Jean-Paul van Gestel

Structure and tectonics of the upper Cenozoic Puerto Rico‐Virgin Islands carbonate platform as determined from seismic reflection studies

The Puerto Rico-Virgin Islands carbonate platform was deposited over an area of 18,000 km2 from early Oligocene to Holocene on top of an inactive and subsiding Cretaceous-earliest Oligocene island arc. Regional single-channel and multichannel seismic reflection lines presented in this study provide the first information on the regional stratigraphy and structure of this platform that has previously been known mainly from onshore stratigraphic sections of a relatively small (2250 km2) portion of the platform exposed by late Neogene tectonic uplift along the north coast of Puerto Rico. Seismic reflection lines are used to map the thickness of the carbonate platform strata and to correlate this thickness with onshore outcrop and well data from northern and southern Puerto Rico, St. Croix (U.S. Virgin Islands), and the Saba Bank. Limestone thickness variations from a little over 2 km to almost zero are used to subdivide the Puerto Rico-Virgin Islands platform into five distinct carbonate provinces: (1) north Puerto Rico area including the onshore exposures; (2) Virgin Islands area; (3) St. Croix and Saba Bank area; (4) south Puerto Rico area; and (5) Mona Passage area. Carbonate thickness and structural information from each area are used to test five previously proposed models for the deformation and vertical movements of the platform. The most prominent feature of the platform in the Puerto Rico-Virgin Islands area is a large, east-west trending arch. The northern limb of this arch exhibits a smoother, more uniform dip than the steeper, more abruptly faulted, southern limb. The core of the arch is responsible for the exposure of arc basement rocks on Puerto Rico. The origin of this arch, which occurs over a 300 km wide area, is best explained by north-south shortening and arching, caused by interaction at depth of subducted slabs of the North America and Caribbean plates. Other important evidence for this model can be found in the Benioff zones observed in the earthquake profiles. Loading of the Caribbean plate results in downward flexing of the North America plate and causes the 4 km subsidence of the carbonate platform north of Puerto Rico.

Vertical Fracture Detection by Exploiting the Polarization Properties of GPR Signal

Vertically oriented thin fractures are not always detected by conventional single-polarization reflection profiling ground-penetrating radar (GPR) techniques. We study the polarization properties of EM wavefields and suggest multipolarization acquisition surveying to detect the location and azimuth of vertically oriented fractures. We employ analytical solutions, 3D finitedifference time-domain modeling, and field measurements of multipolarization GPR data to investigate EM wave transmission through fractured geologic formations. For surface-based multipolarization GPR measurements across vertical fractures, we observe a phase lead when the incident electric-field component is oriented perpendicular to the plane of the fracture. This observation is consistent for nonmagnetic geologic environments and allows the determination of vertical fracture location and azimuth based on the presence of a phase difference and a phase lead relationship between varying polarization GPR data.

THREE-PHASE TECTONIC EVOLUTION OF THE NORTHERN MARGIN OF PUERTO RICO AS INFERRED FROM AN INTEGRATION OF SEISMIC REFLECTION, WELL, AND OUTCROP DATA

Abstract Integration of seismic reflection, well, and outcrop data from the Oligocene–Pliocene Puerto Rico–Virgin Islands platform (PRVI platform), north of Puerto Rico, indicates that three major tectonic phases characterize this seismically-active region within the North America–Caribbean plate boundary zone. Tectonic phase 1 : Cretaceous to Eocene formation and sedimentary infilling from a southern source area into a forearc basin, formed between down-to-the-north normal faults near the present-day coast of Puerto Rico and an outer-arc ridge near the present-day shelf break. Tectonic phase 1 concluded the last volcanic arc activity in Puerto Rico produced by the subduction of oceanic crust of the North America plate beneath the Caribbean arc system. Formation of the outer arc high defining the northern limit of this elongate basin may have been related to the accretion of off-scraped sedimentary rocks beneath the northern margin of Puerto Rico in a manner similar to that observed in active arcs. The end of tectonic phase 1 is related to initial collision between the Caribbean arc and the Bahama carbonate platform. Tectonic phase 2 : Oligocene to Pliocene formation of a ∼1600 m thick, northward-thickening PRVI platform, predominately formed by carbonate rocks. This phase started with a period of non-deposition and erosion, resulting in an Latest Eocene–Oligocene unconformity. The base of the PRVI platform is formed by a ∼400 m thick middle to late Oligocene, basal siliciclastic sequence that prograded northward across the forearc basin. Depositional thicknesses of sedimentary layers deposited during phase 2 are controlled by two large arches: the NNW-trending Guajataca arch appears to have formed as the result of tectonic activity in the Mona Passage area; the northeast-trending San Juan arch cannot be related to any adjacent structure or plate boundary feature. Onlap relations between carbonate rocks of the platform and both arches suggest that the arches were most active in the period from middle Oligocene to early Miocene. Rocks deposited during Tectonic phase 2 are surprisingly devoid of faults and folds given their central position within the North America–Caribbean strike–slip plate boundary zone that is known from regional studies to have been active throughout the deposition of the platform rocks. Tectonic phase 3 : Pliocene to Holocene northward tilting of the PRVI platform, submerged the northern edge of the platform to a depth of 4 km and elevated the southern edge of the platform to several hundred meters above sea level on Puerto Rico. Northward tilting of this area occurred on the northern limb of a large arch or anticline formed parallel to the long axis of the island of Puerto Rico and its shelf areas. The arch formed in response to a post-Pliocene convergence between the North America and Caribbean plates.

Continuous seismic surveillance of Valhall Field

Since the permanent seismic system was installed over Valhall Field five years ago, 11 surveys have been collected—each designed, acquired, and recorded with a high repeatability and efficiency. The time-lapse (4D) data have been processed using a consistent and fast workflow that generates results within weeks. The fast turnaround ensures that information is relevant and has maximum impact on short- and long-term management decisions.

Application of Alford rotation to ground-penetrating radar data

We investigate the application of Alford rotation to ground-penetrating radar (GPR) data. By recording the reflected field amplitudes using four different configurations, we extract information about the orientation of buried objects that have angle-dependent reflectivity. In theory this method can be successfully applied to find the orientation of dipping layers, cylinders, and vertical fractures. Modeling results show angle-dependent reflections in all three cases; as a result, we can exactly determine the orientation of these targets. Analysis of a field survey at a controlled GPR test site in which reflections were collected from an elongate cylinder buried in a homogeneous soil show good prediction of the angle of orientation of the cylinder and confirm the expected theoretical and modeling results. The Alford rotation method requires accurate data acquisition for effective practical implementation. Improved results will require exact knowledge of the radiation pattern of the GPR antennas under different circumstances.

Business Impact of Full Waveform Inversion at Valhall

We apply full waveform inversion (FWI) to produce a high resolution velocity volume from a dataset acquired across the Valhall permanent seismic array. We show how the new velocity model has improved static and dynamic imaging at Valhall, and illustrate how the high resolution velocity model has changed our interpretation and resulted in improved and safer drilling of new wells.

Enhanced dynamic interpretation from correlating well activity to frequently acquired 4D seismic signatures

Quantitative 4D seismic interpretation can be successfully achieved by exploiting the causal link between the temporal variation in well activity and the 4D seismic signatures they induce. This is achieved by capturing in mathematical form the common interpretational practice of identifying the origin of dynamic signals in the 4D seismic volumes or maps on the basis of their association with a particular injector or producer. Thus, for example, a region of reservoir hardening (impedance increase) around a producer may be interpreted as a signal of pressure decrease. Similarly, an area of softening (impedance decrease) around an injector is interpreted as a signal of pressure increase when pressures are above bubble point. In the literature, a hardwired integration between the seismic and engineering domain has been obtained to some extent using methods such as seismic history matching, where the observed seismic and well production history data are simultaneously fit by predictions from a common simulation model. However, this approach is computationally expensive and suffers from nonuniquenesses, inaccuracies in the petroelastic model, and is ultimately only as accurate as the model itself. As an alternate approach, we reconcile here the well production history and time-lapse seismic data in the data domain without the need for a model. This approach involves the use of many frequently repeated seismic surveys shot over the same field, and mathematically correlates changes in the mapped seismic attributes directly to the fluid volumes injected and produced from the wells. Thus, well data normally used exclusively for history matching in the reservoir engineering domain can now also be directly integrated with the time-lapse seismic data.

Valhall life‐of‐field seismic automated workflow

The Valhall Life of Field Seismic (LoFS) project has collected seven time-lapse surveys with very high repeatability. Standard workflows have been established to generate and analyze the resulting 4D images. These workflows have been automated to facilitate a fast and reproducible process that provides standardized products and necessary documentation. A shell script-based workflow was created to replace the existing workflow that was labor-intensive, slow, and error-prone. The final products, extracted attribute maps and 3D volumes, are uploaded to the relevant interpretations system, while key 4D graphic files are generated for each individual wells and linked to an indexed HTML format to allow easy screening of the results.

Migration using multiconfiguration GPR data

Due to the radiation pattern of the GPR antennas, the amplitude of the reflection is not only dependent on the distance between the target and the antennas, but also on the angle of orientation between the antennas and this target. Alford rotation is applied to this radiation pattern of the antennas, and used to extract the location of the origin of the reflected wavefield. This information is used to improve the migration scheme. In conventional migration schemes, recorded data are migrated to all angles. In this method, instead the recorded data are migrated to certain grid points only. This makes the migration algorithm faster and more accurate. This Alford migration method is applied to synthetic data and is shown to be successful below a noise level of about 1% of the amplitude level of the data. A higher level of noise results in an inaccurate estimate of the angle to the location of the target and the signal to noise level to decrease. The result can be improved by migrating over a limited sweep of angles around the estimated angle. Application of this method to field data collected at a controlled test site has shown poor results, as there was too much noise present in the data and errors might occur due to dispositioning of the antennas.

Valhall life‐of‐field seismic: Quantitative analysis of 4D attributes on decimated data sets

Several decimation tests were run on the Valhall Life of Field Seismic (LoFS) survey 1 (November 2003) and survey 2 (March 2004) to determine which decimations may be acceptable while still maintaining a good 4D signal quality. Three different analysis methods were studied: NRMS analysis, 4D resolution and cross-correlation. An automated process to measure 4D resolution was developed using statistical comparisons of 4D attributes. This 4D resolution method combines both 4D signal and background noise and provides a better measure of 4D signal quality than the NRMS method that only looks at background noise.