Field-Scale Stratigraphy and Depositional Elements of a Slope Channel System and Implications for Reservoir Dynamics; A Case Study from the Angolan Margin
Published:December 01, 2012
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Luz Sophia Cobos, Mike Mayall, Tim Primmer, Rosa Osvaldo, 2012. "Field-Scale Stratigraphy and Depositional Elements of a Slope Channel System and Implications for Reservoir Dynamics; A Case Study from the Angolan Margin", New Understanding of the Petroleum Systems of Continental Margins of the World, Norman C. Rosen, Paul Weimer, Sylvia Maria Coutes dos Anjos, Sverre Henrickson, Edmundo Marques, Mike Mayall, Richard Fillon, Tony D’Agostino, Art Saller, Kurt Campion, Tim Huang, Rick Sarg, Fred Schroeder
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The imaging of deep-water turbidite slope-channel systems in Offshore Angola has improved greatly during the past decade through the acquisition of high resolution seismic surveys. Within the Marte Field, located in Block 31 NE, high resolution seismic data provides greater definition of the internal stratigraphy of third-order channel complexes. Fourth-order channel cuts and associated facies, including muddy slumps, within the third-order systems have been mapped in considerable detail. Better definition of the internal depositional architecture reveals a high degree of heterogeneity within the channel systems and offers insights into the fluid dynamics of Marte Field reservoir systems. Improvements on data quality also enable the quantitative use of AVA and inversion products for populating static models with realistic reservoir properties. Given the sparse well data available for rock property calibration, the integration of high-resolution seismic facies mapping into geocellular models has allowed us to constrain further property populations based on well logs and analogs-calibrated net-to-gross estimates. Combining alternate facies descriptions within different polygons is used to provide alternate fluid dynamic scenarios and cases. The range of models produced allows us to establish a reservoir operating envelope that is being used for development well planning and for reservoir management decisions.
The Marte Field is located in the Lower Congo Basin, in Block 31 offshore Angola (Fig. 1). It is part of the PSVM development which ties four fields (Plutao, Saturno, Venus, and Marte) to a floating production facility. The field lies down depositional dip from major developments in Block 15 (Reeckmann et al., 2003; Boles and Mayhall, 2006: Porter et al., 2006). The Marte reservoirs are made up of 3-5 km wide lower Miocene deep-water erosional turbidite slope-channel complexes. Early 3D seismic acquired for exploration purposes has provided enough temporal and spatial resolution to drill successful discovery wells which targeted strong DHIs, and flat spots.
Core data and well logs from the discovery well revealed a higher degree of complexity in the distribution of the architectural elements of the reservoir than suggested by the seismic data at the time. Improvements in temporal and spatial resolution gained from the acquisition of proprietary high resolution 3D seismic surveys resulted in greater definition of the internal stratigraphy within the slope channel complex. Better definition of the internal depositional elements, in line with the core investigation, resulted in the recognition of greater heterogeneity within the channel system, which could have affected the dynamic behavior of fluids within the reservoir. Using a methodology that allowed us to test alternative plausible geological descriptions of internal architecture against transient well test data, we were able to calibrate the range of uncertainty in our reservoir models. Understanding the inherent uncertainty in our models increased our confidence for making intelligent development decisions.
In this paper we describe the internal architecture of the third-order channel complex in the Marte Field as interpreted from high-resolution 3D seismic data. We also describe the methodology used to build a geocellular grid containing the architectural elements within the channel complex and show how we have used the models to investigate the impact of uncertainties in our descriptions of internal stratigraphic architecture on the dynamic behavior of the reservoir.
Our high-resolution 3D seismic data set covers the entire Marte Field; however, there is an area in the center of the field that is characterized by poor imaging below a thick wedge of gas trapped beneath a sub-seafloor hydrate layer. To-date, the only well penetration is the Marte-1 exploration well. A full suite of electric logs, a vertical seismic profile, 75 meters of core, and a DST have been acquired in this well.
The Marte Field contains a four-way salt cored asymmetrical anticline, controlled by a west-verging thrust fault. The depth of the oil-water contact indicated by the flat spot and confirmed by the exploration well indicates that the structure is not filled to spill. A series of east-west trending faults cross the southern half of the field. The northernmost fault is the most significant, potentially sealing the middle of the field, and dividing the field into northern and southern compartments.
The Marte reservoir is made up of a large erosional slope channel complex, about 4 km wide and aligned perpendicular to the field structure (Fig. 2). The channel complex changes systematically as it approaches the structure (Mayall et al., 2010, his Fig. 8); where it crosses the structure, it decreases in width from up to 5 km off-structure to less than 4 km at the present day crest. In a similar way, the incision depth increases from approximately 100 m off-structure to greater than 150 m at the crest. Off-structure, the channel has a regularly layered style of fill; however, toward the crest of the structure the channel fill facies is less regular as individual reflectors appear mounded. Although the structure is interpreted as growing at the time of channel formation, the channel is interpreted as having sufficient erosional power to cut through the developing sea-floor topography.
Slope channel systems in offshore Angola
The general model for large erosional channels is shown in Figure 3. The model is based on Sprague et al. (2005), Mayall et al. (2006), and Jones et al. (2012). The seismic character change, representing the base of the third-order channel complex, constitutes the most extensive surface mapped in the study. Contained within this surface, we can generally map two to four fourth-order channel complexes and within the fourth-order channel complexes, it is generally possible to identify and map a number of specific higher order features. These include:
Basal lags—comprising coarse-grained sands and conglomerates. They are also often well developed above the base of the third-order erosional surface.
Channel axis facies—high net-to-gross (N:G); dominated by thick structureless sands, often having a mounded character.
Channel margin facies—lower N:G lateral to the axis facies; dominated by interbedded sands and muds.
Slumps/debrites—often dominated by muddy deposits derived from mass transport complexes/ debris flows.
Based on outcrop analog data and very high quality seismic data, we interpret the fourth-order channel axes and margins to be composed of a series of individual channels, each of which has axis and margin facies.
Early high quality data on channel complexes in other Angolan fields established our mapping methodology: (1) Map base of the third-order channels and associated basal lag complexes. (2) Map base of fourth-order channels. (3) Within fourth-order channels, map main seismic facies including from base upwards: basal lags, channel axis, channel margin, and slumps/debris flow facies. This mapping strategy has been applied successfully in other Angolan deep water fields having high-resolution 3-D seismic data. In our Marte Field study, high-resolution data over most of the field clearly imaged similar architecture to that illustrated in Figure 3, including fourth-order channel cuts and fills, slumps, and debris flows. However, poor data quality over part of the field made it difficult to interpret the internal stratigraphic components in an area to the east of the field. A combination of detailed trace and line interpretation starting in the areas of best data quality, along with geo-anomaly extraction, allowed us to map the geometry of higher order channels even in areas of high stratigraphic complexity or poor data quality.
Within the Marte channel complex, the major fourth-order elements that can be observed from impedance and reflectivity data are: high reflectivity basal lags covering the whole extent of the third-order channel; a succession of two to three low N:G fourth-order channel cuts and fills in the northern half of the field; a slump and debris flow package to the south of the field; a prominent high N:G fourth-order channel in the central part of the field; and lastly, a low N:G fourth-order channel in the northern edge, representing the latest channel abandonment facies. These facies are described in more detail in the paragraphs below and are shown in Figures 4 and 5.
Basal lag sands
Basal lag sands (Facies 1, Fig. 5) extend continuously throughout the base of the third-order channel, indicating they are more likely related to third-order channel deposition. Two high amplitude events (peaks over troughs), which can be seen in reflectivity data (Figs. 4B and 5B), are tied to the two lag sands at the well. They are readily distinguished on inverted acoustic impedance imagery (Figs. 4A and 5A) where they are characterized by high positive impedance values. At the well, these positive impedances are calibrated to high velocity, high density, and high N:G sands. Log porosity data shows lower porosity for these sands than for overlaying soft sands, perhaps due to poorer sorting and/or the presence of cement (Figs. 6A and 6B). In a map view of an amplitude extraction, these basal lags cover the whole base of the third-order channel and in some areas exhibit clear meandering channel geometries (Figs. 7A and 7B)
Erosional remnant sands
Overlaying the basal lags, in the southern half of the Marte channel complex, high-amplitude soft impedance events conformably overlie the basal lags. These deposits in turn appear to exhibit an erosional, hiatal relationship with overlaying dim events. They are interpreted as erosional remnant sands (Facies 2, Figs. 5A and 5B) that are genetically related to the basal lags and are associated with the same third-order sequence. The remnant sands appear to have been partially eroded by the onset of muddy debris flows and slumps related to deposition of the overlying strata.
Low net-to-gross fourth-order channel cuts and fills
Overlying the northern portion of the basal lags, a fourth-order channel cut can be identified both on impedance and on reflectivity data (Facies 3, Figs. 5A and 5B). The associated change in seismic facies is the basis for mapping this feature. The internal channel fill (Facies 3a, Fig. 5) is characterized by patchy medium-to high-negative impedances, interrupted by dimmer events. (see amplitude map, Fig. 7C). At the well, as seen on electric logs, the base of the channel fill is characterized by thin interbedded sands and shales overlying a thick interval (about 25 m) of continuous shale. Within the fourth-order channel, an internal boundary can be mapped between facies 3a and 3b (Fig. 5) by following a positive event marking the base of channel incision. The change from seismic facies 3a to 3b corresponds to a change from mainly negative impedances to more intercalated positive and negative impedance events that are also brighter, some of which appear to correspond to individual channels seen in a map view of the amplitude extraction (Fig. 8A). These upper facies (facies 3b) are interpreted to be the axial portion of the channel fill. On electric logs, these facies appear as thin interbedded sands and shales interrupted by a thick (25m) blocky sand, which in map view corresponds to the channel shown in Figure 8B. Core has been recovered only from the thin-bedded part of the sequence. In the core, these facies are represented by sands a few meters thick which are interbedded with slumps, debrites, and hemipelagic shales. In some intervals, injected sands cross-cut the interbeds.
Interbeds, debris flows, and slumps
Covering the entire southern half of the third-order complex and overlying facies 2 (Fig. 5) chaotic and dim impedance and reflectivity events are interpreted as representing muddy debris flows and/or slumps (facies 4a, Figs. 5A and 5B) probably associated with the deposition of a mass transport complex. Although these facies lack well calibration at field scale, our interpretation is based on analogous Miocene seismic facies calibrated in other offshore Angola field wells (Reeckmann et al., 2006; Sprague et al., 2005).
Sandy debris flow deposits (facies 4b, Figs. 5A and 5B) are also found in the northern half of the field, overlying facies 3. Only a small portion of this sequence remains, as it appears to be eroded by fourth-order channel number 5. These debris flows are penetrated by the Marte well, and core data has been recovered from them (core and well log imagery, Fig. 6). They are characterized by centimeter-scale sands interbedded with slumps, debrites, and hemipe-lagic sediments. In the core, they appear very similar to facies 3.
High net-to-gross fourth-order channel
Perhaps the most prominent feature observed in Figures 4 and 5 is the high N:G fourth-order channel nested within the third-order channel complex at the center of Marte Field. This feature represents the youngest fourth-order channel in the central part of the field and is characterized by prominent differential compaction at its axis. Its channel fill is interpreted as including facies 5a to 5d (Fig. 5). In reflectivity sections, the fourth-order channel boundary can be mapped easily based on erosional truncations with the underlying events.
The basal part of the high N:G fourth-order channel fill (facies 5a, Fig. 5A and 5B) is characterized by high positive values in the color-inverted acoustic impedance data and medium to dim values in the near trace reflectivity data. Although not penetrated by the exploration well, this basal portion is interpreted as a hard conglomeratic lag due to its high positive impedance character. However an alternative interpretation could be that this is a muddy interval or debris flow associated with facies 4b, rather than part of the fourth-order channel.
Overlying the basal interval and still within the fourth-order channel, a well defined channel axis facies (facies 5b) about 100m thick is characterized by bright amplitudes in the reflectivity data and very high negative impedance values on the inverted data. Differential compaction across the top of this feature makes this fourth-order channel look very prominent in strike section.
Facies 5c strata (Figs. 5A and 5B) flank both sides of the prominent channel axis facies (facies 5b, Figs. 5A and 5B). The channel-flank (facies 5c and 5d) strata exhibit slightly lower amplitudes and impedance indicating lower N:G content compared to the facies 5b axis deposits. However, compared to most other facies recognized in the field, facies 5c and 5d contain relatively high N:G ratios. The facies 5c interval is penetrated by the exploration well, which encountered about 25 m of high N:G strata comprised of sands inter-bedded with silts and shales. The fining upwards log character at the top of the unit represents abandonment facies at the end of the fourth-order channel deposition. We interpret these flanking strata (facies 5c and 5d) to be high N:G channel-margin facies. An RMS amplitude extraction of the fourth-order channel is shown in Figure 9A, where the lateral extent of this massive feature can be observed.
To the south of the high N:G fourth-order channel, a time equivalent interval is characterized by patchy and medium to dim amplitude and impedance values. These are interpreted to be low N:G marginal and over-bank deposits associated with the central fourth-order channel axis.
Low net-to-gross abandonment fourth-order channel
The youngest fourth-order channel located to the north represents the latest stage of the channel fill deposition. This channel is easy to identify in reflectivity data due to sharp channel edge geometries at its northern edge. Compared to the central channel, amplitude and impedance values are lower except for areas around the oil-water contact where fluid and tuning effects create very bright reflections. Dimmer amplitudes, lack of differential compaction and a more continuous succession of positive and negative reflections indicate lower N:G content, more typical of third-order abandonment facies. This feature has not been penetrated by the well.
Tuning effects (high amplitudes and impedances) due to the flat spot being located within this channel could be misleading when predicting N:G content.
A schematic evolution of the Marte third-order channel fill is shown in Figure 10 in an attempt to assign relative ages between the fourth-order internal elements. However uncertainty on the relative ages of some of these fourth-order features is high, especially the relative ages of facies 4a, 4b, 5 and 6.
Once the internal architecture within the channel complex was established, facies polygons where drawn or mapped to further subdivide the regions within the fourth-order channels (basal lag, channel axis or channel margin). Some of the facies polygons could be mapped as three-dimensional surfaces, and some where subdivided with two-dimensional polygons. Additional facies regions were mapped where the well data indicated a change in facies not clearly shown by the change in seismic character. Geologically plausible N:G ranges were assigned to each facies polygon by weighing the N:G at the well, the seismic facies character, the AVO properties and the analogue data. At this stage, the main static and dynamic uncertainties were captured and prioritized; where more than one facies interpretation was possible, the alternative descriptions where carried into the geo-cellular model building for later sensitivity analysis.
Geocellular Model Build
The geocellular model grid was designed to have enough flexibility so that alternative facies interpretations could be incorporated. For this purpose a bucket grid was built from the top of the third-order channel complex to its base. The internal grid cells were made conformable to the top of the reservoir and two other internal surfaces which span the third-order channel. The internal fourth-order elements were later assigned as regions in the model; this allowed for alternative facies descriptions to be incorporated for sensitivity analysis and uncertainty studies.
Figure 11 shows how the final regions in the model honor the facies interpretation, and it also shows a 3D view on the spatial distribution of the internal channel architecture in the model.
Exploring the Uncertainty in Net-to-Gross Prediction
A heterogeneous N:G distribution in the model can be achieved by using rock properties calibrated seismic impedance volumes as proxies for fluid and lithology predictions. A simple linear transform would then relate N:G to impedance values. A great deal of uncertainty is involved in the choice of transform due to the lack of well data for calibration and due to the use of band-limited seismic attributes to derive petrophysical properties. The benefit of surgically mapping the fourth-order seismic facies is that we can further constrain the choice of transform by selecting the one that derives a N:G distribution which matches the target net rock volume assigned to the facies polygons during the model framing stage. This allows adding benchmarked geological constraints to the direct use of seismic for property population. Furthermore, assigning uncertainty ranges to each facies polygon in the model allowed us to test alternative N:G distributions for each fourth-order facies. For the Marte model, a geologically plausible combination of low to medium N:G cases for the facies polygons was used to create a low N:G model scenario. The same logic was applied for the high N:G scenario using a combination of medium to high N:G values for the different polygons to calculate the target NRV. The most likely and mid-value of the N:G range per facies polygon was used to generate the best technical case model.
Additional alternative N:G scenarios were generated to test the uncertainty related to the distribution of facies in the reservoir in cases where we had to carry alternative scenarios into the dynamic modeling phase. One of the uncertainties we considered early during the modeling phase was the connectivity of the thin-beds and debris flows of facies 4b observed at the well and in core (Fig. 12). Two scenarios were tested: one scenario where the thin beds were well connected to the surrounding facies (best technical case) and another one where these facies were very poorly connected to the rest of the reservoir. In this particular case we were able to test these scenarios by trying to duplicate in the simulator a drill stem test (DST), which had been acquired in the exploration well, for both scenarios and comparing to the real DST test. Figure 13 shows the actual test data and the simulated test data for both scenarios and confirms that the scenario for which facies 4b thin beds are not very well connected to the rest of the reservoir is very far from the real DST behavior. In comparison, the best technical case, which assumes good connectivity of the thin beds of facies 4b, more closely matches the actual DST test.
Another sensitivity study carried out was on the uncertainty of the basal facies of the high N:G fourth order central channel (Facies 5a). As mentioned in the reservoir description section, two alternative scenarios were considered: (1) facies 5a were basal lag sands associated with the high fourth-order channel or (2) facies 5a were muddy debris flows associated with earlier deposition of facies 4a (Fig. 14). Very little difference was observed between the simulated DST tests for the two scenarios, they both reasonably match the actual DST data as observed in Figure 13. This result could be attributed to the fact that the DST test was performed some distance away from facies 5a, which means that this sensitivity analysis was inconclusive for selecting alternative scenarios of N:G distribution in these facies. Furthermore, the impact of these scenarios on cumulative oil recovery was small (less than 3% of the cumulative resources). However, both alternative scenarios were carried in the well planning phase because the impact on nearby well recovery could represent a reasonable percentage of the well cumulative oil recovery.
When analysed within its regional setting, Marte Field stratigraphy can be described based on the general model for large erosional channels shown in Figure 3: a large erosional third-order channel with complex internal stratigraphy. The high-resolution seismic facies mapping undertaken allowed us to break this internal stratigraphy into fourth order elements. Within the fourth-order channels, recurring facies have been further identified as basal lags, axis, margin and debris flows, or MTCs (Figs. 5A and 5B). Carrying the highresolution seismic facies interpretation into the modelling stage allowed for the geological calibration of seismic derived net-to-gross population. This systematic approach ensures that the model captures all of the main static and dynamic uncertainties and allows for calibration with dynamic data from existing tests and production data in the future. Based on this current set of scenarios, comprehensive sensitivity analysis and history match can be performed once production data is available.
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New Understanding of the Petroleum Systems of Continental Margins of the World
- Atlantic Ocean
- Central Africa
- clastic rocks
- debris flows
- deep-water environment
- depositional environment
- elastic waves
- geophysical methods
- geophysical profiles
- geophysical surveys
- Lower Congo Basin
- mass movements
- oil and gas fields
- reservoir rocks
- salt tectonics
- sedimentary rocks
- seismic attributes
- seismic methods
- seismic profiles
- South Atlantic
- thrust faults
- Marte Field