the distribution of overpressure at both local and
regional scale. In the overpressured Miocene gas trend of offshore Nile
Delta, the most productive reservoirs are lower in pressure than the
bounding shale and other sandstone units. These have likely been pressure
sinks through time aiding migration to the reservoir. Retention of the gas
charge is also aided by the relatively lower pore pressure of the sandstone.
Primary and secondary migration and trapping of petroleum has been enhanced
in the Nile Delta by large differences in pore pressure.
3-D Acquisition, Perils and Pitfalls
Abstract:
You are in the middle of your 3D seismic acquisition program and have more
problems than you know what to do with! The environmentalists are upset
that you are shooting in the bird sanctuary, the prison warden on whose
grounds you are shooting over wants to go to a Saints game, and the permit
agent has an individual who claims you have damaged ten of his seven foot
marijuana plants valued at $1000 each. The crew is standing by waiting on
a permit that was overlooked and your injunction is going to take another
eight days before you get a hearing. This was supposed to be a piece of cake!
After all, everyone is shooting 3D. You never heard of these problems from
anyone else! Is yours that unique or is it just that no one is talking about
their problems and challenges? Now your partners are threatening to pull out
if you cannot get this shot in the time frame you indicated. Geez, where is
it all going to end? Oh, and by the way, you are over budget!
The advent of 3D seismic has created additional opportunities within the realms of hydrocarbon exploration and production. This new tool allows explorationists to delineate features which they otherwise would not find. However, as with any new technique, there is a learning curve and mistakes are going to be made. Depending on the individuals and the company, these errors can occur in the office while designing the 3D program or in the field during the many stages of acquiring your data set. Though modeling and formulas are an integral aspect of planning the 3D seismic acquisition program, field knowledge and expertise are the key elements between a successful and an unsuccessful 3D program.
Especially when you meet Billy Bob Jim Jack who owns two sections right in the middle of your prospect and he "just don't really want none of them seismographic people runnin' around on my place!" And Rincky Dink Oil Company wants some data for free for a mineral permit. So, while our design looked great in the office when you planned it and presented it to management, it has now taken on a whole new picture, and not necessarily one for the better. How do you overcome this challenge?
While a poorly planned and executed 3D seismic acquisition program will result in dry holes and cost millions of dollars (as well as one's job), a successfully planned but poorly executed 3D program will have the same results. Incorrect control of such issues as mineral and surface permitting can result not only in an unusable survey, but it can also cost hundreds of thousands of dollars and bankrupt an exploration company. Companies must stay current on new federal and state regulations facing the industry or find themselves in a predicament that will take years of litigation to unravel.
And let's not forget the time necessary to execute each individual stage of the geophysical acquisition process. Oh, you mean the contractor didn't mention that the drills were on another job that has been getting rain for two weeks and can't get to yours for a couple of more weeks? Yes, they were promised, but hands are tied and there is nothing that can be done. Hmmm, imagine. Detailed coordination among all contractors, subcontractors and the company forms the cornerstone of a successfully completed 3D program.
Wow! You finally have the field acquisition complete! What do you mean the processor is running behind and can't get to yours right now?! They promised! Yes, but your 3D data was not there at the promised time. Well, you ran into unexpected circumstances, surely they can understand that! Yes, they do understand, but a huge international project came in that has a short fuse due to the timing of the bid round and your 3D will have to wait. Surely you, as a business person, can understand that, right? Now your partners are really hot! Oh, by the way, you are getting calls from some of the landowners who are not happy about what you have done to their land and someone's "prize bull" broke his leg in one of your shotholes. And some of the permits were not paid, so you are also getting calls from other landowners. And you thought that just because the crew was out of the field, you were done with the 3D program. Remember that Murphy's Law applies more so to seismic acquisition than to almost any other component of the oilpatch.
No matter how detailed the planning appears in the 3D arena, a situation will in all probability occur that will affect the procedure utilized in acquiring the data, though not necessarily the data quality itself. Properly planned and properly executed 3D seismic acquisition programs are every exploration manager's dream, a geoscientist's reward, and a company's road to building success.
Biographical Sketch:
Patrick Buckley began his geophysical career over 20 years ago as the youngest
crew manager for Teledyne Exploration, supervising geophysical acquisition for
Amoco Production Company. He assisted their research department in developing
several new geophysical techniques, as well as acquiring their first onshore
3D data set. He joined Seismic Exchange, Inc. in 1983 where his responsibilities
included data acquisition in the Mid-Continent and West Texas regions. During
his tenure, these regions were responsible with the gathering of over 2500
miles of geophysical data. He became the manager of geophysical speculative
programs with Richardson Seismic Services/ Petroleum Information in the spring
of 1991, where his responsibilities included the orientation and acquisition of
geophysical projects. In 1993, he left to form Global Geophysical Experts,
Inc., whose expertise consists of assisting companies in executing all aspects
of geophysical data acquisition of both 2D and 3D seismic programs.
Mr. Buckley has worked in both the domestic as well as the international
arenas gathering high quality 2D and 3D seismic data. He is a member of
SEG, OCGS, DGGS, and was the past chairman for the WTGS governmental
affairs committee.
Interwell Imaging Using Crosswell Seismic
Abstract:
The interwell region is a frontier, poorly understood and lacking
information to optimally apply enhanced recovery techniques. 3-D
surface
seismic lacks the resolution to characterize structure or monitor
property
changes. The barrier to enhanced recovery is an adequate
description of
heterogeneities between wells. The talk will describe crosswell
seismic data
which provides seismic images between wells at the resolution of
well logs
to fill the information void in the interwell frontier, covering the
application and operation of crosswell.