Petroleum Exploration and Production in the Gulf of Mexico I
Presiding: A Cheng, SensorWise, Inc.; R Vines, Shell International Exploration and Production
U41A-01 INVITED 08:30h
Facing Today's Exploration Challenges in the Gulf of Mexico
The Gulf of Mexico represents one of the most intensively explored basins in the world, and yet it still delivers significant new material oil and gas discoveries every year. Because of it high productivity, geologic complexity, competitive acreage access and large profitability margins, the Gulf of Mexico presents many industry-leading challenges to Exploration today. For major companies exploring for oil and gas in the Gulf of Mexico today, their challenge is to "safely, responsibly and profitably find and produce reduced accumulations in increasingly hostile settings." The GoM serves as the sportsman's playground for a significant population located in and around its waters, and therefore, operating safely and responsibly are unassailable moral and operational standards by which we sustain future development, and maintain our license to operate. With that as a backdrop, today's challenges are driven by the nature of where the business is looking for new reserves in this extremely mature basin. These "Opportunities" encompass the following: 1) large, under-explored, sub-salt areas, characterized by poor seismic imaging, uncertain geologic regimes and potentially dangerous overpressures, 2) deep true-vertical-depth opportunities in older rocks that challenge our understanding of reservoir quality prediction and hydrocarbon systems, and are at the edge of today's drilling technologies, 3) access to sensitive areas including the eastern GoM-Florida shelf, coastal areas and international borders, 4) challenging "small accumulation" discoveries that cannot support expensive appraisal or development options, are remote to infrastructure or inefficiently produce the reservoir, and finally 5) new play development, which is challenged by long maturation cycles, small acreage blocks, intense international competition, and rapid lease rolls. This talk will consider what Shell and the Oil & Gas Industry does today to succeed in this arena, and specifically will show examples of the role of technology in meeting these challenges. I will briefly discuss likely future challenges and emerging and upcoming technologies that might help to meet these challenges.
U41A-02 08:45h
The Tectonic History of the Gulf of Mexico
Seismic refraction and gravity data have been used to map three major, deep basement structures in the Gulf of Mexico. Two of these structures have dimensions that are consistent with chains of seamounts, or hotspot tracks, produced by mantle plumes. A Late Jurassic mantle plume may have generated these tracks on the North American plate and Yucatan block as the Gulf of Mexico opened. High-amplitude, distinctive gravity anomalies over these two structures provide the basis for a kinematic reconstruction that restores the western ends of the hotspot tracks with a 20 degree clockwise rotation of the Yucatan block. This rotation, which we estimate lasted 8 to 10 My, represents about one-half of the totals required to open the Gulf of Mexico basin. The third deep basement structure, which is located along the western boundary of the basin, is also associated with a high-amplitude, distinctive gravity anomaly. This anomaly is interpreted to be produced by a marginal ridge, which was created along the ocean-continent transform boundary as the basin opened. The tectonic evolution of the Gulf began with extension of continental crust that lasted from about 160 Ma until 150 Ma and involved approximately 22 degrees counterclockwise rotation of Yucatan. Then, as seafloor spreading began, a mantle plume became active producing hotspot tracks on the North American Plate and Yucatan Block. This phase lasted until 140 Ma during which the Yucatan rotated another 20 degrees counterclockwise. Autochthonous salt appears to be confined to the continental flanks of the hotspot tracks indicating that salt was deposited during continental extension and not after ocean floor had begun to form. The eastern flank of the marginal ridge, and the northernmost, easternmost, and southernmost terminations of the hotspot tracks, are interpreted to coincide with the oceanic-continental crustal boundary in the basin.
U41A-03 INVITED 09:00h
Overpressure and Shale Properties on the Gulf of Mexico Shelf: Stress Unloading or Smectite-Illite Transformation?
Standard methods for estimating pressure from seismic velocity need to be modified on the GoM shelf because overpressure is often not due to undercompaction, but is at least partly due to smectite-illite (S-I) transformation. There are two end-member methods for estimating pressure from velocity for such shales. One assumes that the velocity-stress dependence of the shales during compaction is unaffected by the transformation, and that the transformation only increases the pressure and reduces the net stress. The reduction in net stress implies that the sediment is inside the mechanical yield envelope on an elastic unloading stress path. Therefore this method must also determine the maximum paleo-stress experienced by the sediment. The other method assumes that S-I transformation changes the velocity-stress behavior of the shale, but does not cause a reduction in net stress. Crossplots of shale density and sonic transit time show regularities that are inconsistent with the first model. The very high overpressures observed in some wells are inconsistent with the second model. However, the second model can accommodate reductions of net stress if the S-I transformation involves mechanical failure. The transformation thus might reduce net stress while still allowing the sediment to be on a yield envelope rather than on an elastic path within the yield surface.
U41A-04 09:15h
Computational Requirements For Large-Scale Seismic Modeling On PC-Based Clusters
With the development of deep reservoirs in the Gulf of Mexico, the proper modeling of seismic wave propagation in such an environment has become a huge computational challenge. A potential solution is the use of PC-based clusters. The theoretical peak speeds of these clusters are often impressive. However, the architectures of the clusters and the computers are seldom optimized for numerical computations. In particular the architecture can be very poor for the numerical solution of partial differential equations. Since the wave equation is at the core of many modeling and imaging algorithms, this may be a problem for many applications in exploration seismology. We have performed a series of tests to ascertain the attainable computational speed of PC clusters for large-scale Finite-Difference simulation of acoustic wave propagation. The results show that the most limiting factor is the internal memory bus, followed by the PCI bus. In general the actual peak speed is, by a factor of 4 or more, less than the theoretical peak speed for a single computer; and when the computation is performed in a cluster the actual peak is naturally less than for a single computer, and on the order of a factor of 10 or more less than the theoretical peak speed.
U41A-05 09:30h
Marine EM in GOM: Advances and outlook
Marine electromagnetic (EM) sounding methods provide valuable complementary information to conventional seismic exploration methods and success stories have been claimed by several oil companies: 1) as indicator of hydrocarbon presence derived from strong resistive anomalies 2) as complimentary tool in structural exploration. While 3D seismic identifies geological structures, it does not directly reveal the fluid content (hydrocarbons). Marine EM sounding exploits variations in electrical resistivity, and is directly sensitive to fluid saturation and thus resistive hydrocarbons. Under the right circumstances it can confirm the presence of hydrocarbons by identifying their resistive characteristics. This means that the possibility of drilling dry exploration wells is significantly reduced, as is the need for extensive appraisal drilling. EM data is used to resolve ambiguities in the structural interpretation of seismic data. For example, whereas the top of a diapiric salt body is often well constrained by seismic data, the position of the lower boundaries is often more elusive. Carbonate (or salt blankets, or resistive basalt) layers complicate the detection and characterization of deeper structure because of diffusive scattering in the layer. However, the resistivity contrast between these layers and the sediments below is an ideal target for EM sounding methods. Recently, two marine EM methods have become popular: The controlled source EM (CSEM) method and magnetotellurics (MT). The CSEM method uses an electric dipole source to transmit low frequency electromagnetic signals to an array of receivers that measure the electromagnetic field at the seafloor. Variation in amplitude and phase of the received signal as the source is towed through the receiver array yield the resistivity structure of the sub-surface to depths of several kilometers. The MT method uses naturally occurring electromagnetic source fields to determine the resistivity of the sub-surface. Thus, by studying the variation in response as a function of frequency, the variation in resistivity as a function of depth may be determined. These methods give complementary information about the resistivity structure of the sub-seafloor. Whereas CSEM data are primarily sensitive to resistive structures, and in particular to layers that are thin compared to their depth of burial, MT data can constrain larger scale conductive structure. By combining natural and controlled source methods better constraints on the geometry and properties of the seafloor can be gained than from either data type alone. Several case histories with large salt structures in the section illustrate that the techniques are useful for future exploration in the GOM. We see the technology moving from its present focus of deep water to include shallower water depths (where CSEM sounding is presently restricted). In addition, we envision the integration of complimentary EM techniques to get a better constrained resistivity image of the subsurface.
U41A-06 09:45h
Discrepancies Between LWD and Wireline Acoustic Logging Measurements in an Unconsolidated Formation: a Modeling Study
Acoustic Logging-While-Drilling (LWD) significantly reduces drilling rig time in deepwater development than its wireline counterpart. It can also provide high-resolution formation velocity logs as wireline logging does in most situations. LWD measurements are critical in subsurface imaging, seismic-well tie, and wellbore stability. However, in an unconsolidated formation, data show both compressional and shear wave velocities by a LWD tool are slower than those obtained by a wireline tool. The discrepancies may result from differences in geometries, source-receiver offsets, operating frequencies, and off-centering of the tools and borehole conditions in which both measurements are made. To pinpoint the main cause of the difference and build confidence on application of LWD tools, we extend the propagator matrix method for simulation of acoustic multipole wireline measurements to acoustic LWD modeling in radially layered formations. Both LWD measurements and dispersion properties can be computed analytically. Time and frequency domain semblance techniques are also used to quantify depths of investigation of LWD and wireline tools in boreholes with radially increasing velocity profiles. Extensive modeling results show that the LWD tool investigates about half of the depth of the wireline tool does in this unconsolidated formation. To ensure accurate measurements of formation velocities, particularly shear wave velocities, in unconsolidated formations, a wireline or LWD tool operating at low frequency may be necessary.