Location:
Westchase Hilton, 9999 Westheimer.
Social Hour and Posters or Demonstrations 5:30 PM, Dinner and Talk 6:30 PM
Abstract:
Analysis of pressure data from forty-one deepwater wells in the northern Gulf of Mexico
has revealed consistent patterns in the rates of increase of both pore pressure and
fracture gradient with increasing depth. Several conclusions may be drawn from these
patterns, including: Pore pressure (PP) and fracture pressure (FP) trends are not parallel
with increasing depth. PP and FP converge at the mudline. The small differences
between pore pressure and fracture pressure in the shallow section of a well directly
influence the openhole drilling distance allowable between casing sets. In addition,
these close tolerances can exacerbate problems in controlling shallow water or gas
flows that may be encountered.
Pore pressure is elevated above a "normal" hydrostatic trend at shallow sediment burial depths. Indications are that top seals form with as little as 1500'-2000' of sediment burial. Such early seal formation is favorable to the formation of stratigraphic traps. It also sets a maximum depth for conventional riserless drilling.
Pore pressure and fracture pressure trends diverge with increasing depth in sedimentary sections with high sedimentation rates. This divergence, DPP < DFP, directly limits the column height. As the difference increases, the maximum column height possible also increases.
Conversely, pore pressure and fracture pressure converge, DPP > DFP, in deeper sections associated with lower sedimentation rates and unconformities. As a result, maximum possible column heights decrease in these intervals and may actually preclude sealing any significant volumes of hydrocarbons.
These trends of increasing/decreasing sealing capacity have application to models of generation, expulsion, primary and secondary migration and accumulation of hydrocarbons in this area. Areas of reduced sealing capacity in the deeper sedimentary section will "frac," allowing vertical migration of fluids to zones with higher sealing capacities.
Introduction Pore pressure trends were analyzed in forty-one wells to support deepwater drilling activity, particularly in the Viosca Knoll and Mississippi Canyon protraction areas. The goal was to predict pressure cells as an aid to well design, prior to drilling, because of the high costs of deepwater operations. Pressure trends were analyzed to define regional patterns of pressure increase with depth. As this effort continues, a number of characteristic patterns in the relative rates of increases of pore pressure and fracture pressure were observed in all the wells analyzed. Implications drawn from these patterns extend beyond the original drilling-related focus of the project to include influence on column heights, sealing capacity and hydrocarbon migration issues.
Methods Pressure data were compiled for the existing wells in the area prior to Amoco initiating its drilling program in 1992. As additional wells were drilled they were included in the database. The data collected included direct pressure measurements from drill stem test (DST) and repeat-formation (RFT) or modular-dynamic (MDT) testing tools with pressure equivalents from drilling mud weights and associated leak off tests (LOT). In addition, pressure estimates from empirical relations of travel time and resistivity were applied to acoustic and resistivity logs. The acoustic algorithm was also applied to seismically derived velocity profiles from migration before stack (MBS) data for pre-drill pressure prediction.
All the data were analyzed using PRES-GRAF, a proprietary PC-based program that allows analysis and presentation of pressure data of various types and from multiple wells (Traugott, 1997). The methodology employed was first to create a calibrated pressure profile for known wells and then to extrapolate the profile to new drilling locations, usually using MBS seismic data. A plot from a typical well is pressure (psi) vs. depth and mud weight vs. depth.
First an overburden trend (pressure vs. depth) was created for an existing well. There are two components to overburden in deepwater. First, the water column pressure (water depth x 0.455 psi/ft). The average lithostatic pressure component was established using the density log from the well. Total overburden at any depth below mudline is the sum of the water column and lithostatic overburden components. Estimates of the overburden trend can be compared to measured LOT data which imposes a boundary condition on the estimate.
Second, after establishing an overburden trend, measured pore pressure data from DST or the RFT/MDT log was input. Mud weight and LOT data was also input at this time. These measured values set boundary conditions on subsequent estimates of pore pressure created from wireline log data.
Sonic log data were incorporated next and was processed iteratively with a pressure estimating algorithm within PRESGRAF. The general relation of the algorithm is: pore pressure (PP) is proportional to travel time (DT), porosity at the surface/ mudline (Po), volume clay (Vcl) and a compaction constant (C).
Some of these values may be estimated from log or geotechnical core data (Vcl, Po). The others are varied iteratively to produce a result that conforms to the pre-existing boundary conditions imposed by mud weight and measured pressure data.
Finally, an independent estimate is made using the resistivity data. Though it uses a different algorithm than the sonic estimate, a number of variables are common to both; Po, Vcl and C. The new variables in the resistivity estimate are resistivity (RT) and the cation-exchange-capacity (CEC). A temperature profile for the well is also necessary due to the variations in RT with temperature. The resistivity estimate is computed and compared to the sonic value. The two algorithms are solved iteratively until a close match is achieved using common values for Po, Vcl and C. Once a calibrated model was created for a known well, that model (with adjustments for variations in water depth) was used for pre-drill estimates of pressure for new drilling locations. The sonic algorithm was especially useful for pre-well locations that had MBS seismic. A velocity profile extracted from the MBS velocity volume can be processed in a similar manner to the sonic log. This gives a direct pre-drill estimate of pressure at the well location. Seismic velocity uncertainty will propagate through the model as a resultant uncertainty in the absolute estimated pressure, however the rates of change in estimated pressure and any associated inflection points in the pressure profile, have significance in establishing depths to major pressure cell boundaries.
During drilling operations, the calibrated resistivity model values may be applied to measured-while-drilling (MWD) resistivities to evaluate pressure trends in the well in real time.
Conclusions:
Several general conclusions can be drawn from the data. Most
are easily extrapolated to other areas in the Gulf of Mexico offshore and to other clastic,
passive margin basins. Others are currently specific to the geology of the local area,
and cannot yet be extrapolated to other areas.
Fracture pressure and pore pressure trends converge near the mudline. This convergence sets a physical limit on the amount of open hole that can be maintained before setting additional casing strings becomes necessary. In this shallow section below the mudline, more time and expense are expended setting and cementing casing than in drilling.
These narrow tolerances, typically a few tenths of a pound-per-gallon (PPG) equivalent, between PP and FP can make control of shallow pressure flows difficult. While increasing mud weight to control flows a slight overbalance can break down formation causing loss of drilling fluid. After this loss, the flowing formation comes back into the well. This cycle of flow/kill/breakdown/flow can result in substantial well cost overruns.
Seals form earlier and at shallower depths below mudline in the deep water environment compared to shelf sediments. Water column is a contributing factor, with the water column providing an overburden stress approximately equivalent to a column of rock half this thickness. The water column effect is most noticeable in water depths exceeding ~2000'. Pore pressures are elevated above hydrostatic pressure with as little as 1500'-2000' of sedimentary over burden deposited. This early top-seal formation sets up a favorable system to trap early migrating hydrocarbons. The study area has a relatively large number of fields/discoveries with significant stratigraphic components.
Pore pressure and fracture pressure increase at different rates with increasing burial depths. These differential rates of pressure increase result in variations of potential column heights with increasing depth. In the younger, expanded Miocene sections, characterized by high sedimentation rates, the rate of pore pressure increase is lower than the increase in fracture pressure (DPP< DFP). In the deeper, older section there are transitions into higher pressure cells where the rate of change in pore pressure is higher than the Houston Geological Society Bulletin April 1998 11 fracture trend gradient (DPP > DFP).
In the expanded Miocene section of the study area, pore pressure increases at ~0.8 psi/ft, whereas fracture pressures increase uniformly at ~1.0 psi/ft. This separation results in increased seal potential and therefore greater possible maximum column heights with increasing depth. Three fields in the area are filled to spill, having hydrocarbon columns of 2100' (Neptune), 1900' (Marlin) and 1400' (King). An additional benefit, this pressure gradient differential increases the depth interval that can be drilled after each succeeding casing point resulting in reduced total drilling time.
A few wells drill completely through the expanded Miocene to the Lower Tertiary and Upper Cretaceous sections. This older stratigraphic section has significantly lower rates of sedimentation, as well as significant unconformities, and is associated with a notable transition to higher pressures. The pore pressure gradient increases abruptly (1.8 psi/ft) in the transition zone. The separation between pore pressure and fracture trends is substantially reduced, resulting in diminished seal capacity and an accompanying reduction in maximum possible column height. An additional drilling consideration is that the transition may be quite abrupt with pressure differentials of 2000-3000 psi occurring in as little as 120' of vertical section.
In this local area the succession from the mildly pressured Miocene reservoirs, with high seal capacity, to the lower Tertiary and Cretaceous source rocks, with significantly decreased sealing capacity, provides a probable mechanism for expulsion and vertical migration of hydrocarbons. On deep high relief structures in the deep source section, any significant accumulation of hydrocarbons will result in buoyancy pressures that exceed the fracture pressure sealing capacity. At that point, hydrocarbons can fracture the top seal and move vertically along salt/sediment interfaces or faults that extend up to the Miocene. Once above the pressure transition, hydrocarbons then charge lower pressured sands that have higher sealing capacity.
Biographical Sketch: Bruce Wagner earned B.S. and M.S. degrees in geology from Florida State University. He has worked at Amoco since 1982 in exploration and exploitation assignments on the shelf and in deep water. Since 1991 he has been engaged in exploration and drilling in the GOM deep water, primarily in Mississippi Canyon and Viosca Knoll. In addition to prospecting, Wagner's geoscience interests include petroleum systems, geochemistry and pressure issues.
References:
Authors:
by Raúl A. Ysaccis and A.W. Bally, Dept. of Geology and Geophysics, Rice
University, Houston
Abstract:
The Tertiary evolution of the northeastern Venezuela offshore is dominated by
Paleogene (Middle Eocene-Oligocene) extension and Neogene transtension,
interrupted by Oligocene to Middle Miocene inversions. The pre-Tertiary
basement of the northeastern offshore of Venezuela consists of a deeply
subducted accretionary complex of a Cretaceous island arc system that formed
far to the west of its present location. The internal structure of this
basement consists of metamorphic nappes that involve passive margin sequences,
and as ophiolites.
The Paleogene extension is mainly an arc-normal extension associated with a retreating subduction boundary. It is limited to La Tortuga and La Blanquilla basins. All of these basins are north of and not directly related to El Pilar fault system. On a recon-struction, these Paleogene extensional systems were located to the north of the present Maracaibo basin.
By early Miocene, the leading edge of the now overall transpressional system had migrated to a position north of the Ensenada de Barcelona. This relative to South America's eastward migration is responsible for the Margarita strike-slip fault and the major inversions that began during the Oligocene and lasted into the Middle Miocene The Boconó-El Pilar- Casanay - Warm Springs fault and the La Tortuga-Coche-North Coast fault systems are exclusively Neogene with major transtension occurring during the Late Miocene to Recent. They act independently from the earlier Paleogene extensional system and are responsible for the large Neogene transtensional basins of the area, the Cariaco trough, the northern Tuy Cariaco and Paria sub-basins, and the Gulf of Paria basin.
This latest phase is characterized by strain partitioning into strike slip faults, a trans-tensional northern domain and a transpressional southern domain, which is responsible for the d$#233;collement tectonics and/or major inversions of the Serranía del Interior and its associated Monagas foreland structures. Part of the latest phase (Middle Miocene-Recent) is the formation of a large arch that corresponds to the Margarita-Testigos-Grenada zone, which was subjected to mild lithospheric compression during the Pliocene.
Biographical Sketches:
Raúl A. Ysaccis is a native of Carupano, Venezuela. In 1989, he received
his degree in geological engineering magna cum laude from the School of
Geology and Mines of the Universidad de Oriente in Puerto La Cruz, Venezuela.
He worked for four years as a geologist for Lagoven S.A. before starting his
postgraduate work at the Department of Geology and Geophysics of Rice
University in 1993. He obtained his Ph.D. from Rice in the fall of 1997. He
has returned to Venezuela to continue working with Lagoven S.A. Raúl is
interested in all aspects of petroleum geology and his main ambition is to
contribute to the discovery and development of the hydrocarbon resources of
his native country.
Bert Bally received his Ph.D from the University of Zurich in Switzerland. Between 1954 and 1966 he worked for Shell Canada. Between 1966 and 1968 he was Manager of Geological Research for Shell Development in Houston. From 1968 to 1981 he was first chief geologist and later senior exploration consultant for the Shell Oil Co. Between 1981 and 1996 he was at the Department of Geology and Geophysics at Rice University where he held the Harry Carothers Wiess Chair. He now is retired but still active working with students and as a consultant to several oil companies.
Poster Session:
"Exploration Potential Of Bangladesh" By Abu N. Chowdhury,
Geco-Prakla-Schlumberger
Location:
Hyatt Regency Hotel, Downtown Houston
Speaker:
David Wright, Amoco Production Company, Houston.
Abstract:
In October 1996, Amoco Production Company completed the Parlange #5 well in
Judge Digby Field, Louisiana, with an initial flow rate of 18 million cubic
feet of gas per day (MMCF/D) and 460 barrels of condensate. This well marked
a new phase in Amoco's continuing successful redevelopment of deep Tuscaloosa
gas fields using modern 3-D seismic technology. Since implementing 3-D
technology, Amoco has drilled 18 consecutive discoveries in areas once thought
to be near the end of their productive lives. 3-D seismic technology has added
value to Tuscaloosa gas fields by defining untested fault blocks in known
productive intervals and revealing previously unseen deeper gas-sand packages.
This technology has also helped Amoco avoid the costly and potentially
catastrophic drilling of depleted reservoirs, where current pressures are in
the 4000 pounds per square inch (psi) range. Additionally, 3-D seismic has
provided a tool for prospect risk assessment, leading to better decisions for
very expensive, deep wells, where dry hole costs average about $7,000,000.
In Judge Digby Field, 3-D seismic has been directly responsible for gross gas reserve additions of over 200 billion cubic feet (BCF) .The Amoco Parlange #5 is currently the deepest commercial gas well in the state of Louisiana with the deepest perforations at 21,500 ft. Amoco's Deep Tuscaloosa redevelopment program continues today in Judge Digby, False River, and Profit Island Fields.
Biographical Sketch
David Wright has a B.S. degree in geology from the University of Texas at
Arlington. His career began at Amoco (then Pan American) in 1970 in New
Orleans as an operations/exploration geologist. Projects included work in
the Oligocene and Miocene trends of coastal Louisiana and the Gulf of Mexico
Pleistocene trend. He has also worked as the unitization geologist for Amoco's
New Orleans region. David is currently a senior geological associate in Amoco's
Houston office where he is working the Tuscaloosa trend.
Location:
Westchase Hilton, 9999 Westheimer.
Social Hour and Posters or Demonstrations 5:30 PM, Dinner and Talk 6:30 PM
Speaker:
Robert K. Goldhammer, SONAT Exploration Co., Houston, Texas
Abstract:
Recent advances in high-resolution sequence stratigraphy of carbonate ramp
systems have direct application to enhancing our understanding of Late Jurassic
stratigraphy of the East Texas salt basin. Currently, the East Texas salt basin
is enjoying a revival via the recent Cotton Valley lime pinnacle reef play.
This play element complements the existing traditional Cotton Valley
lime/Haynesville oolite shoal play type. Consideration of Gulf of Mexico
regional Mesozoic sequence stratigraphy and paleogeography aids in linking
the two plays together in an integrated chronostratigrahic framework, thus
providing some predictive capability for reservoir distribution and quality.
Although the pinnacle play is currently perceived as a 3-D seismic play, regional sequence stratigraphic analysis assists greatly in locating favorable play trends and high-grading existing opportunities. In this study, I present a high-resolution sequence stratigraphic analysis from the western shelf of the East Texas salt basin derived from the integration of 2-D and 3-D seismic, with well log and facies information obtained from cuttings.
The Middle Jurassic-Early Cretaceous stratigraphy in the East Texas salt basin consists of four major second-order super-sequences of approximately 15 m.y. duration. These are defined as large, regionally correlative, retrogradational to aggradational / progradational accommodation packages. Each exhibits systematic vertical stacking patterns and associated lateral facies shifts within subordinate third-order sequences lasting between 1-3 m.y., with related facies and systems tracts. The four supersequences are: Supersequence 1(SS1)-Late Bathonian to Early Kimmeridgian (158.5-144 m.y.); SS2-Early Kimmeridgian to Berriasian (144-128.5 m.y.); SS3-Late Valanginian to Early Aptian (128.5-112 m.y.); SS4-Early Aptian to Late Albian (112-98 m.y.).
The Late Jurassic Smackover-Buckner-Cotton Valley lime-Haynesville-Bossier formational stratigraphy make up parts of two second-order supersequences, SS1 and SS2. The Smackover represents the second-order, late transgressive systems tract (TST) and highstand systems tract (HST) of SS1; the Buckner evaporite/red bed facies depicts latest HST condition of SS1 and lowstand systems tract (LST) development of SS2. The Haynesville/Cotton Valley lime paired ramp-shoal carbonate and offshore detached pinnacle reef facies marks the second-order TST of SS2, and the overlying Bossier equates to the second-order interval of maximum flooding.
Within the above framework, the second-order HST of SS1 (Smackover-Buckner carbonate-evaporite facies) consist of four to five regionally correlative third-order sequences, 250-350 ft thick and 1 m.y. duration, which systematically stack in a progradational fashion such that successive ramp margins are progressively offset downdip. In detail, each successive sequence is thinner than the underlying one and each is progressively enriched in blocky highstand carbonates and proximal evaporite-red bed facies. A typical sequence contains an updip anhydrite facies and a ramp margin, high-energy grainstone belt composed of a series of higher-frequency, offlapping, clinoforming shoal packages beneath each third-order sequence boundary. There is little, if any, pinnacle reef development linked to these sequences.
The 144 m.y. super-sequence boundary marks a zone of minimum second-order accommodation (a point of stratigraphic turnaround) and serves as a regional stratigraphic datum useful for hanging well log cross-sections. This surface is recognized in well logs by analyzing the vertical stacking patterns of third-order sequences, as recorded by overall thickness trends, and the ratio of blocky highstand carbonates(low gamma ray response) to spikey, transgressive carbonates (high gamma ray response). By tieing the wells to the 3-D seismic with velocity surveys, the true geometry of the 144 m.y. terminal progradational ramp sequence is defined. Downdip from the terminal ramp margin of the underlying second-order HS T, 2-3 basinally restricted reef cycles are recognized within older, larger downdip pinnacle reefs which were in a mid-slope position. These basinally restricted reef cycles record the initial floodback following the 144 m.y. relative sea-level drop and they have no equivalent ramp carbonate on the shelf, which may have been subaerially exposed.
Updip from the terminal ramp margin, above the 144 m.y. horizon, the second-order TST of SS2 (Haynesville/Cotton Valley lime carbonate shoal-pinnacle reef facies) consists of 4-5 regionally correlative third-order ramp sequences and 4-5 pinnacle reef cycles, each 50-150 ft thick, lasting 1 m.y. Pinnacle reef cycles are detached in plan view from the ramp cycles , yet linked in accommodation space and time. Ramp sequences systematically stack in a retrogradational or aggradational fashion, whereas individual pinnacle reefs progressively decrease in diameter as they aggraded vertically. Each ramp sequence consisted of an updip, proximal evaporite red bed facies, a ramp-margin oolite shoal belt (traditional Haynesville reservoirs), and an outer ramp slope composed of muddy, argillaceous carbonate. During the second-order regional transgression (TST of SS2) older pinnacle reefs, over 1300 ft thick, grew in progressively deeper water and were eventually stranded downdip, passing updip to younger pinnacles, typically less than 300-500 ft thick, which grew in successively more landward positions. Younger pinnacles are missing the earlier reef cycles , are not as tall, and are enriched in shallower-water facies as compared to their older, downdip counter-parts.
Through high-resolution correlation of ramp sequences with reef cycles, guided by integrated seismic and well log control, updip oolite shoal regional porosity can be correlated directly with time-equivalent pinnacle reef reservoirs, casting light on porosity distribution as well as mechanisms for porosity development within the East Texas salt basin. The top of the Cotton Valley lime/Haynesville carbonate is a diachronous surface characterized by appreciable depositional topography, onlapped by the Bossier shale along a well-documented submarine condensed section. Little evidence exists for a relative sea-level drop at this surface.
A high-resolution sequence stratigraphic model which summarizes the Smackover-Buckner-Cotton Valley Lime/Haynesville (Lou-Ark) stratigraphy is presented in Figures 1 and 2 Figure 2 depicts the accommodation history over the temporal interval of concern. In this model, composite accommodation changes are produced by superimposing high-frequency 4th-3rd-order relative sea-level changes and lower frequency 2nd-order relative sea-level changes on background, regional tectonic, subsidence. The horizontal axis (Figure 2) represents time moving for-ward from left to right. The vertical axis depicts changes in sea level. The timing of second-order systems tracts are shown at the top of the diagram. Each high-frequency eustatic cycle (eustatic beat) is numbered from 0 to 12. As each beat floods the ramp top, sedimentation takes place beneath the high -frequency sea-level curve in Figure 2; "PWD" re fers to paleo-water depth and delta X shows changes in PWD). During high-frequency submergence, t he top of the sediment surface climbs from lower left to upper right in the diagram. When high-frequency sea level falls beneath the ramp top (times depicted by darker vertical shading), marine sedimentation ceases.
Due to the effects of composite eustasy, the proportion of marine submergence and concomitant sedimentation to exposure and non-deposition per high-frequency beat varies systematically as the beats migrate through the lower-frequency 2nd-order eustatic cycles. These systematic and sequential changes in accommodation space during eustatic beats result in a predictable stacking architecture of high-frequency stratigraphic cycles. Eustatic beats 0-4 are within the 2nd-order highstand systems tract, and each eustatic beat is capable of generating one stratigraphic cycle.
During the 2nd-order HST, accommodation is progressively declining and submergence-prone eustatic beats pass into exposure-prone eustatic beats. Thus, ramp cycles 1-4 thin upward and prograde laterally into the basin. Each ramp cycle has an updip evaporite facies (Buckner), a mid-ramp quiet-water facies, a ramp crest grainstone oolitic-facies and a ramp slope facies. Small patches of biohermal or reefal facies are depicted by dark grey shading and these biohermal entities are located at the seaward margin of the ramp crest or slightly down the ramp slope. Biohermal masses within cycles 1-3 are spatially restricted and inhibited from becoming pinnacle buildups due to two factors: (1) the declining accommodation with-in 2nd-order HST, each biohermal entity is smothered in carbonate sand from above as the next cycle progrades out and over the bioherm; (2) related to the same accommodation problem, "nasty" bank water of elevated salinities from the Buckner facies washes seaward over the bioherms adversely affecting their growth.
The 2nd-order HST passes into the 2nd-order LST between eustatic beats 4 and 5 where the rate of 2nd-order fall is at a maximum (the inflection point on the 2nd-order eustatic curve). This point marks the 2nd-order super-sequence boundary and equates hypothetically to the 144 m.y. supersequence boundary in the Lou-Ark framework presented previously. In this position of stratigraphic reversal, the system turns around from progradation related to progressive accommodation loss, to retrogradation caused by progressive accommodation gain.
From here on, each high-frequency beat becomes progressively submergence prone and the ramp cycles display a retrogradational stacking architecture with increasing topographic relief as they march updip. Pinnacle buildup develop-ment is now promoted as problems (1) and (2) outlined previously are alleviated. For example, between ramp cycles 4 and 5, biohermal growth which initiated during cycle 4 can continue because the ramp crest of cycle 5 (or rollover point) is now located slightly updip, or landward, of the ramp crest of cycle 4. Because of this relationship, it is hypothesized that the biohermal contribution from cycle 5 will stack vertically on the ready-made foundation of the healthy bioherm from cycle 4.
The 2nd-order TST occurs between eustatic beats 6-12 as the rate of 2nd-order fall declines, and passes through its trough and back into a 2nd-order rise. The composite eustatic effect each of high-frequency beat becomes progressively submergence-prone and overall accommodation increases, promoting pinnacle development. In detail, above cycle 5, each reef cycle is broken into its high-frequency transgressive and regressive phases. The net result is that each pinnacle buildup is cyclic with contributions from 2 to 4 eustatic beats. The furthest downdip pinnacle reef consists of contributions from cycle 4 through the transgressive part of cycle 7. By contrast, the most updip pinnacle only contains contributions from cycle 8 and the transgressive phase of cycle 9. The most downdip pinnacles are therefore the oldest and were drowned during the overall regional 2nd-order transgression prior to the inception of the most updip pinnacle. A lack of appreciation of the true chronostratigraphic and dynamic relations summarized here has lead to the misperception by some workers that the downdip pinnacles are deep water and the updip pinnacles shallow water. With respect to internal facies composition and petrophysical parameters, each pinnacle is vertically heterogeneous.
Inspection of thin sections from cuttings and rotary sidewall cores, coupled with core descriptions from various operators, indicates that the transgressive phase of each pinnacle reef consists of slightly argillaceous lime wackestones (increased gamma ray count) composed of thrombolitic facies or microbiolite facies marked by an abundance of algal binding and clotting. These facies, with associated delicate deeper-water skeletal allochems, suggest moderate water depths related to high-frequency rise in sea level. The maximum flooding surface of each reef cycle is approximated by the highest gamma ray count. The regressive cap or highstand systems tract of the reef cycles is composed of in situ, apparently low-energy Late Jurassic reef-builders, such as sponges and delicate corals. The caps to some of the reef cycles consist of high-energy grainstones with oncolites and abraded, well-washed, skeletal-peloidal sand, indicative of shoaling to very shallow water depths. On well logs, the gamma ray within the highstand portion of a reef cycle cleans upward, becoming blocky to remarkably flat. A lack of core data has hampered a complete under-standing of facies and diagenesis.
Carbonate systems in similar accommodation settings, such as the younger Sligo formation in south Texas, provide stratigraphic analogues useful for driving well log correlations and seismic interpretation . Analogous buildup or pinnacle reef facies typically occur linked to the terminal phase of carbonate deposition near the top of regiona, second-order TST's beneath deep marine shales (second-order MFS) which serve as source and seal facies. Pinnacle geometries are promoted by increasing accommodation within an overall retrogradational stacking of carbonate facies belts. Differential compaction of shaly, onlapping facies around pre-existing rigid carbonate buildups enhances their seismic recognition. Hydrocarbon-pro-ducing examples include the Devonian of Canada, the Miocene of Southeast Asia,the Mississippian Lodgepole of the Williston basin, and the Upper Pennsylvanian Horseshoe Atoll of the Midland basin, among others. Integration of key principles from the Late Jurassic of the East Texas salt basin with these and other examples should fuel the search for other, as yet, unrecognized carbonate buildups and pinnacle reefs within similar accom-modation windows in other areas.
References:
Goldhammer, R. K., Lehmann, P. J., Todd, R. G., Wilson, J. L., and Ward,
W. C., 1991, "Sequence stratigraphy and cyclostratigraphy of the Mesozoic of
the Sierra Madre Oriental, a field guidebook", Gulf Coast Section, Soc.
Econ. Paleont and Mineral. Found., 86 p.
Poster Session for North American Explorationists Meeting:
"Sequence Stratigraphic Framework And Exploration Potential of Lower Permian
(Wolfcampian) Gravity-Flow Deposits, Eastern Midland Basin, Texas." by
Morgan, William A., Conoco Inc., Houston; George F. Kokkoros, Conoco Inc.,
Houston; Bruce H. Wiley, Conoco Inc., Midland; and William W. Clopine,
Conoco Inc., Aberdeen, Scotland. Eight major Wolfcampian sequences (W1-W8)
have been identified on seismic data and well logs in a 1500 square-mile study
area along the eastern shelf of the Midland basin. These sequences are composed
of two lower frequency sequences (W1-W5 & W6-W8) that are separated by a major
erosional surface associated with a sea level fall near the time of the
early-middle Wolfcampian boundary. Sea level falls associated with sequences
W5 and W6 resulted in significant erosion of platform-margin deposits, incision
of a submarine canyon several miles into the eastern shelf, and deposition of
carbonate gravity flows in the Midland basin. Gravity flows entered the basin
via the submarine canyon and fed basin-floor fan systems which extend up to 25
miles basinward of the toe-of-slope. Within the study area, production from
carbonate gravity-flow deposits is mostly from three submarine fan systems within
sequences W5 and W6 - the Credo (10 MMBOER) fan (oldest), the Triple M
(5 MMBOER)-Howard Glasscock (7 MMBOER) fan, and the Hutto (5 MMBOER) fan
(youngest). The maximum gross thickness of the fans ranges from 100 to 200
feet. Regional isolith and seismic amplitude data were used to high-grade a
70 square mile area in the vicinity of Triple M and Howard Glasscock fields.
Model-based velocity inversions, constrained by sonic and density data from
52 wells, were performed utilizing a grid of 2-D seismic data and a 3-D survey.
Both Triple M and Howard Glasscock fields are clearly defined by high-velocity
anomalies within the productive basin-floor fan interval of sequence W6. An
untested high-velocity anomaly with a velocity signature and stratal geometry
similar to the two fields, and pre-drill esti-mated reserves of 5 MMBOER, was
drilled in 1995. The well encountered 7 ft. of carbonate gravity-flow deposits
in the Credo zone and 52 ft. of carbonate gravity-flow deposits in the Triple
M zone (gamma ray < 40 API). Twenty-five feet of gravity-flow deposits had
been predicted for the Triple M zone. However, the deposits were nonporous
and the well was plugged and abandoned. Although the location of carbonate
gravity-flow deposits within the study area can be identified using sequence
stratigraphic and seismic imaging techniques, prediction of porosity within
the flows remains problematic.
