OTC 17360

Beating Caspian Geohazards, Block 1 Development, Turkmenistan M. Galavazi and J. Wegerif, Fugro Engineers BV; Z. Razak, Petronas Carigali SDN BHD; and I. Hamilton, Fugro Survey Ltd

Copyright 2005, Offshore Technology Conference This paper was prepared for presentation at the 2005 Offshore Technology Conference held in Houston, TX, U.S.A., 25 May 2005. This paper was selected for presentation by an OTC Program Committee following review of information contained in a proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Papers presented at OTC are subject to publication review by Sponsor Society Committees of the Offshore Technology Conference. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to a proposal of not more than 300 words; illustrations may not be copied. The proposal must contain conspicuous acknowledgement of where and by whom the paper was presented. Write Librarian, OTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract The geohazard assessment for Block 1 identified multiple geohazards in the field development area. The most significant of these hazards are active faulting, mud volcanoes, shallow gas and earthquakes. Less dramatic but equally important features identified were seabed ridges and channels at or just below seabed.

In many cases, such hazards would seriously jeopardise a successful and safe development of a hydrocarbon field. However, for the Block 1 development, these hazards were identified in a timely manner such that their accompanying risks could be incorporated in the field layout design without major cost implications. Initial field layout was such that mud volcanoes and shallow gas occurrences were avoided, and well planning incorporated the location of the active faults.

In the Block 1 field development, an early understanding of the geological setting combined with good interaction between the structural engineers, drilling engineers and geohazard specialists resulted in a cost-effective field design minimising the uncertainty and financial as well as environmental risks.

This paper illustrates that geohazards need not always have a major impact on hydrocarbon field development. It shows that, with the right approach and timely recognition, successful hydrocarbon development is possible in one of the world's most geohazard prone and complex areas. Introduction The Block 1 area, in which Petronas Carigali intends to develop the Livanov, Barinov and Gubkin field, is located on the eastern part of the Apsheron Ridge, approximately 65 km to 95 km offshore Turkmenistan (Figure 1). The area covers about 2000 km2 and the water depth ranges from 40 m in the east to 100 m in the west of the area.

Figure 1. Location of Block 1 on the Turkmenistan Shelf, Caspian Sea.

Virtually no detailed information has been published on

the geology of the Turkmenistan shelf. However, previous experience along the Apsheron Ridge suggested that geohazards might well pose serious problems for field development in the Block 1 area. This is indicated by, for instance, the presence of many collapsed platforms and wells, which form relicts of the attempted development of the fields during Soviet times.

To deal with the geological challenges, an extensive site characterisation study based on 3D exploration seismic data and regional 2DHR seismic data was performed during an early phase of field development. The resulting 3D site model to a depth of 1000 m sub-seabed was subsequently used to prepare a conceptual field development plan and to perform a realistic conceptual engineering study. Geological Setting The Block 1 area in the Caspian Sea lies on the Apsheron Ridge, which forms the boundary between the South and Central Caspian Basins.

The South Caspian Basin is thought to be a remnant of oceanic crust from the late Mesozoic or early Tertiary Tethys Sea, and is being subducted beneath the Eurasian plate in the

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north and the Arabian plate in the south. The Apsheron Ridge, stretching between the Caucasus and the Kopet Dag Mountains, is the surface expression of the subduction zone between the South Caspian Basin and the Eurasian continental plate (Jackson et al., 2002; Brunet et al., 2003). This subduction zone is one of the main controls of the seismic hazards in the Caspian Sea and particularly in the Block 1 area.

The relative motion of the Eurasian and Arabian continental plates results in crustal compression of the region. The mountain ranges surrounding the southern and central Caspian Sea are fold and thrust belts formed as a result of the crustal shortening from plate collision (Yilmaz, 1997; Jackson et al., 2002). Thrust faulting in the Mesozoic strata caused by subduction of the South Caspian Basin, resulted in the formation of an anticlinal ridge, the Apsheron Ridge, in the overlying Tertiary and Quaternary strata. Extensive, deep and shallow seated faulting occurs along the crest of the ridge and many of these faults are active and have offsets at seabed. Although the South Caspian Basin is thought to have originated in Mesozoic times, the bulk of the sedimentary infill is of Oligocene age and younger. Approximately 10 km of Plio-Pleistocene sediments have accumulated in the basin, representing an average sedimentation rate of 2 km/My (Brunet et al., 2003). This rapid, clay dominated sedimentation combined with hydrocarbon generation and tectonic forces has led to the formation of highly overpressured, underconsolidated clays at Maikopian (Oligocene to Early Miocene) level (Brunet et al., 2003). Mud volcanism occurs where these overpressured mud and fluids escape to the seabed. Methodology In a geologically complex area such as Block 1 in the Caspian Sea, it is impossible to understand local features without an in-depth understanding of the regional geology. It is therefore important to create a regional overview before zooming in on local details. This phased approach has been adopted for the geohazard assessment of Block 1, as shown in Figure 2.

When run in parallel to the phases of field development (Figure 2), this phased approach reduces uncertainties and the associated investment risks by allowing identification of major constraints for field development at an early stage, and results in a cost-effective and realistic field development plan. The successive phases in the geohazards assessment are further explained below. Phase 1: Regional Site Characterisation. The first phase in site characterisation involves a regional screening with regard to geohazards, geology and geotechnics by using all available data to produce a single coherent model of the site.

An initial literature desk study provides a first-pass overview of the area. Based on this desk study, the actual requirements for a site characterisation can be defined. Such requirements could include the need for seismic hazard modelling, slope stability analysis or other specialist input. Interpretation of 3D exploration seismic data and regional 2D high-resolution data further defines the hazards affecting the development and the issues that need to be addressed for successful field development.

Preliminary site characterisation is typically performed in the exploration and appraisal phases of field development and results in a regional site model, which incorporates the type, general distribution, frequency of occurrence and severity of the identified hazards. Based on this model, the most favourable sites for facilities may be selected.

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Figure 2: Flow chart showing phases of site characterisation. Phase 2: Field layout. The prime objective of Phase 2 is to merge the preliminary site model with the conceptual reservoir development plan to incorporate the identified geological constraints on field development into the conceptual field layout.

Combining the site model with the conceptual reservoir development plan at an early stage is essential. During this phase of field development, there is sufficient flexibility in defining and refining the field layout. Changes to the layout plan that are required to avoid or mitigate hazards can be incorporated with minimal cost implications. Additionally, at this stage tailor-made reconnaissance surveys may be performed to further define the risk of specific hazards, when the field layout can still be altered.

For Phase 2 to be successful, a close interaction between the geohazard team, drilling engineers and the exploration and production team is of the utmost importance. An iterative

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process to determine the ideal field layout has proven to be most time and cost efficient.

The ultimate output of Phase 2 is a realistic conceptual field layout, which incorporates all site-specific hazards. Phase 3: Site-specific Data Acquisition. Once a conceptual field layout is in place, site-specific high-resolution or ultra high-resolution well site surveys and pipeline route surveys can be performed. Besides providing detailed information for siting of facilities, the interpretation of such high-resolution seismic data is used to refine the preliminary site model. Feed-back of the high-resolution data increases the reliability and the level of detail of the site model. Due to limitations in the vertical resolution of 2D/3D exploration seismic data, interpretation of this data is limited to relatively large-scale features, which can be refined with the much higher resolution of the site-specific survey data. The increased level of detail may aid in the understanding of complex processes occurring across the site, such as detailed fault offset interpretation for probabilistic fault modelling or stratigraphic mapping for slope stability analysis.

Phase 4: Foundation Design. The high-resolution well site or pipeline route data allows for final siting of facilities. Based on the final field layout, a geotechnical site investigation can be planned to acquire data for foundation design.

A phased approach to site characterisation enables a cost

and time effective development of the site and ensures that geophysical and geotechnical site surveys only need to be performed once to obtain the required results. Results The first phase in the site characterisation study of Block I consisted of a literature study to identify the major geological constraints for development. In-house experience in the South Caspian Basin and an extensive literature search indicated that the Turkmenistan shelf is located in a seismically active zone and shows complex geological features. Based on the desk study, the key parts in the site characterisation study consisted of a probabilistic seismic hazard analysis and mapping of geohazards to approximately 1000 m sub-seabed.

The probabilistic seismic hazard analysis was based on historical seismic data collected from literature in an area approximately 500 km2 around Block 1. The historic seismicity data was grouped into deep events of the Apsheron Ridge region and shallow events of the surrounding region. Using the Gutenberg-Richter formulation (Gutenberg and Richter, 1965) earthquake recurrence for each of the seismic sources could be defined. These results were used to compute a seismogenic model of the Block 1 area, based on which a probabilistic seismic hazard analysis (PSHA) was performed to calculate expected peak ground accelerations.

The PSHA results indicate that peak ground accelerations of up to 0.20 g should be considered for preliminary design. As no detailed fault offset analysis was yet available at the time of study, this value may represent a conservative estimate. Further detailed analysis and mapping may provide a more precise estimate of ground accelerations.

Mapping of geohazards involved the interpretation of - 3D exploration seismic data to 1.5 ms across the entire

Block 1 area - regional 2D exploration seismic data across the western

half of Block 1 - high resolution 2D seismic data covering three 2kmx2km

areas - interpretative charts of regional analogue survey side scan

sonar and sub-bottom profiler data - four detailed geotechnical site investigations varying from

40 m to 80 m below seabed. Based on the seabed pick on the 3D exploration seismic data, a pseudo-bathymetry map (Figure 3a and 3b) was produced which provided an overview of water depth variations and seabed features. Although a 3D bathymetry has some limitations related to the limited vertical resolution of the 3D data, seabed features such as fault scarps, mud volcanoes, pockmarks and carbonate mounds are readily identifiable. The 3D seismic seabed interpretation was the key to a first understanding of the Block 1 area.

Figure 3a. Shaded relief map of pseudo-bathymetry western Block 1

Figure 3b. Shaded relief map of pseudo-bathymetry Eastern Block 1

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Figure 4. Regional shallow geohazard map (to 200 m sub-seabed).

With the use of the pseudo-bathymetry and sub-seabed interpretation of the complete data set, the zone to approximately 200 m below seabed was interpreted with regard to geohazards that could affect facilities (Figure 4). Drilling hazard screening was performed to a depth of approximately 1000m below seabed (Figure 5).

Figure 5.Data example of seismic anomalies to 1000 m sub-seabed. The main hazards encountered in the Block 1 area include active faults, shallow gas, expulsion features, carbonate build-ups and mass transport features.

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A transtensional strike slip duplex was identified in the Block 1 area (Figure 3a), consisting of a zone of closely spaced faults associated with thin-skinned tectonics along the crest of the Apsheron Ridge. The pseudo-bathymetry map shows that many of these faults have offsets in excess of 5m, locally up to 12 m, at seabed. Sea level curves for the Caspian Sea (Kosarev et al., 1994) indicate that during the Mangyshlak regression approximately 10,000 years BP, sea level was well below that of the present, suggesting that the Block 1 area was sub-aerially exposed during this period. It is thought that sub-aerial exposure has resulted in the erosion of previously formed seabed scarps. It is therefore probable that the scarps observed at present represent fault movement over the past 10,000 years. This indicates that fault movement in the Block 1 area may be considered a significant hazard and has an important impact on pipeline routing and facility siting.

Shallow gas in the Block 1 area is present both as a broad zone of disseminated gas and isolated pockets of potentially pressurised gas. The shallow disseminated gas is probably not pressurised as no problems were encountered during drilling of geotechnical boreholes in the past. However, the localised pockets of potentially pressurised gas do form a drilling hazard as they could result in uncontrolled gas blow-outs and unstable hole conditions. Side scan sonar results from the regional analogue survey indicate a number of wrecked platforms, which may be attributed to gas blow-outs due to drilling such pockets of pressurised gas.

Expulsion features are common in the Block 1 area. Several mud volcanoes have been identified and pockmarks and gas vents occur throughout the site. Two types of mud volcano were observed; mud mound type and depression type mud volcanoes, or salsas (Jakubov et al., 1971). The first type is associated with the release of hydrocarbon fluids and mud at seabed (Figure 6a) while at the latter type, only hydrocarbon fluids are released, but no mud (Figure 6b) (Jakubov et al., 1971; Hovland et al., 1997; Planke et al., 2003).

Figure 6a. Data example of mud mound type mud volcano.

Figure 6b. Data example of salsa type mud volcano. The mud volcanoes in the Block 1 area are interpreted to be actively seeping gas, water and locally mud, as is evidenced by a recent mud outflow observed at seabed. The flow is approximately 3 km long and 500 m to 900 m wide. The flanks of the mud volcanoes are thought to be unstable as side scan sonar results from the regional survey show evidence of slumping. The processes associated with fluid and mud expulsion are active and may adversely affect foundation stability and drilling operations and are therefore important to take into consideration for siting of facilities.

Expulsion of gas from isolated vents or seepage of gas along faults occurs extensively in the Block 1 area. Many faults appear to act as migration routes for gas, and seepage is mostly concentrated in the zone of intensive faulting along the crest of the Apsheron Ridge. In the water column, plumes of gas and suspended material are visible on 2D seismic data, indicating active seepage at present. Associated with the active seepage of gas are carbonate concretions, interpreted as methane-derived authigenic carbonates. These carbonates are formed due to bacterial activity in the interface between gas and sea water (Hovland, 1990; Johnson et al., 2003). The carbonates have a rock-like character and form rough patches on the seabed. As these patches may be underlain by soils with reduced shear strength due to a high gas content, they may pose a punch-through risk for facilities emplaced upon them.

Mass transport complexes as a result of slope failures are common in the South Caspian Basin. No recent large-scale slope failures were identified in the Block 1 area, but buried mass transport features occur at various depths and are generally very thick. Mass transport features may be more prone to unstable hole conditions and loss of circulation than undisturbed sediments.

Mapping of the different hazards has resulted in a regional site model for the Block 1 area, which provides an overview of the spatial distribution and zone of influence of the hazards. This regional 3D model was used in the field layout phase to assess the impact of geohazards on field development, and in finding the solution to manage the associated risks.

Based on the regional site model, the initial development concept was changed from multiple directionally drilled wells from a single platform to a combination of directionally drilled and vertical wells from multiple platforms. The zone of closely spaced active faults along the crest of the Apsheron

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Ridge inhibits the extensive use of directional drilling techniques, as the wells would cross active faults, risking drilling operations and the integrity of the wells.

Use of the regional site model has allowed for the selection of well sites with a safe stand-off distance from expulsion features, shallow gas occurrences and faults. It has prevented the performance of detailed site surveys at sites unsuitable for development, thereby saving time a money. Drilling hazards, such as pressurised gas pockets and active faults could be minimised.

The regional site model is also used for routing of infield and export pipelines, such that steep slopes and faults are avoided. Steep ridges are difficult, if not impossible to cross due to the limitation of pipeline curvature, while fault movement may potentially rupture pipes.

High-resolution seismic data for a number of well sites has recently been acquired, and the high-resolution data has been incorporated into the preliminary site model to update and refine it. This allows the site model to become progressively more detailed and reliable, which in turn allows for more detailed assessment of siting of future facilities. Conclusions The site characterisation study for the Block 1 area has shown that, even in a geologically complex area, geohazards need not necessarily have a negative impact on offshore hydrocarbon development, as long as they are recognised at an early stage of development.

The phased approach adopted for this study has resulted in the timely recognition of major constraints to field development. The regional site model, produced in the exploration and appraisal phases of field development, was combined with the conceptual reservoir development plan to optimise field layout.

Assessing geohazards as early as the conceptual phase in field development allows upfront risk assessments for proposed drilling and platform sites and relocation of facilities to low-risk areas before spending large sums on detailed surveys in high-risk areas.

The main result of the study was to change the development concept from a single platform with multiple deviated wells to multiple platforms with vertical wells. The presence of closely-spaced active faults limits the possibilities for directional drilling. At this early stage, changing the conceptual reservoir development plan involved minimal cost implications.

Key to the success of this approach was the close interaction between the geohazard team and the drilling engineers and exploration and production teams. References Brunet, M-F., Korotaev, M.V., Ershov, A.V. and Nikishin, A.M.,

(2003). The South Caspian Basin: a review of its evolution from subsidence modelling. Sedimentary Geology Vol. 156, pp 119-148

Gutenberg, B. and Richter, C.F. (1956). Earthquake Magnitude, Intensity, Energy and Acceleration. B. Seism. Soc. Am., Vol. 46, No.2, pp. 143-145.

Hovland, M., 1990, Do carbonate reefs form due to fluid seepage?, Terra Nova Vol. 2, pp 8-18.

Hovland, M., Hill, A. and Stokes, D. (1997). The structure and geomorphology of the Dashgil mud volcano, Azerbaijan. Geomorphology vol. 21, pp 1-15.

Jackson, J., Priestley, K., Allen, M., and Berberian, M. (2002). Active Tectonics of the South Caspian Basin. Geophys. J. Int., Vol. 148, pp. 214-245

Jakubov A.A., Ali-Zade A.A. and Zeinalov, M.M. (1971), Mud volcanoes of the Azerbaijan SSR: atlas (in Russian). Azerbaijan Academy of Sciences, Baku.

Johnson, J.E., Goldfinger, C. and Suess, E. (2003). Geophysical constraints on the surface distribution of authigenic carbonates across the Hydrate Ridge region, Cascadia margin. Marine Geology vol. 202, pp 79-120.

Kosarev, A.N. and Yablonskaya, E.A. (1994), The Caspian Sea. Planke, S., Svensen, H., Hovland, M., Banks, D.A. and Jamtveit, B.

(2003). Mud and fluid migration in active mud volcanoes in Azerbaijan, Geomarine Letters, vol. 23, pp 258-268

Yilmaz, Y. (1997). Techtonics of the East Anatolian-Caspian Regions. The Leading Edge, p 89-891