Large gas-prospective areas indicated by bright spots
Christian Muller
Results from combined vertical-incidence and wide-angle seismic, as well as ODP data, argue in favor of gas-charged reservoirs in the deepwater Southern Canary basin
Seismic evidence acquired on RV Poseidon in 1997 and 1999 indicates the presence of large, deepwater gas reservoirs in Middle Miocene-age sediments in the Southern Canary basin. Pronounced, seismic bright-spot reflections in the sedimentary column–with areal extents of more than 50 [km.sup.2]–are well imaged on multichannel reflection seismic sections (MCS) and Ocean Bottom Hydrophone (OBH) records. This article examines that evidence that these bright-spot areas are indeed gas prospective.
Normal-incidence reflection coefficients of R = -0.4, calculated from MCS shot records, indicate strong, negative acoustic-impedance contrasts. Further, mud diapirs near bright spots outline vertical gas-migration paths. However, the nature and origin of these prominent bright spots remain unresolved. Circumstantial evidence, such as occurrence of bright spots within the hydrate-stability-zone and existence of large magma volumes in the lower crust and upper mantle, points toward methane or [CO.sub.2]-rich volatiles trapped in the sedimentary sequence.
INTRODUCTION
Reconnaissance seismic surveys were conducted to image deepwater basins at the transition between continental margins off West Africa and oceanic crust. The Canary Islands add complexity to the study area. Ocean island systems such as Hawaii and the Canaries have been built by large volumes of magma intrusions and lava extrusions based on intraplate volcanism. The Canary Islands, off the northwest African continental margin (Fig. 1), have been formed by hot-spot volcanism, where subaerial volcanic activity started on Fuerteventura around 20 million years ago (Ma). [1]
Today, most investigations focus on the subaerial part of the islands and their submarine flanks. [2] Large sediment flows have been found on the flank of Tenerife by the Teide Group, indicating past catastrophic sediment failures. [3] Recent megaslides have also been studied west of El Hierro Island, [4] Seismic surveys in deepwater areas focus on landslide distribution and structure in the Northern Canary basin. [5] A series of well-stratified layers down to basement levels can be interpreted from MCS results and four ODP sites in the Northern and Southern Canary basins. [6]
The main unconformities correlate with major phases of volcanic activities in the area, e.g., Roque Nublo phase (4.3-3.4 Ma) and Mogan Group (14 to 13.3 Ma) on Gran Canarma. [7] Stratigraphic sequences are mainly formed by debris avalanches in the vicinity of the islands. However, no indications for bright-spot reflections in this area have been reported in literature.
During two cruises of RV Poseidon in 1997 and 1999, a number of pronounced seismic bright spots were imaged on a dense grid of 2-D reflection seismic lines, 1,000 to 2,600 ft (300 to 800 m) below seafloor at water depths of more than 12,500 ft (3,800 m) in the Southern Canary basin. The deepwater basin is located at the transition between oceanic crust and the continental margin, indicated by a 175-Ma magnetic anomaly (S1) east of Fuerteventura. [8]
Bright-spot evaluation using forward modeling, Amplitude Versus Offset (AVO) analysis and true-amplitude data-processing techniques require detailed background P-wave velocity information. Most seismic surveys in deep water lack wide-angle reflections for velocity analysis. The MCS maximum shot-receiver distances of 2,600 ft (800 m) at water depths of more than 12,500 ft (3,800 m) do not provide reliable subsurface velocites.
ODP Leg 157, located on the submarine flank of Gran Canaria, provided limited stratigraphic and well control of the main bright-spot areas south and southwest of El Hierro. This was helpful in interpreting major stratigraphic changes in the Southern Canary basin indicated on seismic line 28/97, Fig. 2.
Velocity, density and porosity logs from these sites are available for the upper 2,300 ft (700 m) of sediments, whereas the lowermost 1,000 ft (300 m) mainly comprises volcaniclastic deposits originating from Gran Canaria. Pressure and temperature data provide additional constraints for calculation of the stability field of volatiles. [6] For acquiring local, subsurface velocity structures in bright-spot areas, 15 OBH stations were deployed on the seafloor.
VERTICAL-INCIDENCE REFLECTION DATA
The multichannel reflection seismic data has been processed (12 fold) to preserve relative-amplitude ratios. Seismic sections exhibit a number of pronounced bright-spot reflections with high acoustic-impedance contrasts and clear lateral terminations embedded in well-stratified sediments of low reflectivity, Fig. 3. The bright-spot reflection shows a clear phase reversal compared to the seafloor reflection.
Based on the dense grid of 2-D seismic profiles, the bright-spot structure (line 17/99) south of El Hierro reveals an areal extent of about 25 [km.sup.2]. Bright-spot reflections generally tend to follow sedimentary bedding, thus distinguishing them from bottom simulating reflectors (BSRs). A second bright-spot structure southwest of El Hierro shows an areal extent of more than 50 [km.sup.2].
A prestack MCS shot record, where bubble pulses have been suppressed by applying predictive deconvolution, is presented in Fig. 4. Phase reversal and bright-spot reflection amplitude of about twice the seafloor-reflection amplitude are clearly visible. A seafloor reflection coefficient of about 0.2 has been calculated from primary and multiple reflections in OBH records, as well as velocity and density logs from ODP Site 956. From this, bright-spot reflection coefficients of about -0.4 were calculated, indicating a strong decrease in acoustic impedance and P-wave velocity.
Seismic attribute mapping, particularly amplitude response, has been applied on stacked MCS sections. For example, the average ratio between bright-spot amplitude and seafloor reflection amplitude for bright-spot structure V, south of Tenerife, clearly exceeds 2:1. Amplitude ratios displayed in Fig. 5 show little variation throughout the entire bright-spot area, whereas a slight amplitude decrease observed between traces 200 and 250 is caused by another high-reflective bright spot located directly above this structure. Hereby, the local character of these amplitude anomalies against sedimentary reflections is clearly seen.
One-dimensional forward modeling of the near-vertical bright-spot reflection response, based on Zoeppritz-equations, indicate that preferably negative impedance contrasts (trapped volatiles) produce the high-reflection amplitudes observed in the data. Large, positive impedance contrasts (e.g., sills, lava flows)–including velocity and density constraints calculated from ODP Site 956–do not produce sufficient reflection amplitudes.
Mud diapirs (seismic chimneys) give further evidence of gas-saturated sediments. [9] Three mud diapirs, interpreted as representing different developmental stages, indicate vertical gas/fluid-charged sediment mobility from bright-spot depths into shallow sediments and to the seafloor, where mud volcanoes are observed, Fig. 6.
WIDE-ANGLE SEISMIC DATA
Velocity analysis of the deepwater MCS reflections provided poor resolution. Therefore, an AVO study using OBH stations at the seafloor was designed. In 1999, a bright-spot location southwest of El Hierro Island was covered with 10 OBH stations on two lines at distances of 2 to 4 mi. OBH records with high S/N ratio allowed access to P-wave structure in the sedimentary column and upper oceanic crust, Fig 7. Strong, refracted energy indicates P-wave velocities of about 6 km/s for the acoustic basement at 1-s TWT below seafloor. Here, the phase-reversed bright-spot reflection can be traced to incidence angles of more than 40 [degrees].
Shallow sedimentary reflections in the OBH records are superimposed with high-amplitude direct arrivals comprising the primary pulse and a sequence of bubble pulses dominated by low frequencies. Application of predictive deconvolution was not successful, which was obviously due to trace-to-trace variation of the source signature and the dominating direct-arrival amplitudes.
Application of a band-pass filter of 25 to 100 Hz, followed by subtraction of an average estimate of the direct arrival, led to a significant improvement of S/N ratio for primary wide-angle reflections. Subsequently, semblance-based velocity analysis was performed. Considering P-wave velocity constraints from ODP Site 956 and Hamilton, [10] a subsurface velocity model for the target zone was established, Fig. 8.
Since OBH records are common receiver gathers, shot-to-shot variation source strength has to be corrected before any further AVO is performed. These corrections have been applied after spherical-divergence correction based on amplitude variation of the direct arrival. OBH records from shallow bright spots south of El Hierro provide the full range of incidence angles for AVO analysis. Airgun-OBH geometry implies that reflection points are spread out over about 1 km. but no significant changes in reflection characteristics at the bright-spot interface are observed from amplitude analysis.
Fig. 9 shows the AVO response corrected for spherical divergence and non-elastic attenuation at OBH 04, line 14/99. AVO modeling reveals the best fit for a constant Poisson’s ratio of 0.35 across the interface, indicating presence of liquid volatiles rather than free gas. Local pressure and temperature conditions, calculated from ODP Site 956 parameters, require [CO.sub.2] and [CH.sub.4] to be in liquid phase. Other investigations confirm that [CH.sub.4] and [CO.sub.2] in liquid phase can cause this pronounced decrease in bulk P-wave velocity. [11]
Large, negative normal-incidence reflection coefficients that increase slightly with offset lead to definition as a Class III reservoir. [12]
CONCLUSIONS
The observed phase reversal, high amplitude ratios and modeling point toward pronounced low-acoustic impedance layers caused by local fluid/gas accumulations in the deepwater sediments. Lack of a lower boundary reflection is interpreted as a P-wave gradient zone caused by a decreasing amount of dissolved gas below the bright spots, comparable to BSRs and gas hydrates at shallow and intermediate water depths. At present, origins of these bright-spot reflections in the deepwater environment between the Canary Islands ‘and West Africa remain unresolved.
In this environment, [CO.sub.2] and [CH.sub.4] as main gas components are considered. During island formation, large amounts of [CO.sub.2]-rich magma has been transported from deep crustal and mantle reservoirs to shallow magma chambers, while [CO.sub.2] escapes due to decompression. [13,14] Large amounts of methane-rich gas have been dril1ed at ODP Site 955, southeast of Gran Canaria, originating
From organic-rich sediments transported from the African continental margin, and bright spots following the sedimentary bedding point to low-permeability sequences as reservoir seal.
ACKNOWLEDGMENT
This work was financially supported by the Bundesministerium fur Bildung. Wissenschaft, Forschung und Technologic BMBF (contract number 03G050B) and the Deutsche Forschungsgemein-schaft DFG (MI 558/4-1 and MI 558/4-2).
THE AUTHORS
Christian Muller received his undergraduate degree from the University of Kiel in 1997 where he worked on application of ground-probing radar techniques. Since 1998, he has been working toward his PhD at the University of Kiel interpreting bright-spot reflections in the Southern Canary basin and applying various seismic techniques such as AVO, inversion and modeling. He is student member of SEG, AGU, EAGE, EEGS, EGS and DGG.
Dr. Friedrich Theilen is a marine geophysicist at the Department of Geosciences of the Christian-Albrechts-University in Kiel. His main research focuses on marine seismology and development of methods for acquiring physical parameters of marine sediments. Major ongoing projects are related to investigation of the seismostratigraphic situation in the Canary basin and the Tjoemes Fracture Zone north of Iceland. He is member of SEG, AGU, EEGS and DGG.
Bernd Milkereit is currently director of the geophysical section of the Department of Geosciences at the University of Kiel. He graduated in 1981 from Kiel, where he worked until 1985. He earned his PhD in 1984, joined the Geological Survey of Canada from 1985-’96, and returned to the University of Kielin 1996. His main interests are 3-D seismic data acquisition, processing and interpretation. He is member of CSEG, EAGE, AGU and SEG.
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