GeoArabia, Vol. 7, No. 4, 2002 Gulf PetroLink, Bahrain A successful 3-D seismic survey over Bahrain Island Jean-Jacques Postel, Companie Générale de Géophysique, Abdul Nabi Mukhtar, Bahrain Petroleum Company and Philippe Feugère, Companie Générale de Géophysique ABSTRACT Environmental constraints are critical issues for land seismic operations and require the development of appropriate equipment and methods. In 1998, Compagnie Générale de Géophysique (CGG) acquired and processed seamless seismic data on behalf of the Bahrain Petroleum Company (Bapco) from a complex field on the Island of Bahrain. The operation ran smoothly in spite of the difficulty of working amongst pipelines, wells, and other oil and gas installations; scarps and sabkhas; urban areas; a zoo, golf course, and racecourse; an oil refinery and aluminum smelter; and other industrial and commercial facilities. The survey deployed a wide range of recording equipment including two sets of vibrators and a combination of radio and cable telemetry recording systems, and a fleet-management system. The successful completion of the survey was achieved through dedicated teamwork. The key elements were as follows: (1) collecting the correct baseline information; (2) detailed planning of the timeframe, human and material resources, adaptation of acquisition techniques to varying circumstances, and health and safety requirements; (3) continuous monitoring of external conditions and the impact of the operation on the environment, in full consultation with the appropriate authorities; (4) maximum flexibility in the field operations so as to allow for changing conditions and unforeseen events; and (5) close partnership between CGG and Bapco at all stages of the project. A joint CGG/Bapco team designed sophisticated processing methods, with top priority being given to the numerous surface heterogeneities. In particular, the recording of an intensive up-hole survey allowed a dedicated team at CGG in France to compute a robust subsurface model and a precise set of primary statics. Seven velocity layers from the Quaternary sandstone to the base of the calcareous and dolomitic Eocene were mapped. These showed significant lateral and vertical velocity variations within identified geological units. Surface-consistent signal processing with calibration to well data was used to compensate for the variations. Other critical processing steps included detailed velocity control, the application of dip moveout routines with acquisition-irregularity- compensation schemes, Radon demultiples and 3-D algorithms, designed for optimal de-noising and imaging. These combined efforts led to the delivery of a final 3-D migrated block. As a result, a complete reinterpretation of the area was possible that significantly increased the estimated recoverable oil reserves (particularly by-passed oil) in the Awali field. INTRODUCTION The paper is based on presentations made at the GEO 2000 Conference in Bahrain in March 2000 by Postel and Mukhtar (2000) on the acquisition of the 3-D seismic data, and by Crémiere et al. (2000) on a complementary study of the data processing. Bahrain Island has a surface area of about 586 sq km. It is the topographic expression of the northerly oriented Awali dome that is affected by a complex horst-and-graben fault system. The island has low relief and its highest point is Jebel ad Dukhan (134 m) in the center of the dome. The geological and topographical variety in the survey area (Figure 1), together with the many surface obstructions (Figure 2), made the survey a combined acquisition/processing challenge. 657 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/7/4/657/4564341/postel.pdf by guest on 28 September 2021 Postel et al. Geologic Map Muharraq Island Quaternary To Saud i Ara bia Umm An Na'san Island Arabian Gulf Escarpment of Dammam Formation surrounding Interior Basin Jebel Cap formation (Jebel ad Dukhan 134 m) Interior Basin with Rus Formation Dammam Dammam Formation (Shark Tooth Shale Member) Formation (Khobar Member) Hawar Islands N 010 N (Same scale) km Figure 1: Simplified geologic map of Bahrain Island. 658 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/7/4/657/4564341/postel.pdf by guest on 28 September 2021 3-D seismic survey, Bahrain Island Satellite Image Urban areas Industrial areas Plantations Race course Oil field facilities University of Bahrain Al-Areen Wildlife Park Quarries Sabkha N 010 Hawar Islands km Landsat image courtesy of ChevronTexaco Figure 2: Satellite image of Bahrain Island showing the outline of the 3-D survey area (orange) and the main logistical constraints (red) that had to be overcome in recording the survey. 659 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/7/4/657/4564341/postel.pdf by guest on 28 September 2021 Postel et al. The Eocene Rus Formation crops out in the central erosional depression (the ‘Interior Basin’) of the dome (Figure 1). The Interior Basin is completely surrounded by the Eocene Dammam Formation that forms a low outward-dipping escarpment. Low-angle dip slopes of the Dammam Formation merge into the unconsolidated Quaternary deposits of the coastal lowlands that are most widespread in the north, west and south of the Island. In the center of the Awali dome, the Miocene Dam Formation (Jebel Cap formation) unconformably overlies the Rus Formation to form Jebel ad Dukhan. The geology of Bahrain Island is summarized in a 1996 Exploration & Production Highlights feature in GeoArabia (v. 1, no. 3, p. 376–379). The 3-D survey of 1998 was the first such survey of Bahrain Island. It covered an area of 325 sq km, or about 55 percent of the Island’s surface area, and was centered on the main petroleum producing area of the Awali (or Bahrain) field. The history of the field dates from 1932 when it was the first oil field to be discovered in the Gulf region. More than 600 oil and gas wells have been drilled. The aims of the survey were as follows: 1. Primary targets: (a) Cretaceous carbonates and clastics at a depth of about 2,000 ft (600 ms Two-way Time: TWT)— detection of small-throw faults and fractures and their orientation, and the identification of thin sand bodies and fluid contacts; (b) Permian Khuff carbonates at a depth of about 10,000 ft (1,400 ms TWT)—detection of faults and their orientation, and the identification of porous zones and fluid contacts. 2. Secondary targets: (a) Jurassic carbonates at a depth of about 4,000 ft (900 ms TWT)—detection of faults, fractures and their orientation, and the identification of facies changes and fluid contacts; (b) Paleozoic clastics at depths of at least 12,000 ft (>1,600 ms TWT)—identification of facies changes and fluid contacts. DATA ACQUISITION Survey Constraints The survey area (Figure 2) was characterized by three distinct zones: • The southern zone is mostly desert and covers restricted areas. Some villas and farms are located in this area, which is also a natural wildlife reserve inhabited by animals such as sand gazelles (rheem). A sabkha is located on the western side of the zone. • The central zone covers the oil and gas field and includes 600 wells, many pipelines, and several production plants. The Al-Areen Wildlife Park, the University of Bahrain, and several large private properties are located on the western side of the zone. • The northern zone has urban areas such as Awali, Riffa, Isa, and Sanad. Several compounds and many large private properties are also located in this area. In addition, the Bapco oil refinery and the Alba aluminum smelter are located in the east of the zone. As the survey took place during the summer (from June to November), high temperatures made working conditions difficult. In June, the average temperature was 37ºC, which was 2ºC higher than the record temperatures for the previous 96 years. July was also the hottest for 96 years. Temperatures were very high throughout the month, with a maximum of 54ºC being recorded for 12 days. The base camp for the survey was located 10 km from Manama, and Bapco’s Awali facilities were used for mechanical and geophone and cable repairs (Figure 2). 660 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/7/4/657/4564341/postel.pdf by guest on 28 September 2021 3-D seismic survey, Bahrain Island The extremely sensitive nature of the environment required the preparation of a complete health and safety report that detailed and rated all the risks associated with the operation. The major risk identified was road transportation. Special training sessions were given at the start of the survey to about 200 people who worked on the project. The course was divided into the following topics: • general safety information given by the Safety Advisor focusing on the objectives of the crew, the definition of an accident, and the risk matrix; • awareness of the major risks; • working conditions in the field; • awareness of the environmental impact; and • distribution of personal protective equipment. Parameters In order to fulfill the technical, economic and logistical constraints of this survey, the 3-D parameters were designed as follows: Table 1 3-D Seismic acquisition parameters Group interval (m) 50 Number of active channels 1,088 Receiver line interval (m) 200 Bin size (m) 25 x 25 VP interval (m) 50 In-line fold 17 VP line interval (m) 200 Cross-line fold 4 Number of active receiver lines 8 Total fold 68 Figures 3 and 4 show the parameters in detail and the offset and azimuth distributions. We noted a uniform distribution of the offsets from 500 m to 3,500 m. Receiver lines were oriented E-W and vibrator lines N-S. Although the crew was equipped with two sets of vibrators, the time to move from one salvo to the next was on average equal to the time to record eight Vibrator Points (VP).