Shallow Clues for Deep Exploration

Shallow Clues for Deep Exploration Seismic interpreters study images of present-day sedimentary deposits to recognize similar features in the subsurface. Applying this technique to surveys offshore West Africa reveals features that could be high-quality sands containing hydrocarbons. Douglas Evans Gatwick, England The best place to look for oil is near existing discoveries especially when it comes to exploring offshore. This simple axiom is encouraging governments to offer leases on acreage blocks adjacent to known discoveries, attracting a number of oil and gas companies to explore in ever-increasing water depths. In these environments, a deepwater well can cost $25 million or more, with no guarantee of success. To improve the chance of success, seismic interpreters evaluating unexplored regions rely on high-quality three-dimensional (3D) seismic data that provide a continuous image of the subsurface. This gives a clear picture of promising structures, sedimentary sequences and other key elements of potential drilling targets. Every prospect needs the requisite source rock, reservoir, trap and seal all occurring in the proper geologic timing and successions to become a viable target. However, because of the high risk associated with wildcat exploration, many For help in preparation of this article, thanks to John English, Gatwick, England; and George Jamieson, Houston, Texas, USA. 1. www.angola.org/fastfacts/economic.html 2. Kwanza is sometimes spelled Kwanzaa or Cuanza. exploration and production (E&P) companies delay acquiring proprietary 3D surveys until they have licensed the acreage. In the absence of 3D surveys, companies turn to widely spaced twodimensional (2D) seismic lines, between which the interpreter may have to make unsupported guesses about geologic structures and stratigraphic changes. To satisfy a need for early high-quality 3D seismic data on unlicensed acreage at lower risk and lower cost to E&P companies, seismic contractors offer surveys for use by multiple clients. At their own risk, seismic contractors acquire and process 3D surveys over blocks of acreage not yet licensed for exploration, then sell access to, or license, the seismic data to companies interested in evaluating the block before entering a bid. In the case of WesternGeco, multiclient 3D marine surveys have been acquired over an estimated 500,00 2 [193,000 sq miles] of the Gulf of Mexico, offshore West Africa, the North Sea, Indonesia and Australia. An example of where these surveys have been used extensively is offshore Angola. Here, the Congo River has been depositing sediments for more than 90 million years. The conditions for a productive petroleum system have come together to create some of the largest discoveries of the last decade (next page). In Block 17, at 1350-m [4429-ft] water depth, TotalFinaElf (TFE) is operating the giant Girassol field, which contains 700 million barrels [111 million m 3 ] of oil reserves. Industry experts say Blocks 14 (operated by ChevronTexaco), 15 (ExxonMobil), 17 (TFE) and 18 (BP) may contain up to 10 billion barrels [1.5 billion m 3 ] of recoverable oil. 1 Most of these deepwater discoveries are in channelized sands deposited by the Congo River system during the Tertiary period. The Angolan government has agreements with companies to explore Blocks 31, 32 and 33. Sonangol and Norsk Hydro are exploring the recently licensed Block 34. The deepwater blocks to the south of the giant fields have not yet been licensed. These lie in the deepwater Kwanza basin, where conditions for source rock, hydrocarbon generation, structure and seal have yet to be proved. 2 Given the size of the finds to the north, the undesignated and unlicensed areas, also known as open areas, are of significant interest to exploration companies. 2 Oilfield Review

14 Boma Plutao Marimba Kissanje 31 15 Essungo 1 2 Congo River 16 3 32 Girassol Rosa 33 17 Tulipa Dalia Cromio Platina 18 4 5 A N G O L A 34 19 6 Luanda 20 Kwanza River 7 21 8 22 100 23 9 62 > Large discoveries offshore Angola. In Block 17, at 1350-m [4429-ft] water depth, TotalFinaElf, with partners ExxonMobil, BP, Statoil and Norsk Hydro, is operating the giant Girassol field, containing 700 million barrels [111 million m 3 ] of oil reserves. Blocks 14, 15, 17 and 18 may contain up to 10 billion barrels [1.5 billion m 3 ] of recoverable oil. The deepest discovery so far is the BP Plutao well in Block 31. Winter 2002/2003 3

Several recent wells have been drilled in the shallower water of the Kwanza basin to test play concepts different from those found in the deeper water farther offshore. The wells have come in with mixed results; only the ExxonMobil Semba-1 well flowed, at about 3000 B/D [477 m 3 /d] during testing. 3 This has provided encouragement that there is an active petroleum system in this basin. Farther offshore, WesternGeco has acquired multiclient 3D data over 19,00 2 [6840 sq miles] of the Kwanza basin, including a 7000-km 2 [2700-sq mile] subset of the 3D dataset that we examine in more detail (below). The dominant feature visible in the high-resolution seafloor image is the Kwanza Canyon, an active submarine canyon that serves as a surface example, or analog albeit on the present-day seafloor for understanding subsurface structures interpreted in the 3D seismic images. The canyon is of particular interest because it is presumed to act as the conduit that carries sands from the African continent to the deep waters offshore Angola. This article describes how seismic interpreters use images of present-day deposits in this offshore province to detect similar features in the subsurface, features that could be highquality sands charged with hydrocarbons. We begin with the shallow, seafloor expression of the Kwanza Canyon, and track its manifestation in deeper, more mature sediments, before examining the resultant abyssal-plain deepwater fan 14 Boma 31 15 1 2 Congo River 16 3 A N G O L A 32 17 4 33 18 5 34 19 6 Luanda Kwanza River 20 7 21 8 22 100 23 9 62 > Multiclient 3D seismic surveys (pink) acquired by WesternGeco. Light pink areas show surveys acquired over unlicensed acreage, and darker pink areas show surveys acquired over areas that were licensed at the time of seismic survey acquisition. The trapezoidal area outlined in magenta is the area covered by the bathymetric data shown on the next page. 4 Oilfield Review

by analogy to the Congo Fan. Finally, we show some possible exploration targets that have been identified using the knowledge built on these Kwanza and Congo examples. Starting at the Surface The Kwanza Canyon was first identified on bathymetry data acquired, in association with a gravity survey by ARK Geophysics Ltd., at the time of the 3D seismic data acquisition (below). The underlying salt diapirism is currently active and causes the noticeable north-south grain, or topography, of the seabed. The canyon starts near Luanda, Angola, in 50-m [164-ft] water depth and extends east to west cutting across the regional grain but always following the continental slope for about 30 [188 miles], eventually reaching the abyssal plain in water depths of more than 4000 m [13,100 ft]. Abandoned meanders and oxbow lakes cut off by chute channels correspond to earlier channel cuts of the submarine river before it became as deeply incised as it is today. 4 The canyon is of variable width, but generally about 1 to 2 km [0.6 to 1.2 miles] wide and 400 m [1300 ft] deep in the area covered by the 3D survey. The water depth varies from 1200 m [4725 ft] in the east to 2500 m [8203 ft] in the west. 3. http://www2.exxonmobil.com/corporate/newsroom/ Newsreleases/corp_xom_nr_140601_1.asp 4. An oxbow is a U-shaped bend in a river. The gradual meandering of rivers commonly produces cutoff bends, visible as oxbow lakes, that mark the river s former course. Kwanza Canyon Feeder canyon 10 6.2 Incipient channel Pockmark > Seabed topography of the Kwanza Canyon, first identified on bathymetry data acquired in association with a gravity survey. Color indicates depth below sea level, with yellow as shallow and purple as deep. Underlying salt diapirism causes the north-south trends visible in the seabed. The canyon crosscuts these north-south features. It starts in the shallower water in the east and extends westward for about 30 [188 miles]. This image covers about 700 2 [2700 sq miles] of seabed. Winter 2002/2003 5

1 2 3 4 Feeder canyons Seafloor island Kwanza Canyon 4 3 2 1 > Bathymetry of the Kwanza Canyon (bottom) and cross sections extracted from 3D seismic data (top) in the eastern portion of the canyon. Cross sections are numbered starting at the shallower end of the canyon. Panel 1: Two feeder canyons south of the main canyon are seen close to their point of origin, which is not seen in the 3D volume. Panel 2: The feeders form a subsea island. The bases of the channels show high-amplitude reflections, possible indicators of sand. Panel 3: The main Kwanza Canyon comes in from the east, and at this point is at a depth of 500 msec, or 400 m [1312 ft]. The canyon exploits the crest of a salt diapir in the subsurface (outlined in red), probably a zone of weakness that is more easily eroded. Panel 4: The Kwanza Canyon is less deeply incised and is on the flank of the salt diapir. Round depressions, or pits pockmarks, as they are known in the industry, are visible on the seafloor image throughout this area. These are caused by escaping gas or fluids bubbling up from the subsurface, and can provide direct evidence of present-day hydrocarbon migration from deeper hydrocarbon source rocks. The pockmarks exploit zones of weakness and fracturing, eventually coalescing into continuous linear features, forming the troughs that are commonly seen in the seafloor along the flanks of salt diapirs. These troughs may also have some influence on the routes taken by features such as the Kwanza Canyon. Cross sections through the shallow section of the 3D volume show the canyon at different points along its course (above). In the first two panels, smaller feeder channels, or side canyons, can be seen cutting through shallow sediments before joining the main Kwanza Canyon. In the third and fourth panels, the canyon cuts near a salt subcrop. Interpretation of the seismic lines determined that, at the time they were acquired, there was no sediment in the Kwanza Canyon; there are no rugose or mounded reflections seen at the base of the channel cut, either of which would be indicators of unconsolidated sediments progressing down the canyon. Interpreters speculate that sediment movement along the canyon could be intermittent but rapid, taking the form of a debris- or mass-flow deposit, similar to those seen occasionally onshore. 6 Oilfield Review

5 6 7 8 Filled earlier canyon High-amplitude canyon bottom Oxbow Main canyon 8 7 6 5 > Bathymetry of the Kwanza Canyon (bottom) and cross sections extracted from 3D seismic data (top) in the western portion of the canyon. Panel 5: The canyon follows the predominant north-south grain imposed by underlying salt diapirism. A filled earlier canyon can be seen to the left of the present-day canyon. Panel 6: Earlier traces of the canyon are seen in sediments beneath the current canyon. Salt diapirism is no longer evident. Panel 7: The main canyon cuts across an oxbow, abandoning it and leaving the oxbow higher. Older cuts that have been filled are preserved below the current channel. Panel 8: The main channel widens and deepens in the deeper water near the boundary of the 3D survey. As the canyon moves into deeper water, its course becomes independent of salt-related structures (above). In the first two panels, labeled 5 and 6, an earlier cut of the canyon can be seen to the left of its present-day position. By the second panel (6), salt diapirism no longer seems to control the canyon s location or direction. Panel 7 shows the canyon and a nearby oxbow. Immediately under the canyon cut, high-amplitude reflections point to preserved older channel fill. In the last panel, the canyon widens and deepens just before it disappears across the boundaries of the 3D survey. The meandering form of the canyon is believed to be a consequence of the interaction between salt diapirs and the angle of slope of the continental shelf. Submarine canyons seen on the Nigerian continental shelf beyond the Niger delta tend to be more linear, because there is no salt tectonism to alter the seabed topography and interact with the canyons. What happens to the Kwanza Canyon beyond the boundaries of the 3D survey? Despite a tight grid of 2D data in the nearshore area of Block 6, the canyon cannot be traced back to the continent. This could indicate that it is not related to a present-day river system onshore. However, the canyon may connect to the mouth of the Kwanza River, some 5 [3] to the south via the strong northerly current known as the Benguela Current, which passes along the West African Winter 2002/2003 7

coast here (below). The Kwanza Canyon starts where there is a break in the coastline, near a spit, 5 north of the mouth of the Kwanza River. The Kwanza Canyon may also relate to an earlier and different drainage system than the one that exists today. At its distal, deepwater end, the Kwanza Canyon is not covered by the 3D survey, but can be traced across 2D surveys, eventually reaching a point at which there currently are no data to trace it farther. Interpreters can conjecture that a deepwater fan exists at the end of the canyon system, where sands funnelled by the Kwanza Canyon are deposited on the abyssal plain; an image of this system for the Kwanza Canyon remains unavailable. However, 3D datasets acquired in the abyssal plain of the prolific Congo Fan to the north show the likely depositional regime. These will be discussed later in this article. Evaluating Canyon Fill An interpreted seismic line from the area to the south of the Kwanza Canyon shows a preserved canyon cut in the shallow subsurface about 500 msec below the seafloor, which itself is at about 2.5 seconds two-way traveltime (next page, top). This feature can be used to demonstrate the likely style of deposition and preservation of a recent but now buried canyon. A large channel complex delineated in yellow can be interpreted through the volume. Detailed examination of the data shows multiple channel cuts stacked within and beneath the yellow surface. This system is located on a present-day structural high. Sediment thinning into this high suggests that this was also a high at the time of canyon cutting. The high-amplitude events in the bottom of the canyon indicate sand-rich channel fill. To visualize this canyon and the sediments that fill it, seismic amplitudes measured in an interval including the channel bottom and extending up to 50 to 100 msec above the yellow event are plotted (next page, bottom). The high amplitudes colored in yellow resemble a meandering system within the overall canyon cut. Fine-tuning the time windows selected for amplitude extraction probably would show more detail and complexity within this system. The high-amplitude events 31 14 Congo Canyon 15 1 2 Congo River Boma 16 3 A N G O L A 32 17 4 33 18 5 Kwanza Canyon 34 19 6 Luanda Kwanza River 20 7 100 62 21 8 22 > The Kwanza Canyon, offset about 5 [3] from the mouth of the Kwanza River, and an abyssal fan that might be inferred to exist in the deep water to the west. 8 Oilfield Review

Earlier filled canyon High amplitudes in canyon bottom A A 2.5 Two-way traveltime, sec 3.0 3.5 4.0 2.0 1.2 > An interpreted seismic line showing a canyon (yellow) filled and preserved south of the Kwanza Canyon. The bright reds and blacks represent high amplitudes at the base of the canyon and are signs of sand-rich sediments filling the canyon. The seismic amplitudes at the bottom of the channel are shown in the figure below. A A 4.0 2.5 > Amplitudes measured across a 3D seismic volume in a time interval constrained to the high-amplitude bottom of the canyon interpreted in yellow in the figure above. High amplitudes in yellow show the meandering nature of the preserved canyon. The location of the cross section in the above figure is shown by the A to A traverse. Winter 2002/2003 9

Meandering channel Meandering channel in profile Fan deposit in profile Fan deposit > A horizontal time-slice, or plan view, through a 3D seismic survey north of the Kwanza Canyon, revealing the high amplitudes of a large fan deposit on the right and a meadering channel on the left. are interpreted to be sands; the highest amplitude sands may be hydrocarbon-charged. The gray low-amplitude areas are interpreted to be mudstones or shales. Offshore Angola, sediments of the Tertiary period with high seismic amplitude are generally indicators of sand deposition, whereas low amplitudes indicate mudstone, clay and shale. Later movement of deeper salt has caused uplift and erosion, and controls the limits of the meandering system in the east-west direction; the system cannot be traced any farther, and therefore constitutes a potential hydrocarbon trap. This is interpreted to be the mechanism of reservoir deposition, preservation and trap formation that has occurred in the Congo and Kwanza basins throughout the Tertiary period. However, the exact mechanism for the establishment and abandonment of this type of canyon is not well understood. The seismic expression of a deepwater fan can be seen in some of the seismic volume acquired to the north, over the deepwater end of the Congo Fan system. This provides an example of the probable style of depositional regime that may have occurred or be occurring at the end of the Kwanza Canyon, where no direct evidence exists. The 8000-km 2 [3090-sq mile] northern survey covered part of the abyssal plain of the Congo Fan as well as the salt-diapir province in shallower water. A time-slice through the 3D volume shows a fan and a meandering channel (above). To a first approximation, the abyssal plain is flat and horizontal, so the time-slice is consistent with a depositional horizon. The image displays roughly in plan view the two types of high-amplitude reflections commonly seen in this region, and also shows how these 10 Oilfield Review

look in cross section, or along seismic lines. Long, extensive high-amplitude reflections are likely to be fans or sheet sands. Short, stacked high-amplitude reflections are likely to be channelized sands. This information can be applied in areas of greater structural complexity where horizon-consistent time-slices are difficult to make. Seen in vertical section, the sand amplitudes diminish with depth (below). This is interpreted to be due to a change in acoustic-impedance contrast caused by compaction. Sand channels in this seismic section have been interpreted first in time-slice displays, and then the intersections of the channels with the seismic section are displayed as yellow circles. The channels clearly correspond to short high-amplitude events, indicating that channels in this basin can be picked with confidence from seismic sections, even at 6- to 7-second two-way traveltimes. Below the blue horizon at about 7.5 seconds two-way traveltime, sand channels are not evident on time-slices, so the first input of sand to this part of the basin occurs at or above this level. The regional uplift of the African coastal areas, which occurred in the Oligo-Miocene about 35 million years ago, provides the source of the earliest sand influxes to the basin. Prior to the Oligo-Miocene uplift, little or no sand is present. This information has been used to date the age of these sections, giving the blue horizon an age of the top of the Eocene epoch, just prior to the Oligo-Miocene. The seismic character changes below the top of the Eocene, indicating major changes of environment and deposition. Regional information identifies the interbedded high-amplitude, laterally continuous events found beneath the top of the Eocene as a potential source-rock interval, equivalent to the Iabe formation. The Iabe is a proven source in Block 2 offshore fields such as Essungo. If the Iabe is present as a source rock in the Congo basin, then it is early to mid-mature, given the thickness of Tertiary sediment burying it, and therefore presently capable of generating hydrocarbons. Deeper Cretaceous (Albian) source rocks, close to the basement, would also be mature if they exist. Clues to Hydrocarbons Direct hydrocarbon indicators (DHIs) provide evidence that hydrocarbons can be found in this area. These DHIs typically are anomalously highamplitude reflections resulting from the additional acoustic-impedance contrast generated by hydrocarbon as compared with water in sands. 5 Migration of hydrocarbon fluids occurs through the minor fractures evident on the seismic sections. 5. Acoustic impedance is the velocity multiplied by the density of a rock. Both of these quantities vary, and usually increase, with depth for most rock types. N S 5.5 6.0 Two-way traveltime, sec 6.5 7.0 7.5 Top Eocene 8.0 Top Cretaceous 8.5 4.0 2.5 Break-up unconformity > Seismic image showing several high-amplitude (black and red) sands near the top of the section (down to about 6.5 seconds), lower-amplitude sand-filled channels deeper (down to about 7.5 seconds), and no obvious sand below about 7.5 seconds. The decrease in amplitude of the sand-filled channels with depth is attributed to a lower acoustic-impedance contrast with increased compaction. Yellow circles correspond to intersections of this seismic image with channels that were interpreted on time-slices between 6.0 and 7.0 seconds. Winter 2002/2003 11

Sand-rich deposit Overbank deposit Shale-plugged channel Levee Earlier fan deposit > High-resolution detail of meandering channel from the deepwater Congo Fan, showing high-amplitude sand fill (yellow and red), low-amplitude shale (gray), and middle-amplitude channel levees (green). Dip-closure horizon Canyon 3.0 Fan DHI Fan Channels DHI Channel 3.5 4.0 Two-way traveltime, sec 4.5 Top Iabe 5.0 Top salt 5.5 > Channels, fans and direct hydrocarbon indicators (DHIs) interpreted on a seismic line from the salt-diapir province of the Congo Fan, north of Kwanza Canyon. A horizon with dip closure is interpreted in orange. 12 Oilfield Review

To evaluate the variation in direction and complexity of the channel systems, eight timeslices from the abyssal plain were picked at 40-msec intervals in the range 6620 to 6900 msec, which is about 2000 msec, or 2000 m [6560 ft], below the seafloor (right). The current limit of salt shows the eastern edge of the tectonically active region, although this has varied with time. It is assumed that channels would flow westward beyond the limit of salt. The channels show wide variation in orientation, from north-south to eastwest. Clearly, it cannot be assumed that channels always follow the regional dip away from their source. Exploring for and developing hydrocarbons in these channels require the information contained in seismic data. The 3D seismic data contain high-resolution detail of individual channels and other depositional features on the abyssal plain, even at 1.5 seconds, or 1500 m [4920 ft], below the seafloor, which is 4 seconds or 3 km [2 miles], below sea level (previous page, top). In this example from the northern 3D survey over the Congo Fan, high amplitudes in yellow and red delineate sands. The time-slice shows facies variations within a channel, where channel fill changes from the red and yellow indicating sand to the gray of shale. 6 Where the channel is plugged with shale, sometimes it has levees marking its position. Overbank deposits, or crevasse splays, can also be identified. 7 All the information from the Kwanza Canyon and Congo abyssal plain can be applied in areas that currently are being evaluated for hydrocarbon exploration and future development potential (previous page, bottom). This seismic line comes from the salt-diapir province of the Congo Fan. By applying the knowledge and models developed from the shallow seafloor down to the abyssal plain, we see that two types of sand deposition can be identified in this province: channels appear as short, bright reflectors, and sheet or fan sands as more extended lines of high amplitudes. Recognition of sand-prone intervals, combined with regional knowledge, allows approximate geologic ages to be assigned to different parts of the section in areas where no well information is available. The salt provides four-way dip closures of varying areal extent within the Cretaceous and Tertiary overburden, of which there are examples in this area. Sands can be interpreted by their 6. Facies variation is the variation in rock type within a unit as a result of the depositional process. N characteristic amplitude, and are seen over the crests and on the flanks of the closures, offering attractive hydrocarbon-trapping configurations. Post-salt source rocks, such as the Iabe formation and those of Albian age, are mature in the deep synclines between the salt diapirs and on the adjacent abyssal plain. The DHIs that are apparent in these data strongly suggest that hydrocarbons are present, have migrated into structures and have been trapped making the area highly attractive for future exploration. Abyssal plain Present-day limit of salt > Orientations of channels interpreted in 3D seismic data in the interval 6620 to 6900 msec. The channels do not always follow the east-west regional dip away from their source. Channel orientation cannot be predicted, but it can be mapped from 3D seismic data. 7. An overbank deposit, or crevasse splay, is made of sediments deposited when the channel breaks through or runs over its banks. In this example, the present is clearly a clue to the past. By using information from the present-day seabed and from deeper analogs, we have built a model of processes that have probably been occurring offshore Angola for the last 30 million years. Many of Angola s recent discoveries are in these older channels, but the existence and economic value of surrounding accumulations have yet to be proved. The proof may lie in further examination of the 3D seismic data that form the foundation for the exploration process in this deepwater region. LS 5 3 Winter 2002/2003 13