The relevance of seismic amplitudes in exploring the Niger Delta

Equatorial Guinea's Zafiro Field's Late Miocene,
Early Pliocene sediments provide examples Andrew Radmore Stephens
Gene Dee Monson
Mobil Equatorial Guinea Joseph Micheal Reilly
Mobil Technology Company The Zafiro oil field, located in License Area B, offshore Equatorial Guinea, was discovered by Mobil and partners

Equatorial Guinea's Zafiro Field's Late Miocene,
Early Pliocene sediments provide examples

Andrew Radmore Stephens
Gene Dee Monson
Mobil Equatorial Guinea

Joseph Micheal Reilly
Mobil Technology Company

The Zafiro oil field, located in License Area B, offshore Equatorial Guinea, was discovered by Mobil and partners United Meridian Corporation (UMC) in March 1995. The field was the subject of a fast-track development program and first production was achieved on 25th August 1996. The oil bearing formations are Late Miocene to Early Pliocene in age and were deposited in a deepwater setting on the slope of the Niger Delta by a range of depositional processes that include slumps, debris flows, and turbidity currents. In such settings, reservoir prediction and mapping are extremely difficult. Working with the existing 2D seismic data and a fast-track, on-board processed 3D data set, Mobil and its partners have utilized 2D amplitude versus offset (AVO) techniques, workstation-based amplitude extraction, together with other attribute analysis and 3D visualization techniques to delineate the Zafiro reservoir and the fluid contacts within it. In addition, the spatial relationships of the amplitude packages are providing significant insight into the temporal variations in depositional style in the Zafiro Area, leading to a better understanding of the depositional processes and identifying additional hydrocarbon reservoirs.

As a direct result of this work, Mobil and its partners have successfully planned and executed the first phase of development drilling to meet the start-up goals for the project - less than 18 months from the initial discovery to first production. It is expected that the application of these techniques will yield additional discoveries elsewhere in the block.

The Zafiro Field is situated in Block 18, in the western offshore area of Equatorial Guinea, adjacent to the international border with Nigeria. The field lies approximately 36 km (22.5 miles) southeast of Mobil's Edop Field in Nigeria and 68 km (42 miles) west-northwest of the island of Bioko. The License Area covers approximately 500,000 acres with water depths ranging from less than 300 ft to greater than 5,000 ft. The water depths in the vicinity of the Zafiro and Topacio elements of the Zafiro Field vary from 450 ft to 1,900 ft Mobil Equatorial Guinea operates the License Area and Zafiro Field with a 75% interest, with its partners United Meridian Corporation, who hold the remaining 25% interest.

Geological setting

License Area B is located over the distal portion of the eastern Niger Delta, one of the worlds' largest and sandiest deltas. The Delta comprises a series of thick, shore parallel, southward-migrating depositional sub-basins, often termed depobelts, which have significant temporal and spatial connotations. The present day shelf break trends east to west across the License Area just to the north of the Zafiro Field. However, for the majority of the life span of the Delta, License Area B has been on its distal margins in a depositional environment characterized by mid to lower slope and basin floor settings.

During the Late Miocene and Early Pliocene, the Delta was subjected to a major marine transgression interspersed with short, sharp, regressive cycles. This cyclic sea level fluctuation provided the Qua Iboe Member of the Agbada Formation, with the distinctive sedimentary assemblages observed in such fields as the Edop Field in OML67 in Nigeria. During the marine high stands, sand prone sediments are ponded on the expanded shelf whilst slope sedimentation is dominated by hemipelagic muds. During the marine low stands these sands are redistributed off the contracting shelf and onto the muddy shelf slope where they are carried by a variety of flow processes down the slope towards the deep basin. Irregularities in the shelf margin often give rise to a focusing of the redistributed sediment input, producing sand prone fairways on the slope. Identifying these fairways is critical to a successful exploration campaign. Various transportation modes have been described which cover the full gamut of slope processes from sediment slides through debris flow to more classical turbidities. The transportation processes produce a variety of morphological forms but most are typically elongate in the flow direction. This type of reservoir can be extremely difficult to track due to its propensity to shale out laterally over very short distances. A means of defining the reservoir distribution is critical to a successful development program.

Concurrent with the slope depositional processes described above and as a direct result of the structural extension occurring in the depobelts to the northwest, this area was subjected to structural foreshortening which manifested itself as a series of low angle toe-thrusts in the older Miocene sediments and structural inversion of the Late Miocene/Early Pliocene, Qua Iboe section.

License area history

Mobil farmed into the license in May 1994. Following disappointing results from the Delta-1 well which targeted a pre-Que Iboe interval, the rig moved six km north to drill the Zafiro-1 well in January 1995. The well encountered Lower Qua Iboe sands of good reservoir quality, which were oil bearing. Subsequent flow testing produced a flowrate of 10,500 b/d on a 2 3/8 inch choke. Following the drilling of three successful step-out/appraisal wells the Zafiro Field was declared commercial in October 1995. The Field is today being produced using a floating, producing, storage, and offloading facility (FPSO). First oil was produced on 25th August 1996 and production is expected to reach 40,000 b/d by the end of 1996. Up to the end of August 1996, nine consecutive successful exploration/appraisal/development wells had been drilled in the Field. A large factor in this success has been the seismic attribute analysis of the reservoir zone.

Amplitude versus offset (AVO) Analysts

A study of the AVO response of the Qua Iboe sediments was initiated early in the interpretation of License Area B. The occurrence of both areally large, high amplitude reflections and more confined bursts of high amplitude, in an area which lacked well control, required a tool that was capable of differentiating between a lithological and a hydrocarbon effect. Initial observations of individual common depth point (CDP) gathers indicated that many of the high amplitude events exhibited an increase in amplitude with increasing offset. This classic Class III AVO response is typically associated with gas bearing, poorly consolidated sands. Although this combination does not represent a viable exploration play in this part of the world, it was felt that the analysis of such anomalies would yield valuable reservoir trend information. This combined with the working hypothesis that gas saturated oils should give a similar response to gas, gave confidence that the AVO tool would enhance our probability of success in this area. In order to investigate the areal distribution of these AVO anomalies a number of 1993 vintage 2D seismic lines were reprocessed in-house at Mobil.

Examination of the CDP gathers and pre-drill modeling indicated that the observed AVO response in the vicinity of the Zafiro Field would not be well suited for detection using any conventional statistical linear slope/gradient approximation technique. This fact was confirmed by the Zafiro-1 well. This is primarily due to the fact that the zero offset reflection amplitude of a gas charged reservoir was anticipated to be similar to the surrounding (wet) reflection energy. The AVO amplitude bloom occurs mainly on the far offset data, at arrival angles greater than 25 degrees. Another problem that was anticipated with conventional AVO processing in this area was the fact that significant structural dips were observed at the reservoir level indicating that some form of migrated AVO attribute display (not available using conventional techniques) was highly desirable.

The raw data were processed through to stack with a near angle stack (6 to 19 degrees) and far angle stack (32 to 45 degrees) generated. The two angle stacks were migrated using a reverse time migration algorithm. The AVO attribute was calculated in two steps. Step 1 consists of the subtraction of the near and far migrate angle stacks to produce a difference section. Step 2 takes the difference section and multiplies it by the migrated far offset angle stack to produce the AVO attribute section. This second-step has three effects. First it allows correction of the sign of the display such that an increase in amplitude with offset is always given a positive attribute independent of whether the observed amplitude is negative or positive. Second, the potentially large difference artifacts near zero crossings due to noise, attenuation and NMO stretch are reduced. Third, since a strong gas-related AVO response will dominate the far offset data, multiplication of the difference section by the far offset section serves as an enhancement of a positive AVO response.

The AVO attribute displays were instrumental in demonstrating the presence of hydrocarbon-charged reservoirs in a depositional fairway focused on the Zafiro Field. Subsequent drilling has provided the physical measurements that will hopefully allow more quantitative assessment of these anomalies in the future. At the moment we are able to show that every significant hydrocarbon bearing reservoir encountered in the wells has had a positive AVO anomaly associated with it. But unfortunately the converse is not true - that is to say that we have penetrated positive AVO anomalies that have not had hydrocarbons associated with them.

The additional information that the angle stacks provide is not limited to the amplitude attribute alone. We are beginning to integrate the frequency attribute and other attributes will follow. In order to position ourselves for the future, the full processing of the 3D data, discussed below, was adjusted to allow for the spooling-off of the angle stacks during the main processing flow. We plan to extract the AVO attribute from this data later this year.

3D seismic survey

The Zafiro-1 discovery confirmed the presence of an active petroleum system and secured Mobil's long term commitment to the area. As the appraisal wells were being planned it was recognized that the existing 2D data set was not providing sufficient spatial or temporal resolution for the development of the Field. A 3D survey was planned which was extended to cover most of the structurally inverted section in water depths less than 3,500 ft. At its maximum limits, the full fold coverage extends 64 km in the in-line direction (northeast-southwest) and 21 km in the cross-line direction (northwest-southeast). The acquisition parameters were selected to maximize stratigraphic resolution in the Zafiro reservoir zone and still provide adequate penetration into the deeper toe thrust packages.

The survey was shot between June and September, 1995 by PGS Exploration. On-board processing was applied to a fast-track volume comprising one quarter of the traces that had been decimated such that the processed volume yielded a 25x25-meter grid with full 36 fold. Full 3D dip move out (DMO) correction was applied to the decimated volume and the data stacked on-board. The 3D migration was applied on-shore the result of the fast-track processing was that the interpretation team in Dallas had a 3D migrated data volume with which to work three weeks after the completion of the data acquisition. The on-shore processing of the full data volume was not completed until August 1996. Without the advantage of the fast-track processing it is unlikely that the Field could have been appraised sufficiently to meet the production start-up goal of end August 1996.

Amplitude mapping

The appraisal well program has confirmed the relationship between the oil and gas bearing Lower Qua Iboe sands and a high amplitude seismic response. The upper gas sand, is a fairly heterogeneous unit and gives a classic response on most of the logs irrespective of the phase of hydrocarbon present. The base of this unit is associated with a strong negative seismic trough (SEG normal display polarity). Most of the sands in the Zafiro area behave in this fashion. The Main Zafiro reservoir on the other hand typically comprises two zones. The partition between the two zones is not obvious from the gamma ray and resistivity curves. The sonic and density logs however show a significant increase in value from the upper zone to the lower zone. This increase in acoustic impedance dominates the amplitude response of the Zafiro reservoir irrespective of the hydrocarbon phase and produces a strong negative trough (SEG normal display polarity) associated with the zone interface. The detailed mineralogy of these units is being investigated to see if this interface may represent a shift in sand provenance. We have also observed that the base of the hydrocarbon reservoirs in this area is often marked by a lowering of frequency of the seismic data. This is attributed to the absorption of the higher frequencies by the gas saturated hydrocarbons. This effect provides a good first pass method of evaluating exploration leads.

Following detailed seismic sequence mapping, horizon consistent amplitudes were extracted from the data set using the Landmark StratAmp software. The observed amplitude patterns indicate that temporal variations in depositional style occurred throughout the Lower Qua Iboe interval.

An amalgamation of strong amplitude events were orientated approximately east to west (parallel to the present day shelf margin). Individual sub-units were seen to trend in a more northwest to southeast direction (oblique/orthogonal to the present day shelf margin). The seismic indicated that the individual sub-units may have been offsetting each other with subsequent sand bodies being deflected around previous deposits. The sand bodies are fairly massive in nature. The bulk trend of these seismic amplitudes implies some type of slope control to the deposition, perhaps representing the freezing of the sediments along the base of slope. A low frequency black event indicated the presence of hydrocarbons.

Three depositional systems (1,2 &3) orientated perpendicular to the present day shelf margin. Each system comprises several narrow (150 meters) channel-like forms which emanate from a point source and can be tracked for several km down dip. The individual channels cut back and forwards across each other within a channel belt which varies up to two to three km across. The fast-track processing introduced some noise over systems 1 and 2 (orientated along the shooting direction) but careful examination of the data revealed the presence of the channels. The thin, narrow, but persistent sand bodies probably represent lower slope deposition downdip of a steep, possibly narrow, canyon system that migrated northeast with time.

The amplitude distribution is reminiscent of a modern meandering river system. To the north and south of the display the channel form becomes broader and less distinct. However, over the middle section, extending four km downslope, two distinct channel forms can be observed. The channel form to the west shows only slight sinuosity whereas that to the east exhibits classic meander belt morphologies including cut-offs or ox-bow lakes. Such morphologies have been described from the present day water bottom off the front of the Mississippi and Amazon Deltas. How these channels become filled with sand is more of a problem. Perhaps sand was being actively transported down these conduits and froze due to a shift in sediment supply. One possibility is that these channels were filled by sand during a major slump/density flow event when the transported sands infilled the pre-existing topography and the rest were transported out into the abyssal plain. Individual amplitude anomalies occur on a common surface. This may indicate that a series of events were responsible for the sand accumulations. The sands are fairly massive (over 25 feet thick) and log character indicates a coarsening upward sequence or a lower zone dominated by shale clasts.

The areal distribution of the amplitudes is very distinctive. The axis of the form is characterized by low amplitudes while the flanks of the form exhibit the higher amplitudes. The high amplitude event has a gull-wing cross-sectional form with a very narrow low relief, axial, channel cut. In both plan and cross-sectional view this feature is reminiscent of a channel system with a significant associated splay deposit that extends up to 1.5 km away from the central axis. In this instance, the sands may have been deposited in the splays. Although this feature is yet to be penetrated by a well, the low frequency peak (black) below the amplitude anomaly is indicative of hydrocarbon fill.

Conclusions

Improvements in the way in which we process and display the AVO attribute have provided a means of better differentiating between hydrocarbon fill and lithological effects in the shallow (<6,000 ft), relatively unconsolidated sediments of the niger delta. this, in turn, has allowed us to better define exploration fairways and high-grade exploration leads. as our log data base increases we expect that more quantitative analysis will result. detailed examination of seismic amplitude extractions is not only providing information on the geometries of the known reservoir units but is identifying other reservoir trends which may significantly increase the size of the zafiro field or lead to the identification of new fields.

The ability to provide spatial control of gross reservoir units, in association with high vertical resolution core and electric log data, is allowing the sedimentologists to refine their depositional models. Ultimately this will lead to better reservoir models for use in development drilling and production simulation. This combination of disciplines should lead to a considerable improvement in production efficiency and a substantial boost to the bottom line.

[This paper is being presented at Offshore West Africa '96 in Libreville, Gabon. References on request.]

Copyright 1996 Offshore. All Rights Reserved.

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